Acoustics of the Small Recording Studio

7/29/2019 Acoustics of the Small Recording Studio

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This is the day of the small recording studio. Musicians are interestedin making demonstration records to develop their style and to selltheir sounds. There are hundreds of small recording studios operated

by not-for-profit organizations that turn out a prodigious quantity of material for educational, promotional, and religious purposes. Studiosare required for the production of campus and community radio, tele-

vision, and cable programs. All of these have limited budgets and lim-ited technical resources. The operator of these small studios is oftencaught between a desire for top quality and the lack of means, andoften the know-how to achieve it. This chap. is aimed primarily tothose in these needy groups, although the principles expounded aremore widely applicable.

What is a good recording studio? There is only one ultimate crite-rionthe acceptability of the sound recorded in it by its intendedaudience. In a commercial sense, a successful recording studio is onefully booked and making money. Music recorded in a studio is pressedon discs or recorded on tape and sold to the public. If the public likesthe music, the studio passes the supreme test. There are many factorsinfluencing the acceptability of a studio beside sound studio quality,such as the type of program and the popularity of the performers, butstudio quality is vital, at least for success on a substantial, long-range

basis.

Acoustics of the

Small Recording Stud

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415

C H A P.

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Source: THE MASTER HANDBOOK OF ACOUSTICS


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416CHAPTER TWENTY

Public taste must be pleased for any studio to be a success. Produc-ing a successful product, however, involves many individuals alongthe way whose decisions may make or break a studio. These decisionsmay be influenced by both subjective and technical factors. Theappearance of a studio, convenience, and comfort might outweighacoustical quality, sometimes because the more tangible esthetic qual-ities are better understood than the intangible acoustical qualities.This chap. has little to say on the artistic, architectural, and other suchaspects of a studio, but their importance cannot be denied. They justrequire a different kind of specialist.

Acoustical Characteristics of a Studio

Sound picked up by a microphone in a studio consists of both directand indirect sound. The direct sound is the same as would exist inthe great outdoors or in an anechoic chamber. The indirect sound,which immediately follows the direct, is the sound that results fromall the various nonfree-field effects characteristic of an enclosedspace. The latter is unique to a particular room and may be calledstudio response . Everything that is not direct sound is indirect,reflected sound.

Before dissecting indirect sound, let us

look at the sound in its all-inclusive formin a studio, or any other room for that mat-ter. Figure 20-1 shows how sound levelvaries with distance from a source, whichcould be the mouth of someone talking, amusical instrument, or a loudspeaker.Assume a pressure level of 80 dB measured1 foot from the source. If all surfaces of theroom were 100 percent reflective, wewould have a reverberation chamber toend all reverberation chambers, and thesound pressure level would be 80 dBeverywhere in the room because no soundenergy is being absorbed. There is essen-tially no direct sound; it is all indirect.Graph B represents the fall off in sound

S o u n

d - p

r e s s u r e

l e v e

l - d B

80

70

401 2 3 4 5 7 10 15 20 30 40

50

60

(A) All surfaces 100%reflective

D

Partially absorptive

C(B) All surfaces100% absorptive(free field)

Distance from source - feet

F I G U R E 2 0 - 1

The sound-pressure level in an enclosed space varieswith distance from the source of sound according tothe absorbency of the space.

Acoustics of the Small Recording Studio



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ACOUSTICS OF THE SMALL RECORDING STUDIO417

pressure level with distance from the sound source with all surfaces100 percent absorptive. In this case all the sound is direct; there is noindirect component. The best anechoic rooms approach this condition.It is the true free field illustrated in Chap. 4, and for this condition thesound pressure level decreases 6 dB for each doubling of the distance.

Between the indirect all reverberation case of graph A of Fig. 20-1and the direct no reverberation case of graph B lie a multitude of otherpossible some reverberation cases, depending on room treatment. Inthe area between these two extremes lies the real world of studios inwhich we live and move and have our being. The room represented bygraph C is much more dead than that of graph D. In practical studios, thedirect sound is observable a short distance out from the source, but afterthat the indirect sound dominates. A sudden sound picked up by a

microphone in a studio would, for the first few milliseconds, be domi-nated by the direct component, after which the indirect sound arrives atthe microphone as a torrent of reflections from room surfaces. These arespread out in time because of the different path lengths traveled.

