Acoustics of the Small Recording Studio

Source: THE MASTER HANDBOOK OF ACOUSTICS

CHAP.

20 Acoustics of the Small Recording Studio T

his is the day of the small recording studio. Musicians are interested in making demonstration records to develop their style and to sell their 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. Studios are required for the production of campus and community radio, television, and cable programs. All of these have limited budgets and limited technical resources. The operator of these small studios is often caught between a desire for top quality and the lack of means, and often the know-how to achieve it. This chap. is aimed primarily to those in these needy groups, although the principles expounded are more widely applicable. What is a good  recording studio? There is only one ultimate criterion—the acceptability of the sound recorded in it by its intended audience. In a commercial sense, a successful recording studio is one fully booked and making money. Music recorded in a studio is pressed on discs or recorded on tape and sold to the public. If the public likes the music, the studio passes the supreme test. There are many factors influencing the acceptability of a studio beside sound studio quality, such as the type of program and the popularity of the performers, but studio quality is vital, at least for success on a substantial, long-range  basis.

415 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved.  Any use is subject to the Terms of Use as given at the website. website.

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 Acoustics of the Small Recording Studio Studio

416

CHAPTER TWENTY

Public taste must be pleased for f or any studio to be a success. Producing a successful product, however, involves many individuals along the way whose decisions may make m ake or break a studio. These decisions may be influenced by both subjective and technical factors. The appearance of a studio, convenience, and comfort might outweigh acoustical quality, sometimes because the more tangible esthetic qualities are better understood than the intangible acoustical qualities. This chap. has little to say on the artistic, architectural, and other such aspects of a studio, but their importance cannot be denied. They just require a different kind of specialist.

Acoustical Characteristics of a Studio

80    B     d       l    e 70   v    e     l    e    r    u    s    s    e    r 60    p       d    n    u    o    S 50

40 1

2

Sound picked up by a microphone in a studio consists of both direct and indirect sound. The direct sound is the same as would exist in the great outdoors or in an anechoic chamber. The indirect sound, which immediately follows the direct, is the sound that results from all the various nonfree-field effects characteristic of an enclosed space. The latter is unique to a particular room and may be called studio 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 form in a studio, or any other room for that matter. Figure 20-1 shows how sound level (A) All surfaces 100% reflective varies with distance from a source, which D could be the mouth of someone talking, a Partially  musical instrument, or a loudspeaker. absorptive Assume a pressure level of 80 dB measured measu red C (B) All surfaces 1 foot from the source. If all surfaces of the 100% absorptive (free field) room were 100 percent reflective, we would have a reverberation chamber to end all reverberation chambers, and the 3 4 5 7 10 15 20 30 40 Distance from source - feet  sound pressure level would be 80 dB

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 Acoustics of the Small Recording Studio Studio

ACOUSTICS OF THE SMALL RECORDING STUDIO

pressure level with distance from the sound source with all surfaces 100 percent absorptive. In this case all the sound is direct; there is no indirect component. The best anechoic rooms approach this condition. It is the true free field illustrated in Chap. 4, and for this condition the sound pressure level decreases 6 dB for each doubling of the distance. Between the indirect “all reverberation” case of graph A of Fig. 20-1 and the direct “no reverberation” reverberation” case of graph B lie a multitude of other possible “some reverberation” cases, depending on room treatment. In the area between these two extremes lies the real world of studios in which we live and move and have our being. The room represented by graph C is much more dead than that of graph D. In practical studios, the direct sound is observable a short distance out from the source, but after that the indirect sound dominates. A sudden sound picked up by a microphone in a studio would, for the first few milliseconds, be dominated by the direct component, after which the indirect sound arrives at the microphone as a torrent of reflections from room surfaces. These are spread 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 flowing out from the source excites these resonances, bringing into play all the effects listed in Chap. 15. When the source excitation ceases, each mode dies 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 r oom resonances. Distinguishing between reflections and resonances is an acknowledgment that neither a reflection concept nor a resonance re sonance concept will carry us through the entire audible spectrum. Resonances dominate the lowfrequency region in which the wavelengths of the sound are comparable to room dimensions. The ray concept works for higher frequencies and their shorter wavelengths (Chap. 16). Around the 300- to 500-Hz region is a difficult transition zone. But with this reminder of the basic limitations of our method we can return to analyzing the components of sound in a small studio. The third component of indirect sound is involved with the materials of 

417

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 Acoustics of the Small Recording Studio Studio

418

CHAPTER TWENTY

The sound of the studio, embracing these three components of  indirect sound plus the direct sound, has its counterpart in musical instruments. In fact, it is helpful to consider our studio as an instrument that the knowledgeable musician, technician, or engineer can play. It has its own characteristic sound, and a certain cert ain skill is required to extract from it its full potential.

