Ever wonder why all those things look alike?
Simple. Form follows function. A recording control room has to do three
things which dictate it's shape and size. Specifically, the room must:
1: Keep outside sound out; inside sound in. Control rooms must be
soundproof. That mandates building materials as well as some of the
construction details.
2: Allow the mixer a view of the musicians. Windows are a must.
3: Give the mixer a clean shot at the monitor speakers, without
interference from the room. Acoustically, a control room should do
nothing at all. It is a laboratory environment for the monitor speakers
and the mixer.
A room that does nothing sounds easy. Just wallpaper it in SonexTM or
whatnot. No reflections, no room effect, no problem.
But: Those of us who have tried it know that working in a dead environment
drives mixers out of their minds. You need room sound around you to hear
properly.
Since a control room's reflections can't be eliminated, they have to
be managed so as to avoid messing up the monitor sound. That we know how
to do. In fact, we've been doing it as long as we've been recording.
As a case in point, the writer worked for some years in a control room
built for WGN Radio in 1932 that did nothing about as well as possible.
It was monaural, of course, but it was a dandy.
The room was thirty-odd feet wide and about twenty deep with a sixteen foot
ceiling. The front wall and ceiling were very dead, the back and side walls
had been left untreated and reflective.
At eight feet, the speakers were too close to the mixer, but otherwise
that 60 year old room would probably have met the current standards for
a high tech monaural environment just as it stood.
Only probably, because it was never measured; but probably nonetheless,
because of the location of the treatment material (fluffy brown paper on
the wall, peculiar looking tile on the ceiling), and the size of the thing.
Sounded great in mono and better than most in stereo.
It sounded good for the same reason that a lot of old control rooms sound
good. They're enormous, and dimensions are critical to control room
acoustics. Room dimensions (and geometry) control reflection times, which
are the primary design parameter for modern rooms.
While excellent (although huge) control rooms have been built since the
30's, putting together good small ones was a terrible problem until a
few years ago. They sounded cluttered, the stereo positioning was fuzzy
to nonexistent, and they were very tiring to work in. We did good work in
them anyway, but it was hard. Real hard.
Then, about ten years ago, there was a revolution. Without getting into
detail, several exceedingly bright research people exchanged a raft of
data from differing fields, and worked out why the small rooms sounded
lousy.
It had to do with the difference in time between the arrival of the direct
sound from the speakers and the arrival of the first strong reflection
from a wall as heard by the mixer. Seems that a short reflective second
path to the mixer's ears generates a comb filter which the hearing
mechanism confuses with it's own method for determining the elevation
of a source signal; a series of comb filters generated by/in the fleshy
outer ear (pinnae). This confusion raises hell with hearing as the brain
tries to make sense of false elevation cues. Very muddled, very tiring.
The solution to the problem is obvious. Eliminate the short reflection
paths. Working out a design which would actually do it was not obvious
at all. In fact, it required a room that was totally different from
anything that had been built up to that time.
The first designer/builder to come up with a room that worked was Chips
Davis, and through Synergistic Audio Concepts, who were largely responsible
for the information exchange, trademarked the performance specs as Live End
Dead End (L.E.D.E.), mostly to keep unethical types from building the same
old bad rooms and calling them the hip new name.
Having developed the LEDETM, Ol' massa Davis went out and built a bunch of
them. So did a few others, as Syn-Aud-Con's newsletter had published the
design parameters in considerable detail.
His worked every time. Other designers had somewhat less luck, probably
because he understood the concept better than they did. Some did well,
some not so well, particularly when they just followed the shape of the
Davis rooms rather than designing to the theory. There were several bears
in those woods, the meanest of which was back wall geometry.
The back wall bear was shot dead a few years later when Peter D'Antonio put
Schroeder formula Quadratic Residue DiffusersTM, on the market, as they made
back wall treatment both simple and reliable, and everybody started
building Davis look-alikes.
Any number still do. Some of them are thoroughly competent designers whose
rooms meet all the Davis specifications and can be certified as LEDEs, but
mostly they're studio owners trying to save a few bucks by doing their own
design work. Unfortunately, the latter usually call their rooms "ledes",
as they're not aware of either the trademark or the licensing involved in
the term. Makes things confusing.
The majority of look alike rooms aren't certified, and can't be, because
they were made to look like George's, which was copied from Sam's, which
resembles Fred's, which was a crude copy of a real LEDE. It's the old repeat
the story around a circle of people trick. Doesn't work for stories, doesn't
work for control rooms. I've been in a few that came off worse than an
ordinary living room with a couple of speakers hung in it, and a lot more
that started out as fundamentally sound (sorry 'bout that) designs, but
were royally screwed up by adding unnecessary stuff to the original
furnishings.
The sad thing about bogies is that many of them cost as much as the real
thing and were beautifully crafted by their owners, who spared no effort
to make them perfect, but who ended up with nearly useless rooms because
they didn't understand the basic rules of the game. I really feel for those
guys, but it's too late for the majority of existing rooms. Retrofit is
time consuming. If the studio's busy, the down time losses are killer. If
it isn't, there's no money for the work.
Important Note: There is no substitute for professional help. Designer /
builders not only supervise the construction work, they guarantee the
results. They also know enough tricks to save a client most of their fees.
Sometimes more, as owners usually over-build.
For the dedicated do-it-yourself types who plan to put something together
in the future, or fix a not so bad room, however, the rest of this article
should prove helpful.
Since huge control rooms are old hat, we'll assume minimum space available.
The first critical dimension is from the mixer's ears to the back wall,
which has to fall into a real world Haas zone at about 20 milliseconds.
Eighteen is known OK, fifteen may be, but why gamble?
Don't go too long, as you'll start to hear double attacks at 40 to 50
milliseconds.
The nominal 20 milliseconds works out to a wall 11'-3" back of the mixer's
ears. It's time x 1127 over 2. A foot less is about 18 milliseconds, and it
would be nice if the mixer could push away from the console, so 11-1/2 to
12-1/2 feet is reasonable.
The next bit is speaker distance. The reason for using speakers rather than
'phones has less to do with keeping your ears cool than getting a little
distance between yourself and the instruments. The sound from a mike inside
a bass drum is not only weird, it's not real good for the earbones, so
headphones are out. If they weren't we wouldn't have to spend all that money
on control rooms and monitor speakers.
A little distance has been commonly agreed as something between 12 and 15
feet, which is about right for listening to a real rhythm section. At any
rate, most control rooms put the monitors in that range. It's known to work,
so go for it.
The final question is how far apart to place the speakers. Distance apart
equal to distance from the mixer seems logical, but an awful lot of mixers
like the speakers a little over stereoed to broaden out the panning.
Something in the area of 1.3 to 1.4 times the speaker throw looks and sounds
about right to most, but it's a personal choice.
There's probably a firm rule in there someplace, but I don't know what it is.
In any case, speaker separation governs minimum room width, and distance off
the mixer locates the front wall. For a monitor distance of 15 feet the
separation would be 20 feet at 1.35 to 1.
Adding the width of the speaker cabinets puts the room at 24 to 25 feet wide.
Mounting the speakers is more complex. Simply hanging them in the front
corners of the room is a no-no. For one thing, cornering a speaker brings up
it's bass response by anything up to 8 Db, which is a long way from flat.
For another, the sound will reflect off the corner walls beside the cabinets
at a foot or two, which is a millisecond or two, and there's no way to absorb
speaker sound evenly at that distance. The pressure levels are too high, the
angles of incidence are all over the place, and the woofer waveforms haven't
properly formed; that last being why speakers are maeasured and specified at
one meter or four feet.
Hanging the speakers a few feet out from the corners takes care of those
problems, but it leaves the corners in place, and it looks hokey. Corners
sound funny anyway, so they're usually walled off or not built at all. A
pair of eight or nine foot walls across the front corners of a control room
at 50 to 52 degrees eliminates corner effects and makes a great place for
mounting speakers to boot. That's such a good idea it's nearly universal
practice in control rooms. The walls must be very rigid, though, or they'll
vibrate and muddy up the low end.
Whether the speakers are mounted in doghouses or hung on the speaker walls
is dealer's choice.
Each system has good and bad points.
Monitor speakers hung directly on the corner killing walls with nylon line
and some felt padding on the backs are easy to move, service, or change.
Simple, quick, cheap.
Doghousing is harder. The monitors have to be floated in the doghouse, or
the cabinet's bass vibrations will travel through the walls to the floor
and get to the mixer before he hears bass through the air. Sound travels
a lot faster in building materials than in air.
Additionally, the space around the speaker should be sealed off to keep it
from acting as a Helmholtz resonator. Stuffing it with Fiberglas will lower
the resonator output level, but at the expense of broadening the frequency
range. Lower Q. Since floating dictates no solid contact between the cabinet
and the doghouse, sealing off the cabinet gets interesting. An oversize
doghouse with a closely fitted front panel can be sealed with bathtub caulk,
and there are other solutions, but a doghouse essentially builds the monitors
into the walls, so they have to be unhoused in order to do anything with them.
On the other hand, the bass response is somewhat better, they take up less
room space, you pick up about 18 inches of mixer-speaker distance, and
doghouses look professional as hell.
Dealer's choice. However, bearing in mind that you'll probably be in business
awhile and this year's king of the hill is likely to be next year's turkey,
it would be smart to build doghouses big enough not only to accommodate
different speakers but to change positions and angles as well. A new console
will probably shift the mixer position, which means the monitors will have
to be moved to restore the geometry.
This brings up general design rule one (of one). Keep it flexible. You don't
know what's going to happen nest. A control room is too expensive to rebuild
every time you acquire new equipment or a new line of work, so build it as a
mainframe. Plug in the stuff you own now, plug in new stuff as things change.
Back to the walls. There are two good reasons for not building speaker walls
at 45 degrees. One is that they would play directly into the back corners of
the room (if any) and the other has to do with stereo monitor horizon.
If only one person in a control room needs to hear the monitors properly,
they can be pointed directly at that person. As a rule, however, there will
be at least one other pair of ears that need feeding, and they belong to a
producer. Since the producer is generally paying the bills it makes sense to
let him (her?) hear where the money's going, which makes broadening out the
monitor horizon a Real Good Idea. There are limits as to how wide a near
perfect field can be spread, but the only thing that will spread it at all
is over-convergence of the speakers so that as a listener moves closer to
one monitor he also moves a little out of it's axis and into the axis of the
other.
The degree to which speakers need to be crosseyed depends on the tweeter
type. High Q tweeters of the kind found in UREI monitors, which yield
outstanding imaging accuracy, need more convergence than the much lower Q,
somewhat less precise units used by Tannoy which have merits of thier own.
This writer is not going to get handed his head by engaging in a debate on
the merits of monitor types in this paper, because monitor choice generally
involves the preferences of both studio personnel and clients, and whatever
a studio uses is probably optimum for it's working situation. Nonetheless,
monitor horizon is partly a function of tweeter Q, and since it can require
extreme measures, the monitor type affects control room design. For example,
I know one very competent designer who crosseyes UREIs to the degree that
they look really spooky and then hides them behind big cloth panels. They
sound terrific, but the angle would be wrong for any number of other
speakers. Among others, Tannoys need so little convergence that simply
pointing them at the presumptive producer and second engineer positions
works very well. If you don't know the angle for your monitors, experiment
before you commit to a mounting angle.
You pays yer money and takes yer choice, but it's important to allow for
changes in the angles if you doghouse the monitors. Very big doghouse with
equally big front plates will do it. Want a different speaker? Make a new
plate. Mainframe system.
The only remaining speaker dimension is height, which should be as low as
possible, and will be dealt with shortly.
First, the window(s). Not whether. How big.
Since the only reason for putting windows in the front of a control room is
to allow the mixer to watch the players, and since windows not only transmit
bass but cost a fortune, small is beautiful.
A window six feet above the control room floor will permit a standing mixer
to look a standing vocalist square in the eye. That's high enough. The low
edge should be where the seated mixer's sight line is interrupted by the top
of the console, usually at about two feet up. That's a four foot window, and
while it ain't cheap, it beats hell out of a six or eight footer. It will
also transmit less sound through the glass.
The glass is generally 1/4 inch thick with the outside pane mounted dead
vertical and the inside tilted down at eight to nine degrees for a twelve
foot mixer-front wall distance, to keep the inside glass from bouncing rear
wall reflections into the mixer's ears. The offset distance is eight inches
for a 4 foot window.
If the front wall is further away a lower angle will work. More usefully, a
very strong case can be made for raising the console from a standard desk
height of thirty inches to thirty six or more. The extra altitude reduces
the necessary angle on the window, creates more room under the console (of
which more later), and fights off the most common control room disaster;
monitor sound reflecting off the board into the mixer's head.
Returning to first principals, the fundamental design parameter of a modern
control room is to make sure that the first solid reflection the mixer hears
after the direct speaker sound comes from the back wall at something about
20 milliseconds.
