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.