A second component of indirect sound results from room resonances,which in turn are the result of reflected sound. The direct sound flowingout from the source excites these resonances, bringing into play all theeffects listed in Chap. 15. When the source excitation ceases, each modedies away at its own natural frequency and at its own rate. Sounds of

very short duration might not last long enough to fully excite room reso-nances.Distinguishing between reflections and resonances is an acknowledg-

ment that neither a reflection concept nor a resonance concept will carryus through the entire audible spectrum. Resonances dominate the low-frequency region in which the wavelengths of the sound are comparableto room dimensions. The ray concept works for higher frequencies andtheir shorter wavelengths (Chap. 16). Around the 300- to 500-Hz regionis a difficult transition zone. But with this reminder of the basic limita-tions of our method we can return to analyzing the components of soundin a small studio.

The third component of indirect sound is involved with the materials of constructiondoors, windows, walls, and floors. These too are set intovibration by sound from the source, and they too decay at their own par-ticular rate when excitation is removed. If Helmholtz resonators areinvolved in room treatment, sound not absorbed is reradiated.




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The sound of the studio, embracing these three components of indirect sound plus the direct sound, has its counterpart in musicalinstruments. In fact, it is helpful to consider our studio as an instru-ment that the knowledgeable musician, technician, or engineer canplay. It has its own characteristic sound, and a certain skill is requiredto extract from it its full potential.

ReverberationReverberation is the composite, average effect of all three types of indi-rect sound. Measuring reverberation time does not reveal the individ-ual components of which reverberation is composed. Herein lies theweakness of reverberation time as an indicator of studio acoustical

quality. The important action of one or more of the indirect compo-nents may be obscured by the averaging process. This is why it is saidthat reverberation time is an indicator of studio acoustical conditions,

but not the only one.There are those who feel it is improper and inaccurate to apply

the concept of reverberation time to relatively small rooms. It istrue that a genuine reverberant field may not exist in small spaces.Sabines reverberation equation is based on the statistical proper-ties of a random sound field. If such an isotropic, homogeneous

distribution of energy does not prevail in a small room, is it properto apply Sabines equation to compute the reverberation time of theroom? The answer is a purist no, but a practical yes. Reverber-ation time is a measure of decay rate. A reverberation time of 0.5seconds means that a decay of 60 dB takes place in 0.5 seconds.Another way to express this is 60 dB/0.5 second = 120 dB/seconddecay rate. Whether the sound field is diffuse or not, sound decaysat some particular rate, even at the low frequencies at which thesound field is least diffuse. The sound energy stored at the modalfrequencies decays at some measurable rate, even though only afew modes are contained in the band being measured. It wouldseem to be a practical step to utilize Sabines equation in smallroom design to estimate absorption needs at different frequencies.At the same time, it is well to remember the limitations of theprocess.




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Studio DesignIn a general book of this type, space is too limited to go into anything but

basic principles. Fortunately, there is a rich literature on the subject,

much of it written in easy-to-understand language. In designing a studio,attention should be given to room volume, room proportions, and sounddecay rate, diffusion, and isolation from interfering noise.

Studio VolumeA small room almost guarantees sound colorations resulting fromexcessive spacing of room resonance frequencies. This can be mini-mized by picking one of the favorable room ratios suggested by Sep-meyer (see Fig. 13-6) 1.00 : 1.28 : 1.54, applying it to a small, a medium,

and a large studio and seeing what happens. Table 20-1 shows theselected dimensions, based on ceiling heights of 8, 12, and 16 feetresulting in room volumes of 1,000, 3,400, and 8,000 cubic feet. Axialmode frequencies were then calculated after the manner of Table 15-5and plotted in Fig. 20-2, all to the same fre-quency scale. As previously noted, theroom proportions selected do not yieldperfect distribution of modal frequencies,

but this is of no consequence in our inves-

tigation of the effects of room volume. Avisual inspection of Fig. 20-2 shows theincrease in the number of axial modes asvolume is increased, which of courseresults in closer spacing. In Table 20-2 thenumber of axial modes below 300 Hz isshown to vary from 18 for the small studioto 33 for the large. The low-frequencyresponse of the large studio, 22.9Hz, isshown to be far superior to that of the twosmaller studios at 30.6 and 45.9 Hz. This isan especially important factor in therecording of music.