When monitors are set high enough on the front wall to bounce off the console
control surfaces they will produce a second path time of less than 2
milliseconds. Therefore; If the speakers must be mounted above the window,
keep them as low as possible or raise the console height or, if all else
fails, tilt the console, but on your life avoid monitor-console-mixer
reflections. I've seen this problem in several rooms, and it is absolutely
fatal.
Whether the speakers are put above the window or beside it depends on studio
geometry. If it's long and thin, a single window will allow decent vision.
If it's short and wide or just plain big, the window will have to extend
around the speaker walls with the monitors above the glass.
While you're thinking about windows, think about cleaning the inside surfaces
every few years.
Unless they're hermetically sealed, they'll film over and look terrible after
a while.
If you hinge the wooden frames in which the outer glass is mounted now, it'll
be easy to swing the pane down for cleaning later. If not, you're gonna have
to live with dirty control room glass.
Don't bother with a seamless wraparound window. If you can see around the
front door posts on your car, you can certainly manage the equivalent
supports for a control room window. Just sway a little to one side and away
you go, at half the cost.
It would be cute to tilt the side windows to match the front, but it ain't
really necessary, and involves some very delicate glasswork. Not reccomended.
Having got the front walls, the speakers, and the console position, we go
next to the back wall.
There are at least four ways to handle this one, ranging from Chips Davis's
first design with carefully angled zig zag panels to Peter D'Antonio's
curved wall tastefully decorated in RPGs. All of the treatments work,
because they all do the same things; they disperse most of the speaker sound
reflecting off the back wall while bouncing a smaller amount of flat smack
directly into the mixer's ears from the back corners. This combination of
diffuse and direct reflected sound puts the mixer in an aurally live space
with a clear sense of his/her own position in the room, as well as bringing
about a feeling of "being there" which tends to hold down monitor volume,
while the 20 millisecond delay leaves the sound clean and uncluttered. When
it's right, the sound seems to come through the walls behind the monitors
rather than from the speakers with no straining or reaching for it, let
alone the feeling that it's somewhere back of you, as happens with a dead
back wall. No effort, no fatigue, no clutter.
A couple of points here. First; QRDs and RPGs are called diffusers by their
manufacturer, RPG Diffusor. By definition, a diffuser has something going
through it as opposed to bouncing off it, and having been successfully
accused of being a purist on at least one occasion, I prefer to call them
dispersers. Don't try to order them that way, though, or they'll laugh a
lot. Second, proprietary units are not the only way to disperse sound,
just the most practical.
Any irregularity in a wall will break up sound down to a frequency of it's
depth under 560. That comes to 15 inches for 400 Hz, which is the commonly
used low point for dispersion. Human ears are relatively insensitive to
direction at low frequencies, so there's no point to dispersing them.
As an example of any irregularity, ordinary cardboard boxes taped bottom
out on a wall will disperse nicely down to 400 Hz or better and soak up
gobs of excess bass to boot. Good for basement practice rooms.
Old auditoriums use niches with statues and vases for the same purpose
(without the bass absorption), as do the zig zag walls in movie theaters.
There's nothing new about using dispersion to improve the sound of a room.
What is new is the QRD-RPG design, which makes dispersers small and
efficient. QRDs reflect near perfect dispersed sound at about half the
depth of an equivalent box or wall, and they are so effective that it's
usually cheaper to buy them than to construct the alternatives. They're
also guaranteed to work, which is nice.
The flat smack reflection to the mixer comes from (what else) flat panels
at the back corners.
They can be 3 to 4 foot panels as such mounted on ball joints for aiming,
or they can be built into the room as 45 degree corner killers.
In either case they are called Haas Kickers. No mail, please, I didn't
invent it, I just use it.
There are legitimate arguments about using flat smack Haas kickers, but
I like to use 45 degree back walls as kickers, partly to eliminate back
corners and even up the reflection time, but mostly because they provide
a lot of storage space. As the working types know, there's no damn place
to keep anything in a control room, and with solidly latched doors a pair
of 45's will give you enough room to hide the body. Maybe two or three.
Also handy for HVAC ducts or even a small air conditioner. The kickers
don't have to reach the ceiling; eight feet up is plenty, but everything
in back must be absolutely rigid or the bass won't bounce. If you don't
like the flat reflection, QRD the doors.
I learned about that some years ago while doing a few dozen sessions in
a control room built with a flimsy back wall of Masonite over 2 x 4s. It
reflected the top end perfectly, but the bass thundered past like a freight
train and vanished into nothingness. Found myself continually looking over
one shoulder to see where it was going. Spooky.
More importantly, it was distracting. The disappearing bass trick was more
interesting than the session sound. Blew my concentration.
Anything that makes a control room interesting is bad design. It's a tool,
and like any other tool, it's not supposed to be interesting. It's just
supposed to work. Inconspicuously.
Concerning non-parallel side walls; unless you really lust after being
trendy, don't bother.
These things are so popular I've seen echo chambers built non-parallel,
but on balance they're probably not worth the aggro or the expense, as
flutter echo is remarkably easy to exterminate. All it takes is a tiny
amount of absorptive treatment or a wall offset of one inch in ten feet.
So if the mixer hears slap echo in the front of the room, drop in a couple
of 2 foot absorbent squares to kill it.
Better yet, you won't have ten feet of parallel side walls if you hang a
QRD at either side of the mixer's head as suggested by Doug Jones. Great
idea, strongly recommended, saves a fortune. Buy an extra tape machine
with the savings.
The floor: Since most studios are built into commercial properties, we're
usually looking at a concrete slab. Worst floors in the world. Miserably
uncomfortable, no place to run equipment cables, can't be soundproofed.
Example: When Universal Recording built new studios in the late 50's Doc
Sabin (no less) designed a 90 Db soundproof wall between A and B. The wall
was about four feet deep and worked beautifully, but somebody forgot to cut
the floor slab, so the bass from one studio transmitted almost perfectly to
the other. With two sessions running at the same time it got pretty funny
watching a band trying to hold it's tempo while hearing a different tempo
from the other studio. The clients didn't see the humor in it, though, so
studio B got 2 inches of industrial cork and a second brand new floor over
the first brand new floor. Bah, humbug.
Uni was designed as a second floor addition to a building in construction,
so the slab could have been cut. Most studios are on the ground floor,
though, and slicing up a foundation slab is verboten. The control room
must have it's own floor.
As a first step, seal the slab. Untreated concrete will wick up an amazing
amount of water from the soil beneath it. The water will produce 100%
humidity under your new floor and wreck it quick.
By way of checking, lay a sheet of plastic food wrap on the floor slab
overnight. If it's beaded up in the morning, paint the slab. If not, give
it a wash coat of shellac anyway. It might rain.
There are several specifications for a control room floor. They include,
in no particular order, comfort, soundproofing, good looks, durability,
a place to run cables, and cleanability. (see good looks, above)
Starting with cable space, the floor needs to be elevated enough to make
runs underneath. Standard 27 pair snake cable is about 7/10 inch thick, so
2 x 4s laid flat will allow one cable to cross over another with room to
spare. Unless you want to use the under-floor space for air conditioning
returns there's no advantage to going higher.
Isolation from the main slab is achieved by laying the 2 x 4s down on 1 inch
felt weather stripping to prevent solid contact. The principle here is that
sound travels slower in soft materials than in hard, so going from hard to
soft attenuates it rather like going from high resistance to low in an
electrical circuit. Similarly, soft to hard does nothing.
No nails in the felt, please, but you can use the office stapler to hold it
in place if you bang the staples down deep with a hammer.
Keeping flexibility in mind, use short 2 x 4s with serious space between
them so you can make cable runs in any direction. Two feet long with 6
inches gaps in each direction is about right, as the castors on tape
machines and such concentrate their weight on very small areas, but
whatever spacing you use, build and use a gauge for it. Precision counts.
That's a lot of little wiggly chunks to lay a sub floor on, so run a bead
of vinyl caulk or silicon adhesive between the felt strips to keep them put
until you lay a sub floor.
Before laying the sub floor, get a bag full of nylon clothesline and run it
side to side and end to end between every damn 2 x 4 on the slab. Fishlines.
Given a known starting point and exact spacing, you'll be able to drill
little holes and fish new cables for new equipment from anywhere to anywhere
else in future. Saves ripping up the floor when (not if) things change.
The sub floor will likely be single 3/4 inch plywood or staggered double 1/2
ply. Either is good.
Don't nail it down. Common nails attract rot, especially in the high humidity
one finds under an airtight floor no matter how well the slab was sealed.
The best thing is galvanized building screws. They don't rot the wood and
they grip so well you only need about a third as many as with nails. Very
fast. Start the sub floor at the edges instead of the middle of the room
and shim the panels an eighth inch or so away from the walls so you can
caulk them in without actually touching.
All that produces a floated, airtight, soundproof sub floor. After a coat
of shellac to guard against cleaning leaks, you're ready to consider the
finish flooring.
I like parquet squares. They're not the cheapest thing available, but they
meet all the specifications for a control room floor and present two
outstanding advantages over the competition. The first is that the pattern
of dark and light wood will hide the damage that inevitably occurs when
things get dropped on and/or dragged across the floor. The second is that
if a square has to be replaced because of new cable routing or what not it
won't stick out like a sore thumb. Hardwoods don't change color over time.
Neither do the modern finishes used on parquet tiles when they're properly
treated.
Proper treatment involves wax. Old fashioned, slippery, hard paste wax.
Sounds obsolete, but it's been around forever simply because nobody's come
up with anything better.
Why wax? Because the killer damage to working floors is made by scrapes
rather than gouges.
Scrapes leave big, highly visible marks. Gouges don't. That makes anything
you can use to cut down on scrape marks a Real Good Idea. Dumb 'ol paste
wax not only defends against scrapes by lubricating the floor surface, it
helps clean up small gouges if it's properly applied.
As usual, the proper way is the easy way. Paste wax is best applied using
fine steel wool with a half inch or so of water in the wax can. Old trick,
used by half the world's janitors. The trick is that the steel wool cuts
off the old wax and dirt while the water sets up the small percentage of
Carnauba wax (the hard stuff) in the blend. This results in a thin, hard,
even coat of clean new wax that's half polished as it's put down. The steel
wool also polishes out scuff marks and rounds off the ragged edges of gouges
and nicks, converting them from ugly nasty eyesores into marks of character.
In consequence, a parquet floor, like old leather, looks better and better
as it ages. I wish I did.
All this waxing business looks to be time consuming, but it isn't. One solid
coat on a new floor followed by touch ups in the wear areas from time to
time will do it. Paste wax is surprisingly durable stuff.
Whether the platform floor extends to the front walls or stops at the front
edge of the console depends on how the mike and speaker cables are to be
run, the location of the peanut gallery and whether or not the mixer can
see any part of the floor between the mix position and the speakers.
Last first; If the mixer can see the floor between the mix position and the
monitors there will be a short path reflection to the mixer which must be
eliminated. Short of leaving all that space unused, the only practical
treatment available is carpet, as the usual materials don't take kindly to
foot traffic.
As to the peanut gallery, the writer has a long standing preference for
seating visitors in front. Keeps 'em away from the console (and me),
reduces the conversational level, and lets me keep track of who's where
doing what. Given that the area up front will probably need carpet for
acoustical reasons, it makes sense to set a wide sectional couch up there
as well.
The only downside to a front peanut gallery is the people who move in and
out of it, blocking your view of the players. You can hold down that kind
of traffic by using the kind of low, soft, bean-bag couch that's hard to
climb out of. Better still, take the legs off. That'll really anchor 'em.
Two small problems remain. One is that a couch only holds a few people,
and we occasionally get mobbed with visitors. The other is that ordinary
carpet has virtually no low end absorption because it's so thin. For a
small area that's no problem, but this thing is big enough to unbalance
the room with it's top only effect. In passing, don't use nylon carpet.
It reflects at very high frequencies.
Both these difficulties yield to a single solution. Cut off the platform
floor at the console to make a 3 inch pit up front, and fill the pit with
padding and shag carpet. The bass absorption will be greatly improved,
the carpet will be soft enough to accommodate surplus visitors, and if
you cut it out around the couch you will effectively lower it even further.
They'll never get out of it. Or want to.
Cable runs to the studio for microphones, tie lines, cue circuits and the
like are most practically carried in standard electrical trough. It's not
pretty, but it gives great shield, it's easy to seal at the walls (wet
newspaper), and you can't beat it for running new cable. It'd be a damn
shame to turn down film work because you'd run everything in the walls
and couldn't drop in coax for video monitors in the studio.
The only dimension left is ceiling height. If it's high, leave it high.
There is no acoustical advantage to a low ceiling other than reducing
monitor power a little. Against that, a low ceiling requires a lot of
short path absorption. It's a wash, except that cozy control rooms turn
into claustrophobic coffins on long sessions. A mixer sometimes spends
a lot of time in a control room, so think of it as your home. The bigger,
the better.