Reverberation Reverberation is the composite, average effect of all three types of indirect sound. Measuring reverberation time does not reveal the individual components of which reverberation is composed. Herein lies the weakness of reverberation time as an indicator of studio acoustical quality. The important action of one or more of the indirect components may be obscured by the averaging process. This is why it is said that reverberation reverberation time is an indicator of studio acoustica acousticall 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 is true that a genuine reverberant field may not exist in small spaces. Sabine’s reverberation equation is based on the statistical properties of a random sound field. If such an isotropic, homogeneous distribution of energy does not prevail in a small room, is it proper to apply Sabine’s equation to compute the reverberation time of the room? The answer is a purist “no,” but a practi cal “yes.” Reverberation time is a measure of decay rate. A reverberation time of 0.5 seconds 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/second decay rate. Whether the sound field is diffuse or not, sound decays at some particular rate, even at the low frequencies at which the sound field is least diffuse. The sound energy stored at the modal frequencies decays at some measurable rate, even though only a few modes are contained in the band being measured. It would seem to be a practical step to utilize Sabine’s equation in small

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 Acoustics of the Small Recording Studio Studio

ACOUSTICS OF THE SMALL RECORDING STUDIO

419

Studio Design In a general book of this type, space is too limited to go into anything but  basicc prin  basi principle ciples. s. Fort Fortunat unately ely,, ther theree is a rich liter literatur aturee on the subj subject, ect, much of it written in easy-to-understand language. language. In designing a studio, attention should be given to room volume, room proportions, and sound decay rate, diffusion, and isolation from interfering noise.

Studio Volume A small room almost guarantees sound colorations resulting from excessive spacing of room resonance frequencies. This can be minimized by picking one of the favorable room ratios suggested by Sepmeyer (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 the selected dimensions, based on ceiling heights of 8, 12, and 16 feet resulting in room volumes of 1,000, 3,400, and 8,000 cubic feet. Axial mode frequencies were then calculated after the manner of Table 15-5 and plotted in Fig. 20-2, all to the same frequency scale. As previously noted, the room proportions selected do not yield Small studio perfect distribution of modal frequencies,  but this is of no consequence in our investigation of the effects of room volume. A 0 50 100 150 200 250 300 visual inspection of Fig. 20-2 shows the Medium studio increase in the number of axial modes as volume is increased, which of course results in closer spacing. In Table 20-2 the 0 50 100 150 200 250 300 number of axial modes below 300 Hz is Large studio shown to vary from 18 for the small studio to 33 for the large. The low-frequency response of the large studio, 22.9Hz, is shown to be far superior to that of the two 0 50 100 150 200 250 300 smaller studios at 30.6 and 45.9 Hz. This is FIGURE 20-2 an especially important factor in the

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 Acoustics of the Small Recording Studio Studio

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CHAPTER TWENTY

Table 20-1.

Height Width Length Volume

Table 20-2.

Studio dimensions. Ratio

Small studio

Medium studio

Large studio

1.00 1.28 1.54

8.00 ft 10.24 ft 12.32 ft 1,000 cu ft

12.00 ft 15.36 ft 18.48 ft 3,400 cu ft

16.00 ft 20.48 ft 24.64 ft 8,000 cu ft

Studio resonances resonances in Hz.

Number of axial modes below 300 Hz Lowest axial mode Average mode spacing Frequency corresp. to room diagonal Assumed reverb, time of studio, second Mode bandwidth (2.2/RT60)

Small studio

Medium studio

Large studio

18 45.9 14.1 31.6 0.3 7.3

26 30.6 10.4 21.0 0.5 4.4

33 22.9 8.4 15.8 0.7 3.1

dimension of a room better represents the lowest frequency supported by room resonances because of the oblique modes. Thus, the frequency corresponding to the room diagonal listed in Table 20-2 is a better measure of the low-frequency capability of a room than the lowest axial frequency. This approach gives the lowest frequency for the large room as 15.8 Hz, compared to 22.9 Hz for the lowest axial mode. The average spacing of modes, based on the frequency range from the lowest axial mode to 300 Hz, is also listed in Table 20-2. The average spacing varies from 8.4 Hz for the large studio to 14.1 Hz for the small studio. The reverberation times listed in Table 20-2 are assumed, nominal

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 Acoustics of the Small Recording Studio Studio

