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Acoustical Shrinkage of Large Rooms into Small Rooms
Stephen Harner

Honey, I shrunk the room!

The physics of sound is well understood and the equations that describe the propagation of sound can be derived from the fundamental nature of the molecules of air. The equations of sound can then be used to calculate various parameters for a given set of boundary conditions, i.e. volume, surface area, etc., of a room.

Unfortunately, there aren't many widely accepted parameters that acousticians agree will consistently improve the acoustics of small rooms. However, there are now several widely accepted parameters that can be adjusted to improve concert hall acoustics. In the following sections, I will discuss the six most widely accepted parameters for concert halls (large rooms), and then apply them to small rooms by extrapolation, or so called acoustical shrinkage.


Loudness, as applied to concert halls, really means the relative perceived increase in loudness due to the reflected sound field over the loudness that naturally comes directly from the source. In a large concert hall, not being able to hear the quiet passages, (i.e. soft violin notes) would be bad for the overall sound quality. Since small rooms require listening in close proximity to the source, loudness is not really a factor.

Initial Time Delay Gap - ITDiG

The Initial Time Delay Gap, which I call ITDiG (it dig, you dig?), is the time between the arrival of the direct sound and the arrival of the first reflected sound.

Beranek says that the ITDiG should be less than 20 milliseconds for optimum quality of concert hall sound, while longer values have a negative effect.

Small rooms will generally always have an ITDiG less than 20 milliseconds, and typically will be around 2 milliseconds, unless the sound source is placed within 1 foot of the wall (ignoring the low level edge diffraction effects).

Advocates of live end dead end theory say that early reflections "smear" or "blur" the direct sound, and that eliminating the early reflections improves the sound. This involves treating the front half of the room with acoustic absorbers. Others say to use a mirror moved around on the wall to locate the speaker reflections as viewed from the listening position, then treat these spots with acoustic absorbers. Both of these methods are akin to lengthening the ITDiG.

Does too short an ITDiG make for bad sound, or does it improve the sound by adding width and fullness (like reverberation)? Very short ITDiGs, less than 1 millisecond, do affect the way we localize sound sources. Hafter's Auditory Perception Lab has shown that when the left ear is fed an impulse incrementally increasing from 0.1 to 1.0 milliseconds before or after the right ear, the brain hears the sound as coming from further and further left or right, depending on which ear is fed the first impulse. Certainly, if the 2 impulses smeared each other, then the brain would not be able to discern them. The fact that the ear/brain auditory system can discern impulse signals spaced apart by only 0.1 milliseconds is almost unbelievable!

If the second sound came from the same direction as the direct sound, and at the same time, then some phase cancellation would occur (generally referred to as comb filtering) causing peaks and valleys in the frequency response. In fact, this can be heard when 2 speakers are placed in close proximity and fed the same signal. But the direction and timing of reflected sounds are typically different from the direct sound, and there is no conclusive evidence that people actually hear comb filtering when speakers are placed close to walls. It is widely accepted that the ear/brain auditory system can key in on the direct sound, or first sound, then fuse or compensate for, any subsequent similar sound without detrimental affect to the overall sound.

For small rooms, in light of the localization effects, I agree with Ludwig that it would seem prudent to ensure at least a 1 millisecond ITDiG by placing the sound source at least 2 feet away from the walls. However, placing an acoustic absorber on the wall to absorb the first reflection may actually hurt more than it helps because most absorbers commonly used are not wideband, but rather have very little absorption in the low frequencies, thereby creating a very unbalanced first reflection spectrum.

Bass Ratio - BRat

Bass Ratio, what I call BRat, is a measure of the level of lower frequency reverberation versus the level of higher frequency reverberation. Beranek defines it as the average Reverb time at 125Hz and 250Hz, divided by the average Reverb time at 500Hz and 1000Hz (see below for definition of Reverb time). Although it is defined as a time measurement, it is perceived as a level (or gain versus loss). For example, a room with a high BRat sounds warm or bassy, while a low BRat sounds harsh or bright.

The main factor affecting BRat is absorption. Thin paneling and drywall absorb bass by flexing and vibrating. Most acoustical absorbers absorb treble and midrange, but little bass (just like carpet). Beranek's recommended range is from 1.1 to 1.45, for concert halls with lower reverberation times. Note that most studies of concert hall acoustics use symphonic music as the main subject, which in general has much lower levels of low frequency sounds than does modern music (i.e. rock and roll). This would require a higher BRat for good sounding symphonic music and a lower BRat for good sounding modern music.

Small rooms should sound natural and balanced, and since most rooms inherently have more high frequency absorption than low, a slight emphasis in the bass is desireable, which agrees with the range of 1.1 to 1.45. However, overuse of foam type absorbers is all too common, resulting in too high BRat with too much bass. Since most live end dead end rooms use foam on the dead end, they sound unnatural - half live, half dead (frequency-wise).

Inter-Aural Cross Correlation (subtracted from one) - I-ACk

Inter Aural Cross Correlation subtracted from one, what I call I-ACk, is a measure of Apparent Source Width, or more specifically I-ACk=1-IACC. A sound source directly in front of you without any reflections will have zero I-ACk. A sound source fed to your left ear, while blocking your right ear, will have I-ACk equal to one.