So much for dimensions and geometry. Next; on to materials, lighting,
equipment location, airflow, and (oops) a door.
PART TWO: HOW TO DO IT
MATERIALS: There's a very old gag about making a silk purse from a
sow's ear. Start with a silk sow. Not real funny, but it makes the
point. Use materials that are appropriate to the job at hand.
The job of control room walls is to keep outside sounds out and inside
sounds in. Soundproofing.
Actually, a reasonable degree of soundproofing. Nothing on earth will
make a single room genuinely proof against all sound. Worse, attempting
virtually perfect soundproofing is expensive beyond belief, so it's a
Real Good Idea to define the degree of isolation needed for recording work.
Recording, that is, not gunfights in one room and meditation classes in
the other. The idea is to avoid feedback from the monitors to the studio
mikes, not to preserve the silence of death in the studio.
It's easy to figure. Worst case, you'll have a mixer listening at full
thunder to a quiet singer six inches off a mike. That's 120 Db SPL for
the mixer, 84 Db SPL for the mike. The difference is 36 Db.
Sounds wrong? It is. So add 12 Db for vocal limiting, yielding 48 Db,
and then subtract 20 for the studio absorption. Final necessary iso
for the control room walls is 28 Db, which suggests that we overbuild
control rooms something terrible. We do. And we don't.
As anybody with a little studio experience knows, the usual control room
walls shut out the top end sound from the studio very well indeed, but
if you sky a condenser mike the 30 to 50 Hz feedback will like to send
the woofer cones across the room.
Fact is, most control rooms have marginal isolation at low frequencies
as a result of wall construction, very big windows, and/or sound
transmission through the floors and ceilings. We will deal with each
of these one at a time, beginning with the control room walls.
To avid transmitting sound, a wall must be rigid enough not to drum
head section by section, heavy enough not to move as a whole, and airtight.
The only ordinary construction that meets all those specifications is
masonry. This is not a popular idea. In fact, it's usually met with
disbelief, followed by sputterings about weight, costs, and problems with
the landlord.
Most of the objections are based on misinformation, although the weight
problem can be real. Even with the lightweight cinder block currently
available, masonry walls come in at about three times the mass of the
more conventional double wall with an inch of Gypsum board on each side.
So if the foundation slab won't take the weight, forget it. On the other
hand, if there's any possibility of using masonry it would be real smart
to look into it, as there are a number of advantages to the stuff. Among
these are speed of construction, which makes the total costs very
competitive, the cleanliness of the process in building a wall and tearing
it down, and the fact that cinder block is a good acoustical material in
and of itself. Primarily, though, block is rigid, with the result that
it's inherent low frequency rejection is rather better than flexible
materials.
The rigidity bit is what separates silk sows from standard oinkers.
Building a rigid wall with flexible materials is obviously absurd, and
becomes embarrassing if one looks at the published data on Gypsum
paneled walls. Not the STC figures, which are a form of average loss
intended for rough comparisons rather than design purposes, but the
more complex charts which list losses at various frequencies. A quick
look at those shows that the 125 Hz loss of walls is commonly 20 Db
or so less than the published STC loss. They're supposed to, because
the STC contour has a built in 20 DB slope from 1.25 Khz to 125 Hz,
but even allowing for that, STC is an unreliable figure for music
isolation, where the bass is likely to be as loud as anything else,
if not louder.
As an example, an STC 35 2x4 wall with 1/2 inch Gypsum board on each
side loses 47 Db at 2 Khz, but only 15 Db at 125 Hz. Double the board
thickness, the STC is 39 and the 2 Khz figure rises to 53, but at 125
Hz it's still 15. That's a difference of 24 Db between STC and low end
performance. Build for 39, get 15. We need a minimum of 28, remember?
So if you sky a drum mike......
While a 2x4 frame is nearly as heavy as one layer of Gypsum board, you'd
expect that nearly doubling the mass of a wall would improve it's isolation.
In fact, nothing happens at 125 Hz, and the 15 Db loss through the walls
just detailed is 3 Db worse than for a single sheet on an open frame, which
demonstrates that Gypsum board is a fairly strange material in terms of
soundproofing. That's not to say it's no good; just that it's peculiarities
have to be taken into account when designing for isolation.
Time for some hard data. The most common studio walls are 8x8x16 inch
concrete block and double 2x4, 16 inch O.C. stud walls with 1/2 inch
sheets of Gypsum board outside and 7 inches of Fiberglas between walls.
A third, less common construction is double sheets of 1/2 inch Gypsum
board on either side of single 3-5/8 metal studs on 24 inch centers,
again stuffed with Fiberglas.
Frequency 125 250 500 1K 2K 4K STC
MASONRY 38 44 49 54 58 62 50
DOUBLE STUD 36 48 59 64 66 63 59
METAL STUD 39 46 55 61 63 55 54
All three work at 125Hz, but the low end loss curve for the double wall
construction is about 12 Db per octave, so 60 Hz should be 26 Db and 40
Hz, where the speakers pinch off, about 17.
The metal stud wall is better at about 8 Db/octave, and masonry shows the
flattest response at 6. Unlike the others, masonry follows the mass laws,
allowing it to be calculated at a firm 28 Db for 40 Hz.
That'll work.
At this point we have two basic kinds of wall; dumb 'ol masonry, which any
apartment dweller will testify is soundproof, and lightweight, sophisticated
wallboard construction that depends on design and fabrication to work
properly. Leaving out the Fiberglas, for instance, will cost you about 10 Db
of isolation.
There is nothing in the literature to suggest that using two sheets of
wallboard on each side of a double stud wall will improve low end rejection,
although two 1/2 inchers or one 5/8 on one side would probably be good for
two or three Db. The problem lies in one side's resonating the other across
the gap between them, so dissimilarity helps.
For a real difference, use stiff panels on one side of the wall. Stiff
translates to hard, rigid, makes no sound when you hit it. In short,
particleboard.
Particleboard is not the name of a product, but of a process.
Georgia-Pacific lists 63 varieties of the stuff, ranging from thin, flexi-
ble, incredibly cheap chip board to something called Lamiboard, at 200
pounds for a one inch sheet. The more ordinary products found in building
supply houses comes in thicknesses ranging from 1/2 inch through 1 inch.
They're 17 to 20% heaver than Gypsum wall board, but at 3/4 and one inch
they're rigid.
Also expensive. Don't panic, though, as while a double stud wall control
room without a back wall will use up close on three gross of 2x4s, it
needs only 80 odd sheets of paneling, so the panels are a fairly small
proportion of the material costs.
Size for size, common particleboard costs three to four times as much as
wallboard, and the fireproof versions, which carry the same fire ratings
as Gypsum, are about 2-1/4 times the cost of the standard, but even if
you used the fireproof stuff for both sides, the difference would be
about half the cost of the 2x4s. Best not, as you'd be back to two
identical materials drum heading each other. Wallboard is dandy above 1
KHz, particle's good down low.
The resonance and drumming of wallboard has a nasty tendency to add harmonics
to bass, giving the mixer a warm, smooth, false bass sound which leads to
surprises when listening to the product in other rooms. One inch particle
board doesn't drum much, and two inches won't drum at all, so using the
stuff inside a control room, if only on the speaker walls, makes sense from
a monitoring standpoint.
One inch board is easy to set in place and seal, as it has nice big square
edges. At 125 pounds a sheet, you need a little help, but it supplies some,
and it's less than half the work of two individually caulked 1/2 inch
sheets of wallboard.
A 3-5/8 inch metal stud wall is not only superior acoustically but does
better on costs. For one thing, the number of studs is about 30% of the
double wall construction with an equivalent reduction in labor. For
another, the studs themselves are about the same price as standard 2x4's
but half that of the fireproofed version. If you're building in an area
with fire codes that specify no combustible materials in a compound wall,
metal stud construction reduces the frame costs to 15% of a wooden double.
Finally, metal studs can be bought at any length, which makes building 16
to 20 foot walls simple, and for high walls or a little better isolation
2x6 metal studs are available at 50% over the cost of the 2x4's.
To summarize, if you can stand the weight and are willing to hire the work
done, cinder block is probably the best choice for control room wall
construction. If not, a single 2x4 or 2x6 Fiberglas stuffed metal stud wall
with fireproof particle board inside and either that or two sheets of
Gypsum board outside will likely prove lighter, easier, better and cheaper
than double wood stud construction.
WINDOWS: While 1/2 inch glass set just inside the wall edges is common
practice for control rooms, it is not optimum.
The figures for 1/4 inch glass are:
SPACING 125 Hz 250Hz 500 Hz 1KHz 2KHz 4KHz
2" 25 28 36 41 46 -
4" 28 34 38 42 40 -
6" 31 37 43 48 44 -
8" 40 42 49 56 43 -
While figures are not available for double 1/4 inch glass at 4 KHz, those
given for other thicknesses suggest that the isolation is higher than for
2KHz. In any case, the gap's the thing, and given a 40 Db loss at 125 Hz
with a 2 Db per octave curve for an 8 inch spacing there seems little need
to use thicker, more expensive glass.
It seems wrong to use the same size glass on both sides of a window, but
measurements show otherwise. In addition, with one window tilted eight
inches there is no probability of a strong drum head effect between panes
at any single frequency.
The angle of the inner window, however, makes it take up eight inches on
it's own, so the two windows will need sixteen inches between them at the
top. Since most walls aren't a foot thick, this necessitates mounting the
windows on, rather than in the wall. It's easily done by building a second
2 foot high wall outside the main wall to support the outside window or
making a very deep set of sills and sides as an equivilent. The top of the
outside window frame also makes a handy shelf for odds and ends. Put the
excess width on the studio side. Nobody gets close to that wall anyway.
You can increase a windows' losses by 5 Db or more by lining the inside
frame edges with Fiberglas. One inch ceiling panels or Linear Glass Cloth
over 6 to 8 inches of 703 board will do, both reducing transmission and
tidying up the gap between panes. Cute idea. Not mine, but cute. If that's
not enough, 1/4 inch laminated (safety) glass shows 6 Db more loss than
standard 1/4 inch plate at all frequencies. Costs more, does more.
The reason for using 703 board rather than glass wool inside the window
is that for wall mounted material it's the thinnest treatment available
for absorption down to 60 Hz. In fact, hard backed 703 takes about half
the space of it's nearest competitor.
Once again, the figures are:
A: 3-1/2" Fiberglas Building Insulation
B: 6-1/4" Fiberglas Building Insulation
C: 3-1/2" Fiberglas Noise Barrier Batts
D: 2" Fiberglas 703 Insulation Board, Unfaced
E: 4" Fiberglas 703 Insulation Board, Unfaced
125 Hz 250 Hz 500 Hz 1KHz 2KHz 4KHz NRC
A: 0.34 0.85 1.09 0.97 0.97 1.12 0.96
B: 0.64 1.14 1.09 0.99 1.00 1.21 1.05
C: 0.38 0.88 1.13 1.03 0.97 1.12 1.00
D: 0.22 0.82 1.21 1.10 1.02 1.05 1.05
E: 0.84 1.24 1.24 1.08 1.00 0.97 1.15
As can be seen, 4 inch 703 is a better low end absorber than 6-1/4 inch
glass wool, and about equal to seven inches of Noise Batts.
According to the good folk at Owens-Corning back when they had a testing
lab, doubling the thickness of a treatment should shift the figures down
an octave. Real measurements show it's not exact, but the principal holds.
Segueing neatly into control room treatment, don't equalize the speakers!
Build a flat room, install flat monitors. Sounds better, costs less. Sell
those third octave things to somebody with a room problem.
In order to build a flat room, however, it's necessary to use flat treatment.
Except for a hung ceiling, that means thick treatment, with about eight
inches of 703 board as a minimum, which at least in theory will put you down
1.6 Db at 62.5 Hz. Close enough.
So how is it everybody doesn't use thick treatment? Especially since all
those other rooms seem to sound great with thin stuff?
Those other rooms are surfaced with open backed wallboard on 2x4's which,
without any Fiberglas to damp vibration and suck out transmitter sound,
show acoustical absorption of 4% in the midrange and 29% at 125 Hz. Since
wallboard's not porous, it's sure as hell not absorbing the bass, so it
must be transmitting it through the wall. It is, and the rooms are not
particularly soundproof.
Same applies to glass, and a few other non porous flexible materials. The
point is that with the walls drum heading and leaking bass out of the room
you won't hear a lot inside.
Less helpfully, if you can feel the walls vibrating to every bass note,
you can bet your boots they're faking up the bass sound by adding lots of
harmonics. Gives a nice, warm, false bass sound that is not repeat not what
you're putting on tape.
Put in a proper non vibrating wall, you'll get no added harmonics and true
bass sound, but you'll need to soak up as much low end as top, so you'll
need to use thick treatment.
A foot or more of glass wool is fine for a studio, but takes up too much
space in a control room, so 703's better there.