ACOUSTICS OF THE SMALL RECORDING STUDIO

for the small studio. The advantage of closer spacing of axial modes in the large studio tends to be offset by its narrower mode bandwidth. So, we see conflicting factors at work as we realize the advantage of the mode skirts overlapping each other. In general, however, the greater number of axial modes for the large studio, coupled with the extension of room response in the low frequencies, produces a response superior to that of the small studio. The examples of the three hypothetical studios considered above emphasize further the appropriateness of musical instrument analogy of  a studio. We can imagine the studio as a stringed instrument, one string for each modal frequency. These strings respond sympathetically to sound in the room. If there are enough strings tuned to closely spaced frequencies, and each string responds to a wide enough band of frequencies to bridge the gaps between strings, the studio-instrument responds uniformly to all frequency components of the sound in the studio. In other words, the response of the studio is the vector sum total of the responses of the individual modes. If the lines of Fig. 20-2 are imagined to be strings, it is evident that there will be dips in response between widely spaced frequencies. The large studio, with many strings, yields the smoother response. Conclusion: A studio having a very small volume has fundamental response problems in regard to room resonances; greater studio volume yields smoother response. The recommendation based on BBC experience still holds true, that coloration problems encountered in studios having volumes less than 1,500 cubic feet are severe enough to make small rooms impractical. For reasons of simplicity, the axial

421

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 Acoustics of the Small Recording Studio Studio

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CHAPTER TWENTY

8 ft and a width of 16 ft means that the t he second harmonic of 16 ft coincides with the fundamental of 8 ft. This emphasizes the importance of proportioning the room for best distribution of axial modes. The perfect room proportions have yet to be found. It is easy to place undue emphasis on a mechanical factor such as this. I urge you to be well informed on the subject of room resonances and to be aware of certain consequences, but let us be realistic about it—all of the recording that has ever taken place has been done in spaces less than perfect. In our homes and offices, conversations are constantly taking place with serious voice colorations, and we listen to and enjoy recorded music in acoustically abominable spaces. The point is that in striving to upgrade sound quality at every stage of the process, reducing sound colorations by attention to t o room modes is just good sense.

Reverberationn Time Reverberatio Technically, the term “reverberation time” should not be associated with relatively small spaces in which random sound fields do not exist. However, some first step must be taken to calculate the amount of absorbent needed to bring the general acoustical character of a room up to an acceptable level. While reverberation time is useful for this purpose, it would be unfortunate to convey the impression that the values of reverberation time so obtained have the same meaning as that 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 deterioration of speech intelligibility and music quality results. If rooms are

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 Acoustics of the Small Recording Studio Studio

ACOUSTICS OF THE SMALL RECORDING STUDIO

   s     d    n    o    c    e    s      e    m    i    t    n    o    i    t    a    r    e     b    r    e   v    e    R

1.4 1.2 1.0

 c  i c   M u s

0.8 0.6 0.4

 peec h S pe

0.2 0

1,000

2,000

5,000

10,000

20,000

30,000

Room volume - cu ft 

FIGURE 20-3 Suggested reverberation times for recording studios. The shaded area is a compromise region for studios in which both music and speech are recorded.

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

Diffusion Before the advent of the Schroeder (diffraction grating) diffusor, there was little advice to give regarding diffusion in the small studio. Splaying walls and the use of geometrical protuberances have only a modest

423

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 Acoustics of the Small Recording Studio Studio

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CHAPTER TWENTY

Noise Noise is truly something in the ear of the “behearer.” One person’s  beautiful music is another person’s noise, especially at 2 AM. It is a two-way street, and an d fortunately, a good wall that protects a studio area from exterior noise also protects neighbors from what goes on inside. The psychological aspect of noise is very important—acceptable if  considered a part of a situation—disturbing if considered extraneous. Chap. 18 has already treated the special case of air-conditioning noise.

Studio Design Procedure We 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 noise producers, the air-conditioning equipment (Chap. 18). All of these are integral parts of studio design. The would-be designer should also sample the literature to see how others have solved similar problems. 1–3

Some Studio Features A glance into other people’s studios often makes one aware of  “things I want to do” or “things I definitely don’t like.” Figures 20-4 and 20-5 show the treatment of a budget 2,500 cu ft studio. Built on

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 Acoustics of the Small Recording Studio Studio

ACOUSTICS OF THE SMALL RECORDING STUDIO

FIGURE 20-4 View of a 2,500-cu ft voice studio looking into the control room. World Vision, International.

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CHAPTER TWENTY

FIGURE 20-6 A 3,400-cu ft studio used for both recording and editing voice tapes. Decorator-type fabric provides an attractive visual design for the absorber/diffusor wall modules. Mission Communications Incorporated.

design, the other more subdued. The stu-

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 Acoustics of the Small Recording Studio Studio

ACOUSTICS OF THE SMALL RECORDING STUDIO

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FIGURE 20-10 Music studio with a volume of 3,700 cu ft employing two 9 x 11 foot suspended ceiling frames that hold a total of 28 Helmholtz resonators. Far East Broadcast Company