We must remember that modes otherthan axial are present. The major diagonal

Small studio

Medium studio

Large studio

0 50 100 150 200 250 300

0 50 100 150 200 250 300

0 50 100 150 200 250 300

F I G U R E 2 0 - 2

Comparison of the axial-mode resonances of a small(1,000 cu ft), a medium (3,400 cu ft), and a large(8,000 cu ft) studio all having the proportions 1.00:1.28: 1.54.




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420 CHAPTER TWENTY

dimension of a room better represents the lowest frequency sup-ported by room resonances because of the oblique modes. Thus, thefrequency corresponding to the room diagonal listed in Table 20-2 isa better measure of the low-frequency capability of a room than thelowest axial frequency. This approach gives the lowest frequency forthe large room as 15.8 Hz, compared to 22.9 Hz for the lowest axialmode.

The average spacing of modes, based on the frequency range fromthe lowest axial mode to 300 Hz, is also listed in Table 20-2. The aver-age spacing varies from 8.4 Hz for the large studio to 14.1 Hz for thesmall studio.

The reverberation times listed in Table 20-2 are assumed, nominalvalues judged fitting for the respective studio sizes. Given these rever-

beration times, the mode bandwidth is estimated from the expression2.2/RT60. Mode bandwidth varies from 3 Hz for the large studio to 7 Hz

Table 20-2. Studio resonances in Hz.

Small Medium Large

studio studio studioNumber of axial modes below 300 Hz 18 26 33Lowest axial mode 45.9 30.6 22.9Average mode spacing 14.1 10.4 8.4Frequency corresp. to room diagonal 31.6 21.0 15.8Assumed reverb, time of studio, second 0.3 0.5 0.7Mode bandwidth (2.2/RT60) 7.3 4.4 3.1

Table 20-1. Studio dimensions.

Ratio Small studio Medium studio Large studio

Height 1.00 8.00 ft 12.00 ft 16.00 ftWidth 1.28 10.24 ft 15.36 ft 20.48 ftLength 1.54 12.32 ft 18.48 ft 24.64 ftVolume 1,000 cu ft 3,400 cu ft 8,000 cu ft




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for the small studio. The advantage of closer spacing of axial modes inthe large studio tends to be offset by its narrower mode bandwidth. So,we see conflicting factors at work as we realize the advantage of themode skirts overlapping each other. In general, however, the greaternumber of axial modes for the large studio, coupled with the extensionof room response in the low frequencies, produces a response superiorto that of the small studio.

The examples of the three hypothetical studios considered aboveemphasize further the appropriateness of musical instrument analogy of a studio. We can imagine the studio as a stringed instrument, one stringfor each modal frequency. These strings respond sympathetically tosound in the room. If there are enough strings tuned to closely spacedfrequencies, and each string responds to a wide enough band of frequen-

cies to bridge the gaps between strings, the studio-instrument respondsuniformly to all frequency components of the sound in the studio. Inother words, the response of the studio is the vector sum total of theresponses of the individual modes. If the lines of Fig. 20-2 are imaginedto be strings, it is evident that there will be dips in response betweenwidely spaced frequencies. The large studio, with many strings, yieldsthe smoother response.

Conclusion: A studio having a very small volume has fundamentalresponse problems in regard to room resonances; greater studio vol-

ume yields smoother response. The recommendation based on BBCexperience still holds true, that coloration problems encountered instudios having volumes less than 1,500 cubic feet are severe enough tomake small rooms impractical. For reasons of simplicity, the axialmodes considered in the previous discussion are not the only modes,

but they are the dominant ones.

Room ProportionsIf there are fewer axial modes than are desired in the room under consid-eration, sound quality is best served by distributing them as uniformly aspossibly. The cubical room distributes modal frequencies in the worstpossible wayby piling up all three fundamentals, and each trio of mul-tiples with maximum gap between. Having any two dimensions in multi-ple relationship results in this type of problem. For example, a height of




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422 CHAPTER TWENTY

8 ft and a width of 16 ft means that the second harmonic of 16 ft coincideswith the fundamental of 8 ft. This emphasizes the importance of propor-tioning the room for best distribution of axial modes.