You'll need to treat the front and speaker walls to kill bounce to the mixer
from the back wall, possibly some treatment on the forward side walls to
soak up speaker bounce to the mixer from them, and unless the ceiling is
really up there, you'll have to treat it for reflections from the speakers
to ceiling to mixer position.
Locating treatment is easy. Buy a cheap plastic decorating mirror at a tile
store, run it along the walls, and hang some absorption anywhere you can see
the speakers from the mix or producer/second positions. Ditto on the front
and speaker walls for the back walls. Ditto again for the ceiling. You'll
probably wind up with most of the front and a good deal of the ceiling
fuzzed, which is why it's called the Dead End.
How much fuzz involves the studio. Specifically, the control room should
have a shorter reverberation time than the studio because if it's equal or
longer you won't be able to hear the sound of the studio room as such. For
medium and big studios ranging from a half to one second it's no problem.
For small ones it can be big trouble, as while one can hang fuzz all over
a studio, the back third of a control room has got to be left live to feed
the QRD's.
Treating everything possible, a standard control room such as the one
illustrated at a base 25 x 25 x 16 feet will come down to about a quarter
second. For a rationally treated live studio the minimum size for a quarter
second (.266) is 14,400 cubic feet.
For smaller studios, one can lower the control room ceiling, which reduces
the volume and so the reverb time. As it happens, using a ceiling height of
studio volume over 1000 comes out pretty close, although with an 8 foot high
control room for an 8000 cubic foot studio things get very, very cozy.
If the control room's time is too long, it's not a disaster. It's just that
you'll play hell hearing the live sound of the studio itself. Basic laws of
physics. All the other good stuff works nicely.
Where to put treatment is easy. How's not much harder.
The writer strongly recommends hanging acoustical treatment on the walls
rather than building it in. It doesn't have to look horrible, it's less
expensive, and it's the mainframe way of doing things. You might need to
put equipment up there one day.
One simple way to hang treatment is to box it. Use 1/4 inch plywood to build
8 inch deep frames, screw the corners together using 2x2's, staple some
chicken wire across the back, and cover the front with Upholsterer's Burlap.
The burlap is good looking, won't sag, takes paint (non sealing, please),
and compensates the nonlinear response of 703 at very high frequencies.
Don't like burlap?
Try ceiling panels. For damageable areas there's no substitute for Tectum.
Tough as nails, takes paint, looks very good. Looks rather like Shredded
Wheat made up of wood fiber. Too thin and heavy for standalone panels, but
terrific for protecting Fiberglas against public clumsiness.
As drawn, there should be no glancing blow, low incident angle reflections
in the room. If you plan to make some with small speakers, lights and such,
use Sonex to kill them. Nothing touches it for the purpose, but restrain
yourself from wallpapering the room with it, as the base is so thin it's
got no absorption at the low end. Read the specs.
Ceiling treatment is best done with a conventional grid system using 1-1/2
inch Omega Fiberglas panels or whatever 3 Pound glass board.
Omega is the best available, but the panels are 20 x 60 inch. The 3 Pound
generic stuff is nearly as good, and can be got a 2 x 4 feet. Get the numbers
from the supplier, read the specs. Some ceiling panels are much more equal
than others.
THE DOOR: There are a number commercially available steel soundproof doors
which come in their own steel frames. They work every time, but I have a
couple of problems with them. First, they're not a wide as I'd like and
second I've seen three tape machines trip on the thresholds and fall flat
on their faces. One was a 24 track which made very impressive noises in the
process and took about half an hour to restore to service.
It takes two people to get wheeled equipment over a threshold, and you
haven't always got the help available.
On balance, it may well be best to build your own. The design parameters
come down to how soundproof you want it, starting with a standard 1-3/4 inch
solid core door at about 4 pounds per square foot and well under 20 Db of
isolation at 125 Hz.
You may want to end there, as beating it will take some effort.
Doors are pretty much mass law devices, so building one for low losses gets
you into walk-in refrigerator hardware for openers. If that's acceptable,
go for it. If not, stick with a single solid core. Don't even think about
double doors on either side of the wall. If they're properly sealed it'll
take two men and a boy to get them open.
Vacuum.
A massive door can be made of multiple sheets of 3/4 inch particleboard at
3 Lb/Ft, one inch at 4 Lb, or one inch Lamiboard at 6-1/4. That last is 200
pounds a sheet, 175 for a 4x7 foot door. A couple of those will get you 24
Db at 60 Hz, and probably better if they're mounted on a flat 2x4 (2 ea 2x6
at the hinge side) frame stuffed with glass wool. On the other hand you're
looking at a near 400 pound door, so moderation may be in order.
Laminated construction allows door to be built like a safe, with a 1/2 to
3/4 inch setback for each layer. That takes care of sealing three sides with
no effort, and a rubber blade sealer can be used for the bottom. You can get
them with an automatic push down mechanism, but it's probably overkill.
Hiding the blade between layers in the middle of the door is real cute until
it wears out and you have to take the door down to replace it. Not so cute
then.
As a purely personal matter the writer has preferred to leave the control
room door open during about 30 years as a full time music mixer, and some
may agree. Others not. Dealer's choice, but sound proof doors are a pain.
LIGHTS: The purpose of lights is to see what you're doing.
Sounds real obvious, but engineers do a lot of things in a control room, and
there's no one lighting system suitable for all of them.
For recording, you generally want the lights down so the musikers aren't
distracted by movement in the control room, but up enough so you can see
all those little damn knobs. Can't be done, and all of us have occasionally
gone half blind trying to set equalization in low light situations.
Recording takes two lighting systems, one for the equipment, one for the
general room. Equipment lights are easy. Get two or three little theatrical
spotlights with barn doors, put them up on the ceiling, and set the barn
doors so the light covers only the tops of the console and machines. The
Fresnel lenses in those things are so efficient you can nearly burn the
paint off a console at 12 feet and with the barn doors you can do it with
no light on anything else.
For room at large indirect lighting is best, as it's so even that light
levels can be held really low. The troughs in the drawing are an old film
mix lighting system. Both the trough and the wall just behind it are lined
with crumpled aluminum foil for maximum reflection; the lights are store
showcase tubes or dimmable fluorescents. The showcase tubes are nicer,
giving a peculiarly warm, soft, even light.
Bouncing the trough lights off a white enameled ceiling is pretty efficient,
so they don't heat up the room a lot, and they produce perfectly even light
without any bright spots to contract one's pupils, so the lights can be
taken to amazingly low levels during sessions.
So low, in fact, that it's a good idea to hold down on the lighting just
outside the control room. Makes a sort of visual decompression chamber
between control and studio.
Soft, sexy lights are great for recording, but they suck for setup, cleaning,
maintenance, repairs and the like. For that kind of work you need harsh,
nasty fluorescents. So hang a bunch of industrial fixtures a foot or two
below the ceiling back of the mixer. A bunch in this case means enough so
the sun comes out when you hit the switch. As a minor bonus, the fixtures
do a decent job of dispersing sound in the area, but make the spacing a
little irregular to avoid a comb generator. Three to six inches is plenty,
but it's important.
TOYS: All God's children got toys, even purists who won't use anything more
than outboard equalizers and limiters. The problem with toys is where to put
them, and some of the solutions are outrageous.
If the office girl can grab a sheet of stationary faster than you can
get to a limiter threshold control, you might look at how she does that.
Having looked, consider that the Brits call a console a mixing desk.
I've been supporting consoles on toy racks for years, and am astonished
that so few other people do, as it's such a sensible idea.
Cheap, too. Bud 31 inch open frame relay racks cost well under a hundred
bucks per, mount 28 inches of equipment, and raise the console height a
few inches. They're twofers.
The extra height opens up the mix position and adds more comfort than you
might think, but the chief advantage of mounting toys like desk drawers is
that you can diddle them while facing the speakers. The alternative
positions make no sense to me, as tweaking sidecar gear puts the mixer
into monaural with one ear toward the speakers, and turning 'round to use
equipment behind the mix position hardly bears thinking about.
The most frequent objection I've heard to this toy mounting method is that
people will kick the knobs off. Maybe, but several years of experience says
it doesn't happen. Ever. Setting the racks back 9 or 10 inches from the
front of the console may have helped.
Second objection is that you can't see the knobs. Yes, you can, especially
if you hang a shielded showcase light under the console to light them.
Two 31 inch racks at the front of the console will give you 4-1/2 feet of
toys, with another 4-1/2 feet at the back for things you don't need to reach,
such as power supplies and monitor amps. If nine feet of racks isn't enough
you may be in the processing business, not recording. However: The racks are
not particularly handsome, so you might want to cover the sides. Easy.
Cement flat refrigerator magnets to a couple of nice looking panels, stick
'em on the sides. Quick on, quick off.
Don't forget to mount a fluorescent light stick back of each rack. Makes
calibration and service simpler.
Try a computer monitor arm for machine remotes. There are a dozen or more on
the market, and one of them should suit your situation. At worst, it will
at least get the damn things off the console or out from in back of you.
Might even have enough room for the slate sheet board.
They'll also swing Auratones and/or near field monitors out of the way if
they can't be hinged so as to drop them in front of the console during
sessions. Takes more Sonex that way, but you'll be able to see the players.
Finally, while a good deal of the foregoing is a little vague, construction
details are available in the Owens-Corning Noise Control Manual, if you can
find a copy.
Other sources include Armstrong, PPG, Georgia-Pacific, the USG Group, Klark
Teknik's Audio System Designer manual, Davis's Sound System Engineering book
(Sam's Publishing) and last and maybe best, your local building supply store.
Selasa, 14 Juli 2009
MODERN CONTROL ROOM DESIGN AND CONSTRUCTION
DEFINITIONS OF TERMS USED IN COMPUTER PROGRAMS.
DEFINITIONS OF TERMS USED IN COMPUTER PROGRAMS.
a Absorption coefficient of materials. Listed as the amount of sound
absorbed at frequencies from 125 Hz to 4KHz and ranging from .002 (2%)
to 1 Sabin per square foot.
Airloss The frequency at which the absorption of the room's air equals
it's surface absorption, the room becomes muddy, and loudspeakers must be
boosted to compensate.
ALcons(ALs) Articulation Loss, Consonants. Given as percentage of top end
loss. For best quality speech, 10%, with 15% maximum; for music, 5% and a
10% maximum.
Db SPL Sound Pressure Level, in (power formula) decibels. Three Db is
twice the power, ten Db is ten times the power and twice the loudness. 6
Db, 4x PWR, 20 Db, 100x PWR.
Dc Critical distance.The point at which the reverberant sound of a room
equals the source sound driving it and sound level becomes nearly constant.
(Echo=Source) Measured at the 3Db point.
Dl 3.16 x the Dc, the last 3Db of Dc power loss.D1 does not show as a
program listing, nor is it apparent to the ear, but is used in the program
as it is important to power calculations as power requirements double at
that distance.
Dx Distance from sound source (loudspeaker) to furthest listener.
Speaker throw.
EAD Equivalent Acoustic Distance. Apparent listener distance from source.
Ln(1-a) Conversion from standard Sabins, in which construction materials
are specified, to Norris-Eyring Sabins in which 1 Sabin per Sq Ft yields
total absorption of sound. Always used, and critical in rooms with high
percentages of absorption. Shortens RT-60, raising Dc, amplifier power
and isolation figures while lowering AlCons. Output is listed as NERT,
a shorter echo time.
NAG Needed Acoustic Gain: To produce X Db SPL at X feet of loudspeaker
throw from Y Db SPL at Z feet from the microphone.
PAG Potential Acoustic Gain. An isolation figure, mike to loudspeaker.
Usually the acoustic feedback figure for a mike-speaker system.If NAG is
6Db less than PAG no feedback will occur.
PWR T his program arbitrarily multiplies the calculated amplifier power by
10 to allow for transient peaks not read by standard VU meters, which have
a .2" rise time. A close miked piano, for instance, peaks at 30 times meter
reading, but 10 is a reasonable compromise.A high quality compressor at 3:1
will reduce 10:1 peaks to 3:1 to help prevent amplifier clipping. Properly
used, 4:1 is undetectable on voice, but can be heard on music. For general
work, compression at 2.5:1 will reduce the peak power margin to 4:1, and
is undetectable with true RMS detector units.
Q The directivity of a source. Voice is 2.5, cone speakers when ceiling
mounted about 4, (2 if free-hung) most brass instruments and common speaker
horns about 10. Specified for high quality speaker horns; maximum Q
available about 50, minimum 8. Dome tweeters have a Q of 2.8
RT60 Reverberation time decay to 60Db below starting level.Echo time in
seconds.
SEN Loudspeaker sensitivity. Specified as Db SPL at one meter or at four
feet (about 2 Db less) for one watt of amplifier power.
SPL Used in program as desired Db/SPL at furthest listener's ears.
VOLUME, As in"Turn up the volume". Level as heard by the human ear.
Not used in the program as such, but it is essential to know that to
double the perceived volume, one must increase power by 10. Twice as
loud, ten time the watts, one Bel.
RELATIONSHIPS IN ACOUSTICS: CAUSES AND EFFECTS
CAUSES; THE THINGS YOU CAN CHANGE.
1: Room size. Bigger is generally better, and much easier.
2: Sabins. The amount and kind of absorptive material in the room.
3: Source (usually loudspeaker) Q. The beam width/height of the sound source.
4: Loudspeaker Sound Pressure Level. Affects listeners, but not acoustics.
EFFECTS; HOW CAUSES CHANGE WHAT IS HEARD IN THE ROOM.
The relationships take four forms. 1: DIRECT; More x, more y. 2: INVERSE;
More x, less y. 3: SQUARE; Double x, multiply y by 1.414, halve x, divide
y by 1.414 (or multiply by .707). LOGARITHMIC; small changes in x yield
small changes in y, large changes in x yield enormous changes in y.
1: RT-60 to Sabins. RT-60 is room echo time. Depends on the amount of
absorption in the room, and is an INVERSELY LOGARITHMIC relationship.
Doubling the amount of physical Sabins in a room will reduce room time
by more than half. Often much more. See NORRIS-EYRING elsewhere.
2: RT-60 to Critical Distance (Dc). Dc is the distance at which the level
of a sound source becomes constant in a room. The relationship is INVERSLY
SQUARE. Double the RT, Dc multiplies by .707, Halve the RT, Dc multiplies
by 1.414, BUT Dc is also an absolute function of Sabins in that x Sabins
will yield y Dc in a room regardless of the RT produced by the Sabins.
3: Sabins to Acoustical Loss of Consonants (AlCons). AlCons, expressed as
a percentage at a given distance, amount to clarity of sound. The
relationship is simply INVERSE to effective Sabins. Double the Sabins,
double the distance for a given percentage of AlCons.
4: Source Q to Dc. DIRECTLY SQUARE relationship. Double the Q, Dc
multiplies by 1.414
5: Source Q to AlCons. INVERSLY SQUARE relationship. Double the Q, AlCons
multiply by .707
6: Room size to Sabins/RT. Simple DIRECT relationship. Double the room
VOLUME, double the time given the same number of effective Sabins.
7: Sound Pressure Levels, which involve room losses, loudness and source
power will be dealt with elsewhere, as they are too complex to detail here.
MATERIAL SABINS/Sq FOOT AT (Hz) 125 250 500 1k 2k 4k
ACOUSTIC TILE, GLUED ON 0.20 0.35 0.60 0.70 0.80 0.75
ACOUSTIC TILE, SUSPENDED 0.40 0.50 0.65 0.70 0.80 0.75
BRICK, UNGLAZED 0.03 0.03 0.03 0.04 0.05 0.07
BRICK, UNGLAZED, PAINTED 0.01 0.01 0.02 0.02 0.02 0.03
CARPET, 1/8 INCH, NO PAD 0.05 0.05 0.10 0.20 0.30 0.40
CARPET, 1/4 INCH, NO PAD 0.05 0.10 0.15 0.30 0.50 0.55
CARPET, 3/16 COMBINED PILE & FOAM 0.05 0.10 0.10 0.30 0.40 0.50
CARPET, 5/16 COMBINED PILE & FOAM 0.05 0.15 0.30 0.40 0.50 0.60
CINDER BLOCK, UNPAINTED 0.36 0.44 0.31 0.29 0.29 0.25
CINDER BLOCK, PAINTED 0.10 0.05 0.06 0.07 0.09 0.08
CONCRETE/MARBLE FLOORS 0.01 0.01 0.01 0.02 0.02 0.02
CONCRETE/STONE WALLS 0.02 0.02 0.02 0.03 0.04 0.04
FIBERGLAS, UNFACED, 3 1/2 IN ON WALL 0.34 0.85 1.09 0.97 0.97 1.12
FIBERGLAS, UNFACED, 6 1/4 IN ON WALL 0.64 1.14 1.09 0.99 1.00 1.21
FIBERGLAS, PAPER OUT, 6 1/4 ON WALL 0.94 1.33 1.02 0.71 0.56 0.39
FIBERGLAS, UNFACED, 12IN ON WALL 1.14 1.09 1.09 0.99 1.00 1.21
FIBERGLAS 703 BOARD, 1 IN ON WALL 0.03 0.22 0.69 0.91 0.96 0.99
FIBERGLAS 703 BOARD, 2 IN ON WALL 0.22 0.82 1.21 1.10 1.02 1.05
FIBERGLAS 703 BOARD, 3 IN ON WALL 0.53 1.19 1.21 1.08 1.01 1.04
FIBERGLAS 703 BOARD, 4 IN ON WALL 0.84 1.24 1.24 1.08 1.00 0.97
FIBERGLAS 703 BOARD, 6 IN ON WALL(est) 1.19 1.21 1.13 1.05 1.04 1.04
FIBERGLAS GRID CEILING, 1 1/2 INCH 0.97 1.00 0.86 1.01 1.04 1.06
GLASS, 1/8 INCH 0.35 0.25 0.18 0.12 0.07 0.04
GLASS, 1/4 INCH 0.10 0.06 0.04 0.03 0.02 0.02
GYPSUM BOARD, 1/2 INCH 0.29 0.10 0.05 0.04 0.07 0.04
LINOLEUM, ASPHALT FLOOR TILE, ETC. 0.02 0.03 0.03 0.03 0.03 0.02
MARBLE/GLAZED TILE 0.01 0.01 0.01 0.01 0.02 0.02
HEAVILY UPHOLTERED SEATS, EACH 3.54 0.55 0.05 0.55 0.54 0.50
PEOPLE IN UPHOLSTERED SEATS, EACH 2.53 0.54 0.04 0.55 0.04 0.50
PEOPLE IN PADDED SEATS, EACH 4.05 0.05 0.56 0.57 0.07 0.00
PLASTER OVER MASONRY 0.013 .015 .02 0.03 0.04 0.05
PLASTER ON LATHING 0.02 0.02 0.03 0.04 0.04 0.03
SONEX, 2 INCH 0.08 0.25 0.61 0.92 0.95 0.92
SONEX, 3 INCH 0.14 0.43 0.98 1.03 1.00 1.00
SONEX, 4 INCH 0.20 0.70 1.06 1.01 1.01 1.00
TECTUM GRID CEILING, 1 INCH 0.40 0.42 0.35 0.48 0.60 0.93
ABOVE WITH 6 INCH FIBERGLAS OVERLAY 1.01 0.89 1.06 0.97 0.93 1.13
TECTUM ON WALL, 1 INCH 0.06 0.13 0.24 0.45 0.82 0.64
TECTUM ON WALL, 2 INCH 0.15 0.26 0.62 0.94 0.64 0.92
TECTUM 32 X 3 INCH BLOCKS PER UNIT 0.45 0.71 1.87 2.94 2.90 2.91
VELOUR, LIGHT, HUNG STRAIGHT ON WALL 0.03 0.04 0.11 0.17 0.24 0.35
VELOUR, MEDIUM, DRAPED TO HALF AREA 0.07 0.31 0.49 0.75 0.70 0.60
VELOUR, HEAVY, AS ABOVE 0.14 0.35 0.56 0.72 0.70 0.65
WATER, SURFACE .008 .008 .013 .015 .020 .025
WOOD FLOORING 0.15 0.11 0.10 0.07 0.06 0.07
WOOD PARQUET IN ASPHALT ON CONCRETE 0.04 0.04 0.07 0.06 0.06 0.07
WOOD PANELING OVER 2-4 INCH AIR SPACE 0.30 0.25 0.20 0.17 0.15 0.10
WOOD DECK, UNSEALED TONGUE & GROOVE 0.24 0.19 0.14 0.08 0.10 0.15
SOUND TRANSMISSION LOSS IN Db SPL. 125 250 500 1K 2K 4K
BRICKWORK, PLASTERED 4 3/8 INCH 31 36 41 50 55 61
BRICKWORK, PLASTERED 8 3/4 INCH 41 45 50 56 63 62
CONCRETE BLOCK, LIGHT, 12 INCH 38 44 49 54 58 62
REINFORCED CONCRETE, 4 INCH 38 44 49 54 58 62
GLASS, SINGLE STRENGTH, 3/32 INCH 13 14 21 26 32 30
GLASS, DOUBLE STRENGTH, 1/8 INCH 14 17 23 28 33 32
GLASS, PLATE, 1/4 INCH 21 24 27 31 25 32
GLASS, LAMINATED PLATE, 1/4 INCH 30 28 31 35 36 41
GLASS, PLATE, 1/2 INCH 20 28 31 27 36 44
GLASS, LAMINATED PLATE, 1/2 INCH 31 33 36 35 40 49
GLASS, 1/4 & 1/4 INCH, 6 INCH SPACE 31 37 43 48 44 56
GLASS, 1/2 & 1/4 INCH, 6 INCH SPACE 32 38 39 38 38 54
GLASS, 1/4 & 1/4 INCH, 8 INCH SPACE 40 42 49 56 43 59
GYPSUM BOARD, 1/2 INCH 17 20 23 23 23 24
GYPSUM BOARD, 5/8 INCH 19 22 25 28 22 31
PLYWOOD, 3/4 INCH 19 23 27 25 22 30
PLASTER AND LATH CEILING 22 27 31 36 34 42
WALL CONSTRUCTION FIGURES 125 250 500 1K 2K 4K
2X4X16 O.C. WOOD STUDS, 1/2 INCH WALLBOARD EACH SIDE 15 27 42 47 47 40
AS ABOVE WITH FIBERGLAS FILLING 15 31 40 46 50 42
AS ABOVE WITH DOUBLE 1/2 INCH WALLBOARD EACH SIDE 21 37 45 50 55 51
2X4X16 O.C. WOOD DOUBLE STUDS, 1/2 INCH EACH SIDE 30 41 45 50 55 49
AS ABOVE WITH FIBERGLAS FILLING 32 48 57 63 64 61
AS ABOVE WITH DOUBLE 1/2 INCH WALLBOARD EACH SIDE 36 48 59 64 66 63
2.5X24 O.C. METAL STUDS, 1/2 INCH BOARD EACH SIDE 17 24 36 45 45 41
AS ABOVE WITH FIBERGLAS FILLING 22 38 51 57 47 44
AS ABOVE WITH DOUBLE 1/2 INCH WALLBOARD EACH SIDE 36 49 60 62 64 55
3.3X24 O.C. METAL STUDS, 1/2 INCH BOARD EACH SIDE 25 28 42 49 50 40
AS ABOVE WITH FIBERGLAS FILLING 28 39 52 56 58 46
AS ABOVE WITH DOUBLE 1/2 INCH WALLBOARD EACH SIDE 39 46 55 61 63 55
IF ALL ELSE FAILS, THE MASS LAW FOR RIGID MATERIAL IS: 20X 10 LOG(HzXKg/M2)-47
LOSS IN Db SPL EQUALS 20 TIMES THE 10 LOG OF FREQUENCY IN HZ TIMES MASS IN
KILOGRAMS PER SQUARE METER, MINUS 47 Db. IF IN LB/SQ FT, MULTIPLY BY 4.89
NOTE: This is a list of typical professional loudspeakers. Not all
manufacturers, not all models, not recently updated. It is useful to
see what's generally available for early design parameters. For any
specific model, type, or manufacturer CALL THE MAKER OR DEALER.
Tannoy time aligned studio monitors.
MODEL# MAX PWR SEN Q ANGLES xOVER NOTES SIZE WEIGHT
-FSM 500w 95 6 90x90 1KHz Dbl woofer 42x29x22 198 lb.
-15X 300w 95 6 90x90 1Khz. Lo end 52hz 40x26x15 112 lb.
-15XB 300w 93 6 90x90 1Khz. Lo end 40hz 40x26x15 112 lb.
-SRM10B 150w 91 6 90x90 1.2Khz Lo end 55hz 21x15x10 40 lb.
-NFM8 100w 90 5 100x100 1.8Khz Lo end 55hz 18x12x8 25 lb.
U.R.E.I. Time aligned studio monitors.
-811C 150w 95 12 90x45 1.5Khz Lo end 70hz 21x26x19 110 lb.
-813C 150w 99 12 90x45 1.5Khz Lo end 50hz 36x31x23 198 lb.
-815C 150w 101 12 90x45 1.5Khz Lo end 40hz 32x14x21 260 lb.
Altec-Lansing small room monolithic systems: Not time aligned.
-604-16X 100w 105 18 60x40 1.5Khz Lo end 20hz 40x26x18 136 lb.
-9844-8E 60w 103 13 90x40 800hz Lo end 35hz 24x31x16 90 lb.
-A7500-8E 50w 101 13 90x40 500hz Lo end 40hz 54x30x24 174 lb.
-9849-8B 60w 95 13 90x40 1.5Khz Lo end 40hz 24x21x15 60 lb.