The perfect room proportions have yet to be found. It is easy toplace undue emphasis on a mechanical factor such as this. I urge youto be well informed on the subject of room resonances and to be awareof certain consequences, but let us be realistic about itall of therecording that has ever taken place has been done in spaces less thanperfect. In our homes and offices, conversations are constantly takingplace with serious voice colorations, and we listen to and enjoyrecorded music in acoustically abominable spaces. The point is that instriving to upgrade sound quality at every stage of the process, reduc-ing sound colorations by attention to room modes is just good sense.

Reverberation TimeTechnically, the term reverberation time should not be associatedwith relatively small spaces in which random sound fields do notexist. However, some first step must be taken to calculate the amountof absorbent needed to bring the general acoustical character of a roomup to an acceptable level. While reverberation time is useful for thispurpose, it would be unfortunate to convey the impression that thevalues of reverberation time so obtained have the same meaning asthat in a large space.

If the reverberation time is too long (sound decays too slowly),speech syllables and music phrases are slurred and a definite deterio-ration of speech intelligibility and music quality results. If rooms aretoo dead (reverberation time too short), music and speech lose charac-ter and suffer in quality, with music suffering more. These effects arenot so definite and precise as to encourage thinking that there is a spe-cific optimum reverberation time, because many other factors areinvolved. Is it a male or female voice, slow or fast talker, English orGerman language (they differ in the average number of syllables perminute), a stand-up comic or a string ensemble, vocal or instrumental,hard rock or a waltz? In spite of so many variables, readers need guid-ance, and there is a body of experience from which we can extracthelpful information. Figure 20-3 is an approximation rather than a trueoptimumbut following it will result in reasonable, usable conditions




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for many types of recording. The shaded area of Fig. 20-3 represents acompromise in rooms used for both speech and music.

DiffusionBefore the advent of the Schroeder (diffraction grating) diffusor, therewas little advice to give regarding diffusion in the small studio. Splay-ing walls and the use of geometrical protuberances have only a modestdiffusing effect. Distributing the absorbing material is a useful meansof not only achieving some diffusion, but increasing the absorbing effi-ciency as well.

Modular diffusing elements are on the market that really diffuse asshown in Chap. 14. There are even 2 ft- -4 ft-modular units that offerhigh-quality diffusion and excellent broadband absorption (0.82 coef-ficient at 100 Hz, for example), all within a 2-in thickness (the Abffu-sor). The application of this new principle of diffusion, with orwithout the absorption feature, contributes a feeling of spaciousnessthrough the diffusion of room reflections and the control of reso-nances.

1.2

1.4

1.0

0.8

0.6

0.4

0.2

01,000 2,000 5,000 10,000 20,000 30,000

S peec h

M u s i c

R e v e r

b e r a t i o n t i m e - s

e c o n

d s

Room volume - cu ft

F I G U R E 2 0 - 3

Suggested reverberation times for recording studios. The shaded area is a compromiseregion for studios in which both music and speech are recorded.




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424 CHAPTER TWENTY

NoiseNoise is truly something in the ear of the behearer. One persons

beautiful music is another persons noise, especially at 2 AM. It is a

two-way street, and fortunately, a good wall that protects a studio areafrom exterior noise also protects neighbors from what goes on inside.The psychological aspect of noise is very importantacceptable if considered a part of a situationdisturbing if considered extraneous.Chap. 18 has already treated the special case of air-conditioningnoise.

Studio Design ProcedureWe have considered reverberation and how to compute it (Chap. 7),the reality of room resonances (Chap. 15), the need for diffusion(Chaps. 13 and 14), various types of dissipative and tuned absorbers(Chap. 9), and as mentioned, one of the most serious studio noiseproducers, the air-conditioning equipment (Chap. 18). All of theseare integral parts of studio design. The would-be designer shouldalso sample the literature to see how others have solved similarproblems. 13

Some Studio FeaturesA glance into other peoples studios often makes one aware of things I want to do or things I definitely dont like. Figures 20-4and 20-5 show the treatment of a budget 2,500 cu ft studio. Built onthe second floor of a concrete building with an extensive printingoperation below, certain minimum precautions were advisable. Thestudio floor is 3 4-inch plywood on 2- -2-inch stringers resting on1 2-inch soft fiberboard. Attenuation of noise through the double 5 8

drywall ceiling is augmented by a one inch layer of dry sand, a

cheap way to get amorphous mass. The wall modules, containinga 4-inch thickness of Owens-Corning Type 703 Fiberglas (3 lb/cu ftdensity), help to absorb and diffuse the sound.