-937 1 50w 97 6 110x60 3 Khz Lo end 70hz 24x18x16 49 lb.
James B Lansing small room monolithis systems: Not time aligned.
-4612OK 200w 95 5 100x100 3Khz Lo end 60hz 17x22x10 45 lb.
-4671OK 200w 95 13 90x40 800hz Lo end 40hz 31x22x18 113 lb.
Music quality loudspeaker components for large rooms.
Altec Lansing woofers
-8124 500w 92 2 1-12" Lo end 40hz 30x19x15 61 lb.
-8154 500w 93 2 1-15" Lo end 40hz 36x30x15 90 lb.
-8184 600w 97 2 1-18 Lo end 40hz 36x30x26 124 lb.
-8256 500w 100 3* 2-15" Lo end 65hz 36x30x15 106 lb.
-817A 150w 108 3* 2-I5" Horn load Lo end 60hz 34x38x27 224 lb.
-210 150w 108 3* 2-15" Horn load Lo end 50hz 34x84x40 386 lb.
James B Lansing woofers
-4646 300w 94 2 1-12' Lo end 65hz 16x19x11 40 lb
-4647 400w 95 2 1-15" Lo end 35hz 31x22x18 80 lb.
-4648 800w 98 3* 2-15" Lo end 35hz 42x27x18 109 lb.
-4560BKA 300w 101 2 1-15" Horn load Lo end 45hz 36x30x24 137 lb.
-4550BKA 600w 104 3* 2-15" Horn load Lo end 40hz 36x60x33 241 lb.
Altec Lansing horns.
With #290 100w 8Khz driver. Others available from Altec.
MODEL# MAX PWR SEN Q ANGLES xOVER NOTES SIZE WEIGHT
-MR-64 A 100w 112 18 60x40 500hz 1.4" Dvr 29x21x28 13 lb.
-MR-94 A 100w 110 12 90x40 500hz 1.4" Dvr 35x25x28 16 lb.
-MR-542 100w 110 50 40x20 500hz 1.4" Dvr 20x15x29 12 lb.
-MR-564 100w 108 20 60x40 500hz 1.4" Dvr 13x13x13 5 lb.
-MR-594 100w 106 10 90x40 500hz 1.4" Dvr 13x23x13 5 lb.
-MR-5124 100w 105 9 120x40 500hz 1.4" Dvr 13x24x13 5 lb.
J.B.L. large horns 2445 100w driver.
-2360A 100w 111 12 90x40 500hz 2" Dvr 32x32x32 27 lb.
-2365A 100w 113 20 60x40 500hz 2" Dvr 31x31x32 25 lb.
-2366A 100w 116 46 40x20 500hz 2" Dvr 31x31x55 36 lb.
J.B.L. compact horns 2445 driver.
-2382A 100w 108 8 120x40 500hz 2" Dvr 11x18x9 4 lb.
-2380A 100w 110 11 90x40 500hz 2" Dvr 11x18x9 6 lb.
-2370 100w 108 12 90x40 600hz 1" Dvr 7x18x7 3 lb.
-2385A 100w 112 19 60x40 500hz 2" Dvr 11x18x9 6 lb.
-2386 100w 116 45 40x20 400hz 2" Dvr 11x18x14 12 lb.
CONSTRUCTION NOTES: FIBERGLAS BOXES
CONSTRUCTION NOTES: FIBERGLAS BOXES
MATERIALS ARE:
ONE: 1/4 INCH PLYWOOD, standard 4x8 foot sheets, construction as opposed
to finish grade unless it is not planned to cover the sides in fabric,
which is normal practice, and keeps all of the wood out of sight.
TWO: FABRIC, normally Jute Burlap, which is dimensionally stable, can be
painted (water base) to any color, available at 60 (or more) inches wide,
and inexpensive. In addition to it's other good points, the thicker grades
are a nearly perfect foil to Fiberglas, which gets a little reflective
at very high frequencies. The combination of the two is as close to a
perfect/practical absorber as possible. Open cell foam plastics can be as
good or better, but are both expensive and delicate, and present a very
serious fire hazard in terms of fumes even when fireproofed.
THREE: FIBERGLAS, Corning 703 (3 pound) semi-rigid board or equivalent.
The Corning product is better at the low end than others, but also nearly
double the cost of competitive products. Since low frequency absorption
as such can be managed using polycylindrical diffusers in small numbers,
a case can be made for using less costly fiberglass products, but they
require that polys be used in the room.
CONSTRUCTION: Boxes are normally made at 4x8 feet to minimize cutting and
6 inches deep to provide adequate bass absorption. However: The actual
size of glass boards varies a little between manufacturers, so the
material in real use should be measured and boxes built to fit.
2x4 boards are usually 1/4 scant, so when inserted 2 wide and 2 high
(8 ea 3" boards) will fit neatly in a 4x8 foot O.D. 1/4" box touching
but not forced. Don't squeeze the boards!
Thickness is another matter as glass board won't compress, so it is
usually necessary to build the boxes at 6-1/2 inches depth to allow for
a 3 to 4 x 1/4" strip across the middle of the box front and back to
capture the internal board seams. Alternately, the boards take silicon
sealer/caulk very well and allow a 6" box. Either will prevent the
board's bowing out in the middle.
Box sides, top, and bottom are screwed to 2x2s at the corners (1-5/8"
wallboard screws), wrapped in chicken yard fencing or equivalent to
support the glass board, and all visible surfaces covered in Jute Burlap
cloth, both stapled to the 1/4 plywood. If the boxes are mounted
separately from each other and the Jute won't reach the back, small trim
on each side will conceal the problem, OR one can buy wider Jute.
Boxes can be hung with screen door hooks, picture hanging wire or what
not anchored to the top 2x2s.
Trimming the glass to allow the 2x2s is easily done with a big, cheap
scalloped edge bread knife. Long cuts, however, are not recommended
unless really needed, and it is well to keep in mind that acoustical
treatment is generally +/- 10% so filling the last few inches is a waste
of time and energy.
CONSTRUCTION NOTES: POLYCYLINDRICAL DIFFUSERS
MATERIALS ARE:
ONE: TEMPERED MASONITE, usually 1/8 inch as thicker is more difficult to
handle and requires more strength in frame construction. Pegboard can be
used but may fracture and has no advantages unless pegboard hooks are
needed for hanging cables and so forth.
TWO: 2X4 AND 2X2 inch lumber for an open mount OR 6x1 inch finish and
2x2s for a shadowbox. The lumber needs be reasonably straight, otherwise
select for appearance.
The purpose of the unit is to provide high end dispersion and low end
absorption in one device.
These ends are accomplished by suspending a sheet of Masonite by it's
vertical edges (only), bowed out about 6 inches between the vertical
sides of the mount, and free to vibrate at all points except the vertical
edges.
When Masonite is compressed between side boards at 46-1/2 inches, it
takes the shape of an ellipse, reflecting medium to high frequency
sound at all possible angles from 0 to 30 degrees, as an ellipse is
made up of parts of an infinite number of circles, as with the edge of
a football.
While 6" is not the only possible build-out, is a rational design as it
is reasonably inconspicuous, forms a good ellipse, and exerts manageable
side forces on the mount. These forces become excessive at lower figures,
although they lessen as the Masonite warps to form, and at about one
foot the panel starts to become a semi-circle, which yields poorer
diffusion, and probably less bass absorption.
PLACEMENT, MOUNTING, APPEARANCE: Although polys will work as dispersers
in any location and size, both are important to bass absorption.
Defining a corner as two walls at 90 degrees, bass collects in corners
as it compresses into them, so polys in or across corners yields maximum
absorption. Additionally, corners reflect multi phase comb filtered top
end, which sounds bad, and polys cure that as well as killing excess bass.
Still further, polys in corners appear to work at lower frequencies than
when wall mounted, provided only that the walls forming the corner be
(at least) as long as the polys' maximum diagonal, which makes ceiling
to wall installation attractive as short polys can be mounted over the
length of a wall at the ceiling line. Known effective.
Mounting as such can be done by simply beveling 2x2s, nailing them onto
a wall and squeezing Masonite between them, but it is usually better to
make them removable by hanging them. It is very important to keep the
Masonite clear bottom and top, and allow generous air flow behind it,
as either done wrong will partially cripple bass absorption.
Masonite sheets in polys can be painted, wallpapered, cloth covered, and
so forth, but don't add serious weight or thickness.
TRIVIA: A full 4x8 sheet, wall mounted, calculates to 63 Hz. Hum is 60
or 120, but rumble in big rooms goes much lower, so if it is present
(as with being next to trains) they will require big units. 12x12
contiguous will absorb to 33 Hz, 12x14 to 21. Lowest frequency for a
room can be calculated as 1/2 1130/longest dimension. 60 feet, 9.42 Hz.
Masonite hung top and bottom potbellies within a year. Ugly, and so not
reccomended.
If you want to see polys, look at bandshells and old stage theaters. They
are not a new idea, just neglected.
STUDIO BASICS IN BRIEF
Studio isolation depends on several things, some obvious, some not.
To state the first obvious factor, instruments are directional,
mikes are directional, and instrument amplifiers are directional.
Again obviously, the directionality of these three can be used in an
intelligently designed setup to keep the sound of one instrument
out of another's mike.
The second gimmie is room treatment. There are three ways to go wrong
on this, and two of them are common.
The most prevalent treatment problem is the stuff used to deaden the
room. So many studios have put cheap carpet on every available surface
that it has come to be regarded as a good thing to do. In fact, it is
a terrible thing to do. Carpet, drapes, acoustical tile, and thin
Fiberglas all share the same characteristic. They don't absorb bass.
They do absorb top end. Some better than others. Carpet and drapes
absorb about one-fifth as well as Fiberglas, and are many times more
expensive. Cheap acoustical tile absorbs very well, but only in the
middle of the spectrum. Fiberglas and Sonex absorb to the maximum,
but except in suspended ceilings, thin fuzz drops dead below 400 to
500 HZ and Sonex is unthinkably expensive for ceilings.
So unless the studio walls are so flimsy as to vibrate with bass and
send it outside, the result of using a lot of thin studio treatment
is a working space that ranges from boomy at bass frequencies to an
absolute grave on top. The sound of the room is reflected in the
microphone isolation characteristics, with the low end sound falling
through every mike in the place, and a dry, sterile top end.
Needless to say, it's very hard for musicians to work in a room like
that as they can't hear each other bouncing off the walls. All boom,
no tinkle.
Overtreated, dead rooms are a frequent problem, and a near inevitable
result of using thin acoustical treatment, as too much of it is usually
laid on in an attempt to pull down the room bounce at medium
frequencies.
The third standard problem is, of course, insufficient treatment and a
room that is too live to yield adequate isolation.
Which brings up the subject of how much treatment is needed in a studio.
Interesting, as not only is there no simple answer, there has been
virtually no valid material published on the matter.
The difficulty here lies in the fact that the amount of treatment
for a studio varies, but not directly, with the size of the room, so
how much is a complex calculation. However, the result of the treatment
can be stated in reasonably simple terms, and the amount can be arrived
at by experiment.
First, the result. A vocalist needs a minimum of about 26 db isolation
on mike to get decent results, as normal limiting eats 10 to 12 db of
the available margin.
Other instruments should therefore die away by at least that much
before hitting the vocal mike.
Simple distance won't do it, as a performance room must be at least
somewhat live so the players can hear each other. Anechoic chambers are
out for recording.
If there is sound bouncing around in a room, the bounce will at some
point be as loud as the sound source that created it.
That point is called the critical distance, and the Dc of a sound
source is a statement of how far it travels in a room before it goes
constant volume. Dc varies with a number of factors including the
directionality of the source but, as rhythm section instruments have
about the same directionality as voice, a general treatment can be
made for rhythm and vocal isolation.
The treatment needs to yield 26 db or better of die off before the
source goes constant volume. Sound dies off by 6 db per distance
doubling until the Dc is reached, with the last figure a 3 db point.
Therefore, assuming a mike to mouth distance of six inches, 1ft=6db,
2ft=12db; 4ft=18db; 8ft=24db, and as the last figure adds only 3db,
a voice Dc of 16 feet adds 3 db more for 27 db of acoustical loss at
that figure.
You can live with less, as little as 10, but it's not quite
satisfactory. 12 is definitly OK for general work, and usually used,
but 16 feet is just bloody wonderful, as you can put a vocal anywhere
in the room, and allows the mixer to do anything he(she) wants instead
of the room's dictating all kinds of weird stuff to keep garbage out
of the mikes.
That's 16 acoustical feet, not straight line, and unless you have a
low live ceiling, pretty easy to set up.
To find the amount of treatment necessary for a given room, acoustical
math is handy if you have it, and available either in a book titled
SOUND SYSTEM ENGINEERING by Don and Caroline Davis published by Howard
W.Sams, or in the program on this site.
Otherwise, just measure the room's Dc for a human voice. This is done
by walking toward someone in the middle of the room while chanting one
one one one, test, oom, or whatever turns you on as long as it's
constant in volume and tone.
The listener will hear the talker at constant volume until the room's
Dc is reached, at which point the talker will become suddenly and
obviously louder.
Passing thru the Dc a few times in each direction will nail it down
pretty good, and the trick of using a voice for this is better than
it might seem at first blush. For one thing the equipment (none) is
readily available.
For another the system works in any language, and the results can be
'phoned in. Mostly,however, it checks the room at about 100 Hz, where
bad rooms go live, and yields remarkably accurate results as a result.
Clapping one's hands while looking wise and murmuring hmm and aha is
much more impressive, but as handclaps and the like normally trigger
the ear well up into the midrange, it's not uncommon to clap out a
room at one second and find that it's three or more at voice
frequencies.
Finally, it is a simple fact that rhythm instruments were designed
to work with the human voice, and it is rational to set up a room
around that centerpiece.
Studio design per se is dealt with in some detail elsewhere in these
texts but in passing it should be pointed out that isolation is
mostly a function of Dc, and getting a voice Dc of 12 to 16 feet in
a room with less than 10,000 cubic feet of volume produces a room
so dead as to be damn near impossible for musicians to play in.
Generally, small rooms have big problems and vice-versa.
Some less obvious isolation factors include proximity of sound
sources to walls and instrumental volume.
The first is easy. Don't build or set up a studio with instruments
within 4 to 5 feet of a wall. The wall will reinforce the sound
just like any theater back wall, and you'll hear it on every mike
in the room.
Taking the above one at time, room volume, in cubic feet, puts an
absolute limit on how much instrumental volume, in db/spl, the room
will hold without creating problems. This limitation can produce some
astonishing situations.
Some years ago, the writer tried to record an operatic soprano in a
livish 9000 cubic foot room. And failed. The lady produced so much
sound on her loud notes that she loaded the room, and every loud note
distorted the mike, as it heard her voice from three or four directions
at once. She turned a nice little studio into a horrible echo chamber.
The session was moved to a bigger room, worked fine, and the lesson
remembered.
There is probably a firm rule for ceiling height, but I haven't seen
it. So to make a rough estimate, for standard instruments and normal
seating, a live session studio should have a ceiling height in the area
of 14 to 20 feet. It does not follow that a low ceiling ruins the sound
but to get a reasonable volume of 12 thousand cubic feet or more, you
need a whole lot of floor space with a low ceiling.
Low ceilings work fine, but place a lower limit on the number of people
that can work in the room. Additionally, the room volume works against
total instrumental volume, so one can pack strings in like sardines,
but 20 brass or three 200 watt amps can be a disaster. The room volume
versus instrumental volume effect may account for the fact that small
garage studios seem to have less trouble than small basement studios.
THE ACOUSTICAL DESIGN OF RECORDING STUDIOS
THE ACOUSTICAL DESIGN OF RECORDING STUDIOS
When one considers that the recording industry has been building and
using studios for about 70 years, it is remarkable that so little basic
theory has been published on the subject. To be sure, there are plenty
of "here's how we did it" articles in print as well as a number of
"here's how to do it" examples to be found in books and magazines,
but none of these provide enough of the underlying design principles
to enable a reader to duplicate the performance of such studios unless
he also duplicates the studio. For that reason, while such publications
are interesting and even entertaining, they are of little use to a
studio owner who wants to improve an existing room or build a new one
in a space different from the exemplars given.
The situation grows even more extraordinary in light of the massive
amount of experimentation and research devoted to control rooms over
the past few years.
Those efforts have resulted in enough published material to allow a
studio to select from at least two demonstrably excellent generic
control room designs, both of which spring from the same clearly
expressed theoretical underpinning. While the general case design
will need some cleanup and tuning to achieve optimum results, a
studio owner can use the published theory to modify a given plan,
adapt it to his particular situation, and come up with a fundamentally
decent room. In short, we know how to build good control rooms.
We sure as hell don't know how to build good studios. In fairness,
there are some designers who appear to know something of the subject,
but they don't give away their stock in trade, so a studio owner is
faced with the problem of separating the good designers from the good
talkers. With the near future of his business at stake, that's a
serious problem, made worse by the fact that even very good acousticians
have been known to make very bad mistakes when dealing with recording
rooms.
As an example, the two worst studios the writer has ever encountered
were designed from scratch by a Phd named Sabin. (There were three of
them.) The rooms were retreated within a couple of months, but we
turned out some pretty marginal work in the meantime. Since this
happened in the city's premier recording facility, marginal was a bad
case of egg on face.
Doc Sabin was not at fault in that mess. In fact, nobody was. The whole
thing was a terrible mistake.
The mistake was to confuse a recording studio with a normal acoustical
envronment.
Acousticians ordinarily think of large rooms in terms of theaters and
auditoriums, which have a definite sound source feeding a definite
audience. That applies equally to control rooms, theaters, auditoriums,
and almost everything else acoustical designers get into.
It does not apply to studios, which are completely different from most
other rooms. A studio has any number of sources in the persons of the
musicians, and an audience comprised of those same players. Multiple
scattered sources, ditto listeners. Peculiar room.
Keeping strictly to acoustical performance, the primary function of a
recording studio is to provide adequate isolation between microphones
while allowing the players to hear each other as well as possible.
Acoustical isolation is by far the most often discussed of these two
areas, but since the parameters involved are addressable by acoustical
mathematics, producing satisfactory isolation levels is a fairly
straightforward process. All that's needed is a knowledge of what
constitutes adequate isolation and several pages of mathematical
computations. Happily enough, there is a way round that last item.
Treating a room for multidirectional listenability is a good deal more
difficult, as it is not a direct function of the room's global
characteristics and therefore cannot be treated mathematically. General
solutions are available, and they work nicely, but they have more to do
with old fashioned intuitive acoustics than with the glitzy new computer
aided stuff.
Among other things, this means that the merits of a suggested treatment
cannot be readily confirmed by punching up one's handy dandy number
cruncher.
Getting on with it, the specific design parameters are:
1: What is a reasonable isolation level and how much treatment is needed
to get it?
2: What constitutes acceptable listenability and how is that managed?
3: What's the catch? (There's a bear in every woods.) ((Sometimes several
bears.))
Item one. Acoustical isolation between instruments is a function of the
degree to which the sound of one dies away before getting to the next.
When the die off is inadequate the sound of one instrument falls through
the mike of the next and trashes it.
If it is excessive the musicians can't hear each other properly, which
makes group playing difficult and ruins section sound. Everything in the
real world is a compromise, and acoustical isolation is no exception.
The amount of acoustical attenuation for a given instrument in a room
depends on the room's global characteristics. As with any radiated
field, sound pressure levels diminish as the square of the distance
from the source. Double the distance, lose 6 Db SPL. The equation holds
for any distance in a perfectly dead room or out of doors.
In a normal room, however, the walls reflect some of the sound. Since the
source sound level diminishes with distance, at some point the reflections
from the walls will equal the source level. Beyond that point the sound
no longer dies away, and the level becomes constant at any further
distance. The distance at which this transition takes place is currently
called the Critical Distance. It has been called other things in other
centuries, as it's existence has been known for a very long time. It is
easy to observe, easy to measure, and a remarkably accurate indicator of
a room's acoustical performance.
The Critical Distance (Dc) of a sound source depends on the reflectivity
of the walls and how much wall surface the sound hits. As an example, a
firecracker hung on a string in the middle of a room produces a spherical
sound field which will bounce off all six walls of the room. Six, because
sound has no sense of direction, and can't tell a floor or ceiling from
any other surface. This spherical source is assigned a "Q" (figure of
merit) of 1, meaning it has no directionality at all. Hang the cracker
against a wall, and it radiates a hemispherical pattern. That's a Q of 2.
Halfway up the wall and in a corner it's a half hemisphere, and the Q is
4. On the floor and in a corner, Q =8. Q represents the beam width of the
sound source. The higher the number, the narrower the beam.
The narrower the beam, the less wall surface is struck by a source's
sound. Therefore, the higher the Q, the longer the Dc. And the higher
the surface reflectivity, the shorter the Dc.
It follows from the above that low Q instruments will have the shortest
Dc's, and the poorest isolation. As it happens, low Q describes both the
human voice and the entire rhythm section. A moment's thought will
explain that. Bass, piano, guitar and drums were used to accompany the
human voice for several centuries before mikes and such were invented,
and were designed to match it. They match quite well, which leaves us
with a kit of Q 2.5 instruments as the basis of isolation design.
The most difficult instrument in terms of isolation is the voice. Not
because it's so soft, but because of limiting. Unless a studio wants
to turn out 1940's records, there is no choice but to limit vocals,
and the limiter costs about 12 Db of isolation as it pulls up the
consonants in the singer's words. What this amounts to is that the vocal
channel should show something approaching minus 20 Db when the vocalist
is quiet; 12 db for limiting, and 10 to 14 to clear the consonants and
allow a little dynamic range for the singer. Since other instruments
work nicely with a clearance of 6 to 10Db adequate vocal isolation
becomes the criterion for acoustic design in studios.
Vocal isolation is made a little easier by the small size of the
instrument, which allows miking at a half foot without running into
serious proximity effects, and generally presents about 86 Db SPL on
mike. Hardly thunderous, but the peak level differences between voice
and the rhythm instruments are not as great as commonly assumed. It's
limiting up the minus 12 Db consonants that give rise to vocal iso
problems. Still, since other instruments can be 6Db or more over the
voice's 86 Db, the room characteristics have to lay for about 26 Db
of acoustical loss from a vocal to mike distance of six inches to any
other mike twenty acoustical feet away, keeping in mind that the 20
feet may simply be a few feet in front of the vocal mike. It is not a
straight line measurement.
The problem is made harder by a simple but nasty fact. A SOURCE GOES
CONSTANT VOLUME AT IT'S Dc, AND THE VOLUME IS THE SAME EVERYWHERE IN
THE ROOM. Distance beyond Dc makes no difference in fallthrough, and
directionality has no effect, except to mud up the fallthrough.
Dead flats don't work. Hyper cardioid mikes don't work. Nothing works.
The levels of the rhythm instruments have to fall about 26 Db before
going constant volume, or you can't work a vocal anywhere in the room.
Item one is 26 Db. More is nice, but getting much over 26 in a small
room involves so much treatment that the studio turns into an anechoic
chamber, with fuzz covering every wall.
Which brings up item two; listenability of the room.
Totally fuzzed walls return no sound to the players, who respond by
playing louder. And worse.
It's very difficult for a group of musicians to work in concert (pun
intentional) unless they can hear each other. Outdoors, with nothing
otherwise coming back to the players, stage monitors are used to supply
the sound of the group to the group. An engineer can use the studio
playback speakers for the same purpose, and it works surprisingly well,
but in both cases the players hear the mixer's balance, not their own.
They play in one balance and hear another, which creates some subtile
but nasty musical corruptions.
As an example, no mixer will let a solo ride at too low a level. It's
the mixer's job to maintain a proper balance, and mixers do their jobs.
So if a musician plays a tentative first solo, the mixer raises it's
level as needed, and it plays back in proper balance. After a few takes,
the soloist gets used to the idea that his solos will come out right no
matter what he does, and lays back on all of them. It's easier to play
soft. The mixer also adjusts to the situation, and bumps the level for
each solo. All this sounds pretty good at the time, but after a few days
both parties discover that the solos don't sound like solos. They sound
like lifted fills. That's because a solo is generally a high energy item,
and when a player lays back rather than putting out the energy, the solos
lack drive and intensity.
Technically speaking, this is a matter of harmonic content. When an
instrument is played hard or loud, the energy shows up as an increase
in harmonics, and the result is a loud sound. When not, not, and
artificially boosted soft solos just don't make it.
While this is one of the less obvious problems involved, musicians who
can't hear their overall sound well enough to maintain solo and section
balances during performance are very unlikely to play at their full
potential, and they need to hear themselves directly. That's especially
true if the mixer is tricking up the sound as it goes through the console
to the tape.
Since the usual studio setup points the musicians and their instruments
at the control room, the obvious (and normal) way to supply direct
feedback is to bounce the player's sound off the control room wall.
Control room walls are left reflective as a matter of conventional wisdom,
and are even somewhat optimized by stacking the musicians cases against
the wall under the control room window. The cases offer a fair degree of
dispersion to the strong boundary layer sound traveling along the floor
to the control room wall, and integrate it before reflecting it back to
the players. That won't work with a rug on the floor, but improves things
quite a lot otherwise.
Given that a primary function of the control wall is to supply a live
surface to the musicians, it can be made more effective by using some of
the techniques employed in the backs of control rooms. These involve
substituting RPGs for the stacked cases, retreating the wall for maximum
reflection, moving the control room window to the upright position, and
installing a reflector above the window angled to bounce even more sound
back to the rhythm section. The combination of flat and dispersed
reflections has been shown to be optimum for critical listening, and if
it's good enough for engineers, why not supply it to the people who are
doing the actual work in a studio?