The studio of Fig. 20-6 with a volume of 3,400 cu ft has a coupleof interesting features. The wall modules (Fig. 20-5) feature care-fully stained and varnished frames and a neat grille cloth. Those of Fig. 20-7 are of two kinds, one sporting a very attractive fabric




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F I G U R E 2 0 - 4

View of a 2,500-cu ft voice studio looking into the control room. World Vision, Interna-tional.

F I G U R E 2 0 - 5

Rear view of the 2,500-cu ft voice studio of Fig. 20-4. Wall modules containing 4-inchthicknesses of dense glass fiber contribute to diffusion of sound in the room. World Vision, International.




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426 CHAPTER TWENTY

design, the other more subdued. The stu-dio of Fig. 20-6 has a rather high ceiling,hence a virtual, visual ceiling was estab-lished at a height of 8 feet. This consists

of four 5- -7-foot suspended frames asshown in Fig. 20-8, which hold fluores-cent lighting fixtures and patches of glassfiber. The plastic louvers are acousticallytransparent.

The voice studio of Fig. 20-9, with avolume of 1,600 cu ft, employs wallabsorbing panels manufactured by theL.E. Carpenter Co. of Wharton, New Jer-

sey. These panels feature a perforatedvinyl wrapping and a 3 8-in rigid composi-tion board backing. The concrete floorrests on soft fiberboard with distributedcork chips under it. The low-frequencydeficiencies of carpet and wall panelsrequire some Helmholtz correction, and

F I G U R E 2 0 - 6

A 3,400-cu ft studio used for both recording and editing voice tapes. Decorator-typefabric provides an attractive visual design for the absorber/diffusor wall modules.Mission Communications Incorporated.

F I G U R E 2 0 - 7

Close view of the wall modules of Fig. 20-6. MissionCommunications Incorporated.




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thirteen 20 40 8-inch boxes are mounted in the suspended ceil-ing frame out of sight.

Figure 20-10 is a 3,700-cu ft music studio that is also used for voicework. Low-frequency compensation is accomplished by the same

Helmholtz boxes mentioned above, 14 of them in each of two sus-pended ceiling frames.

Elements Common to All StudiosChap. 4 of Reference 1 treats sound lock treatment, doors and theirsealing, wall constructions, floor/ceiling constructions, wiring pre-

F I G U R E 2 0 - 8

The high structural ceiling of the studio of Fig. 20-6allows the use of four 5 X 7 ft suspended frames to

bring the visual ceiling down to 8 ft and to supportillumination fixtures and absorbing material. Theplastic louvers are acoustically transparent. MissionCommunications Incorporated.

F I G U R E 2 0 - 9

Voice studio with a volume of 1,600 cu ft. A 7 X 10 ftsuspended ceiling frame hides 13 Helmholtz res-onators for low-frequency absorption. Wall modulesare proprietary units covered with perforated vinyl.Far-East Broadcasting Company.




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cautions, illuminating fixtures, observation windows, and otherthings common to all studios and which can create serious problems

if not handled properly.

Endnotes1Ballou, Glen, ed., Handbook for Sound EngineersThe New Audio Cyclopedia ,Indianapolis, IN, Howard W. Sams & Co., 2nd ed. (1991): Chap. 4, Common Factors in All Audio Rooms , by F. Alton Everest, p 67-101; Chap. 5, Acoustical Design of Audio Rooms , byF. Alton Everest, p. 103-141; Chap. 6, Contempory Practices in Audio Room Design , by F.Alton Everest, p. 143-170, Chap. 7, Rooms for Speech and Music , by Rollins Brook, p. 171-201.

2Allison, Roy F. and Robert Berkowitz, The Sound Field in Home Listening Rooms , J. Audio.Eng. Soc., 20, 6 (July/Aug 1972), 459-469.

3Kuhl, Walter, Optimal Acoustical Design of Rooms for Performing, Listening, and Recording , Proc. 2nd. International Congress on Acoustics, (1956) 53-58.

F I G U R E 2 0 - 1 0

Music studio with a volume of 3,700 cu ft employing two 9 x 11 foot suspended ceilingframes that hold a total of 28 Helmholtz resonators. Far East Broadcast Company


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Acoustics of the Small Recording Studio