In any case, the amount of acoustical treatment in a studio is limited
by the need to leave the major part of the control room wall reflective.
And there advantages to a live floor in allowing solid boundary layer
sound at the control wall, in addition to making it easier to move things
around in the studio.
The side walls are far less critical. Because of that, they are commonly
either left untreated or given some kind of uniform treatment. Neither
is a good idea.
Flat, straight walls have been known to be acoustically unacceptable for
centuries. That's partly because sound reflects off such walls as a flat
smack, which sounds bad, and partly because it bounces so strongly. If
the side walls are either untreated or evenly treated the sound will
ricochet around the room like a ball on a billiard table until it finds
an open mike to get into. That was the problem with Doc Sabins' room.
It is probably possible to control the results by putting an absorbent
flat behind every mike in the room, but it's a tedious process, and
interferes with player communication. Much better to clean up the bounce.
Since the villains in the piece are flat, evenly treated walls, the
obvious remedy lies in knobbing up the walls and installing absorptive
treatment in patches.
Both objectives can be accomplished by hanging live sided boxes filled
with Fiberglas on the walls. (See drawing.) Floor to ceiling treatment
is unnecessary, as mikes rarely point up. The boxes should start high
enough off the floor to clear chairs and other clutter leaned against
the wall, and will generally top out at eight or nine feet above the
floor.
A box with reflective sides will act as a disperser, and at a foot or
so deep will disperse down to about 550 Hz. Not ideal, but not bad,
and at a foot the boxes are pretty manageable. They are normally spaced
at three to six feet apart, leaving the walls reflective between them.
This presents a combination of dispersion, absorption and reflection to
both the musicians and the mikes, and cleans up the billiard ball
syndrome quite nicely while presenting an optimum listening environment
to the hard working types in the studio.
The back wall can be treated in the same way in small rooms, although
it is best to leave the back as live as possible, as reflections from
it give the players a sense of being in a room rather than working
with their backs to a vacuum.
In cases where a great deal of absorption is needed, the wall area
above eight feet and below about two can be totally treated without
ruining the generally live sound of the room, as the ear only needs
a little encouragement to think it's in a normal environment.
The ceiling is another matter, and needs be almost entirely dead,
because it is almost never high enough to establish a decent modes
structure. The standard literature lists acceptable room proportions
of up to two to one as an extreme case, and the vast majority of
ceilings are well over that. As always, the best way to deal with an
unsolvable problem is to eliminate it, and since a non-reflective
surface generates no modes structure one way or another, dead ceilings
are the norm in most studios.
The ceiling also presents the largest area available for serious
treatment, especially as it can be totally absorbent without making
a room sound dead. Short, yes. Dead, no.
Unless a ceiling is extremely low the ear ignores it, preferring to
take it's cues more or less horizontally.
It is critical that the ceiling treatment be acoustically flat in it's
absorption. Given an ordinary grid hung 16 inches below the structural
ceiling, flat response can be accomplished with 1-1/2 inch Fiberglas
ceiling panels or with thinner panels and a Fiberglas batt overlay.
It is wise to check manufacturer's literature for exact specifications,
as the low end absorption of the ceiling must extend far enough into
the bass range to avoid the common fault of acoustical treatment that
soaks out the top end of the room while leaving the low end live. That
kind of treatment results in a muddy room with terrible isolation probs
in terms of bass, floor tom, and bass drum.
The need for flat low end response applies to all room treatment unless
the studio has big windows or it's walls are so flimsy as to transmit or
absorb bass by vibrating to it. Even then, bass attenuation will seldom
exceed 30%, leaving 70% to be supplied by other means. While there are
any number of bass absorbing devices which can be built or purchased,
they are inconsistent in operation, inefficient except in corners, and
very difficult to analyze as to the number and size required.
On balance it's more practical to install the general treatment in such
way as to absorb uniformly from bass to cymbals.
Controlling high frequency reflections is easy, but bass absorption is
largely a matter of absorber depth, and it takes considerable thickness
to get flat down to 60 Hz. Hung ceilings manage it with thin panels and
the 16 inches between panels and the real ceiling, but a wall mounted
absorber needs a minimum depth of 6 inches for Fiberglas (703) board,
and a foot for glass wool.
DON'T USE THIN TREATMENT! Carpeting and drapes absorb 2 to 14% of bass
while soaking out 60 to 70% of the top end, yielding a room with no
presence and extreme boominess. Bad for playing, worse for recording.
At 70% efficiency, they also require an excessive amount of treatment
and wall area. Interestingly enough, both products cost far more than
proper acoustical materials, and are not necessarily more attractive.
By and large, Fiberglas in one form and another is probably the most
practical treatment available, and it can be covered in any number of
handsome fabrics or in Tectum if a durable wall is needed.
With the type and location of studio treatment in hand, we can finally
address the question of how much absorption is needed.
The following data are not hypothetical. The Dc figures were determined
during extensive reality testing of a newly written acoustical design
computer program.
The test method consisted of retro engineering a number of recording
studios, in each of which the writer had done some hundreds of sessions.
The majority of the studios were acceptable, a few were marginal, two
were bad, and two superb.
The object of the exercise was to find a common parameter that related
to actual studio performance, and the voice Dc proved to be a figure of
merit for isolation in properly treated rooms. Other correlations became
evident over several years of repeated computer runs on these and other
studios in an acoustics course taught by the author at a local college.
Designing for isolation is both simpler and more difficult than it first
appears. The simple part is very simple indeed, as voice Dc and therefore
isolation turns out to be function of the amount of absorption in a room
regardless of room size.
The absorption required for 26 Db of acoustical loss from 6 inches to 20
feet (a voice Dc of 11 1/2 feet) is about 2700 Sabins. Sounds easy.
If the practice were as straightforward as the theory, one could stuff
2700 square feet of Fiberglas into a studio and open for business without
further ado.
Unfortunately, what's wanted is 2700 Sabins of absorption, and the actual
amount of treatment for that figure can vary from less than 1500 to just
over 2500 depending on the size of the room. First bear in the woods.
The reason for a difference between actual treatment and effective
absorption is that the standard Sabin formula is linear, and absorption
in highly treated rooms is not.
In fact, when 80% of the wall surface absorbs at 1 Sabin per square foot,
the effective absorption of the treatment is doubled. There is a formula
for this effect, (Norris Eyring) which is reasonably accurate, but since
it involves the use of natural logarithms, it is tedious to use.
Second bear.
The third bear is the well-populated acoustical forest is the difficulty
of accurately assigning absorption values to various materials already
in the room. Most standard materials can be looked up in tables printed
for the purpose, but there are always a few things that aren't listed.
Additionally, it is very easy to mistake one kind of acoustical material
for another and come up with significant errors in calculations.
Calculations are a pain anyway, so it's best to circumvent the bears by
measuring the acoustical performance of the room.
There are several thoroughly scientific ways to do this, and any number
of manufacturers eager to sell equipment for the purpose, but as a
practical matter such measurements are of little or no use to the studio
owner. Cheap equipment yields cheap results, and the data gleaned from
upscale equipment require expert (and costly) interpretation.
In the first instance, the figures aren't completely trustworthy, and in
the second routinely repeating the tests will cost a fortune.
Following the KISS (keep it simple, stupid) rule, the writer prefers to
measure a room by determining it's voice Dc. The equipment costs nothing,
it takes about two minutes, and the results are more than accurate enough
for real world use.
Better still, being a simple-minded test, it reports simple-minded
figures with no interpretation, no ambiguity. Best of all, a Dc check
makes it's measurement at about 100 Hz, where improper treatments cause
a the majority of isolation problems.
Measuring a voice Dc is child's play provided one keeps in mind that the
purpose is to determine the global characteristics of the space. Toward
that end, it is essential to make the measurement in the acoustical
center of the room. Given normal treatment, that will be in the physical
center as well.
In cases where the absorption is considerably greater on one wall than
another, the acoustical center will have to be found.
Again, dead easy. Using the incredibly sensitive instruments found on
either side of the human head, one sidesteps away from one wall toward
another until the reflected sound from the two are equal in each ear.
If the reader has not done this in past, he may find it useful to
calibrate his ears to wall sound by stepping up to a live wall and
varying his wall to head distance from a couple of feet to a couple of
inches until the wall sound is firmly fixed in mind. It is usually
perceived as a kind of pressure on the ear, and will very reliably
inform the listener of his position in a space. No sound other than the
room's random noise is needed, and once the listener knows the sound of
a close wall he will find that he can walk to within a foot or so of any
live wall with his eyes closed, This is simply a case of practicing a
normal human ability into a skill. The blind do it all the time. So do
the rest of us, but unconsciously.
The writer once deadened one wall of a hallway, and sighted people veered
into it to the point of wearing out the treatment.
Having determined the acoustical center of the room, Dc is measured by
two people more or less astride the room's center starting at a distance
of 15 to 20 feet. One of them walks toward the other droning one, one,
one as the other waits for the sound of the talker's voice to suddenly
get louder. The process works both ways, with the talker's voice abruptly
going constant volume as he retreats, but the writer's experience with
several hundred students indicates that toward is easier to hear than
away, particularly in the learning stage. It is also easier to hear if
the talker walks briskly at first. He can slow down for greater accuracy
once the listener has the sound of the transition in mind.
While rare, there is one case in which it is nearly impossible to get
a decent Dc measurment, i.e. a room with a very high ceiling and a
dead floor. Short a couple of tall ladders and considerable time, it is
not possible to get to the vertical center of the room, so it's back to
plan B. (drop back 10 and punt)
The pair can also check room's frequency response by measuring the Dc
using the word six, leaning on the s and x and suppressing the vowel,
so that most of the sound is at 3 to 5 Khz. This is a pretty rough test,
but if the Dc's are wildly disparate, they indicate a room with more
absorption in the midrange than at the low end.
While Dc is a square root function of a room's global characteristics
and therefore a rather short ruler, the breakover is sufficiently abrupt
to make measurements to within a few inches quick, easy, and repeatable
by any number of talker-listener pairs. Other than the one above, the
only conditions under which it doesn't work properly are rooms in which
the Dc is greater than the wall spacing, (rare) and huge rooms which
appear to divide themselves into several acoustical areas due to
extreme losses between one wall and another.
In the first instance the room will be too small and dead to be of any
practical use, and in the second the room volume will be well in excess
of a million cubic feet. The writer knows of one at 6 million that acts
funny, but it's in no danger of being used for studio work.
Once the Dc of a room has been measured, some acoustical modifications
may seem in order. If so, a few cautionary notes should be kept in mind.
First, the Dc varies as the square root of the room absorption, so
doubling the effective treatment and thereby halving the reverberation
time will extend the Dc to only 1.4 times it's previous figure. This
presents no problem in a medium to large room, but good isolation in a
30x20x10 foot studio would require some 1550 square feet of Fiberglas
scattered over only 2400 square feet of surface area.
Even with the floor thickly carpeted, leaving a 20x10 foot control
room wall reflective would require a 75% treatment of the other walls,
and result in a reverberation time of just over one tenth second.
Some rooms are simply too small to treat for live studio work, as they
get too dead. The 6000 cubic foot case in point is probably the workable
minimum.
Second, a big studio is rarely allowed more that about one second of
reverberation time, which results in a voice Dc approaching 20 feet.
Obviously, such a room needs no help in isolation, and is best left
alone. It is a general rule in acoustics that big rooms are easy. It's
the little ones that give you the pip.
Third, professional engineers commonly do good work in bad conditions.
The writer has done any number of sessions in studios with 7 to 8 foot
Dc's which turned out well enough to sell bags of records. It's not
impossible to record in a room with poor isolation, it's just damn hard
work.
The point of proper treatment is that it allows one to get decent sound
with any reasonable setup, and it eliminates time lost in fooling around
trying to correct the room's faults.
Fourth, none of the figures given are engraved in stone. A twelve foot
Dc is better than ten, and less good than sixteen, but acoustics are
inherently inexact, and there is no sharp point at which rooms switch
from bad to good; they just glide from exasperating to no problem, with
the latter occurring and something around a 12 foot voice Dc for the
bulk of studio work.
In summary, a few minutes spent in measuring the real world acoustical
characteristics of a recording studio may reveal unnecessarily poor
isolation, and some of the treatment methods suggested herein may
improve it's general usefulness.
Since the measurement involves no expense and the treatment is designed
to make experimentation easy, these techniques offer a practical way for
a studio to confirm or optimize it's recording rooms.
THEN AGAIN, THERE'S ALWAYS THE ACOUSTICAL DESIGN PROGRAM (AD.BAS) WHICH
ALLOWS EXPERIMENTATION FOR STUDIO IMPROVEMENT OR DESIGN WITHOUT DOING
ANYTHING PHYSICAL AT ALL. MATHEMATICS ARE WONDERFUL.
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