Lateral reflections from the side walls increase I-ACk, because sounds coming from left or right will arrive at the ears differently due to that big fat head in the way. The exception being if the source is dead center in front of you, and you are positioned exactly half way between two identical walls, but even then tiny movements of your head will increase I-ACk. Of course, stereo speakers playing sounds panned left or right will also increase I-ACk.

Since live end dead end theory reduces early lateral reflections, favoring later reflections from behind, it also reduces I-ACk. This has a negative effect on acoustical quality. For small rooms you should make the walls as live as possible, with any acoustic absorption used only on the floor or ceiling.

Reverberation - Reverb

Everybody loves a little ambience, and some love it a lot. Reverberation Time, or simply Reverb, is the time it takes a sound to drop in level by 60dB. It is the most obvious difference between indoor and outdoor sound.

Talking in a dead room or an anechoic chamber is immediately disconcerting. Music played thusly is universally considered abnormal and some may say it is actually unpleasant. So there is no question that rooms should have at least some Reverb, but how much is best? Beranek and many other acousticians agree that about 2 seconds is best for most symphonic music in concert halls. They also agree that different types of music require different Reverb times.

Beranek, among others, goes on to say that the early decay time (EDT) is more important than the later decay time, and defines EDT as the time it takes a sound to drop in level by 10 dB then multiplied by 6 to normalize it. There are some concert halls, like the Meyerson/McDermott in Dallas that have very different early and late decay times. The three most widely praised concert halls in the world, Symphony Hall in Boston, Concertgebouw in Amsterdam, and Musikvereinssaal in Vienna, have Reverb times close to 2 seconds, with some, but not much, variation in their early versus late decay times.

The consensus is that smaller rooms should have less reverberation than larger rooms. They typically do anyway because reflections are attenuated each time they bounce off a surface, and sound will have more bounces per second in smaller rooms. Thus the density of reflections will be higher in a small room but sound will decay more rapidly.

The most important factor in determining the optimum Reverb time is the type of music. Modern music is faster and more impulsive than classical music. This requires less Reverb time.

Acoustical absorbers may be used to reduce the Reverb time, but most absorbers work on higher frequencies with very little effect on lower frequencies. Of course there are specialized bass traps that absorb narrow bands of lower frequencies, but their absorption is proportional to their area, so covering a large area (and a wide band of frequencies) will require a lot of them. Contrary to popular opinion, putting one or two bass traps in the corner won't do much.

The right amount of reverberation in a small room is whatever sounds good to the listener, which may vary from one person to the next (along with the type of music). Beware of excessively dead rooms with Reverb times much less than 0.5 seconds which will have a negative effect on sound quality. A possible exception would be a control room for monitoring a recording, where you might want to hear exactly what is being recorded without any pleasant reverberation added. Although, in that case, headphones would be a better solution.

Surface Diffusivity - SurDif

Surface Diffusivity, which I call SurDif, is a property of highly irregular walls. Flat walls have no SurDif. Diffusion is not directly related to absorption, but absorption tends to limit the effects of diffusion. That is, dead surfaces cannot have good diffusion because they absorb most of the sound. The degree of diffusivity is proportional to the amount of surface irregularities, but is inversely proportional to the absorption. Unfortunately, there is no objective way to measure it.

Beranek says that diffusivity is an architectural feature that must not be underestimated. The three most widely praised concert halls in the world have excellent surface diffusivity.

Diffusion is always sighted as very important to good sound. Haan and Fricke say that it is the definitive quantity for concert hall sound, and the following is how they defined the Surface Diffusivity Index:

High Diffusivity (1.0) - coffered or checker-designed with deep recesses or deep beams [greater than 4 in.], or random diffusing elements over the full area [greater than 2 in. in depth], and all of the area must not embody any sound absorbing materials.

Medium Diffusivity (0.5) - angled array of broken surfaces, or ornamentally decorative treatment applied with shallow recesses [less then 2 in. in depth], or flat concrete surfaces behind a semi-acoustically transparent screen with mostly reflective materials.

Low Diffusivity (0.0) - large separate paneling, or smoothly curved surface, or large flat and smooth surface, or semi-acoustically transparent mesh screen, or heavy absorptive treatment applied.

SurDif is the most important factor in small room acoustics, so the higher the diffusivity, the better. There are several diffusors available on the market, but they tend to be quite expensive. Some of them use mathematical formulas to help ensure random frequency reflection distribution. Fortunately, there are some household objects that make great diffusors, like bookshelves!


Irregular surfaces, such as bookshelves, brick fireplaces, cabinets, or manufactured diffusors are the panacea for small room acoustics. They will help your room sound as marvelous as it can. Moderate use of acoustical absorbers may help in some rooms, but as far as placing the absorbers following live end dead end theory, forget about it!

The following ranks the parameters discussed above according to relative importance for room acoustics. However, equally important as perhaps all of these parameters combined, is standing waves, which has been covered extensively by many other authors. I do have a somewhat unique solution for the problems caused by standing waves that I may present in the future.

Harner's Subjective Preference rankings for small rooms

1. SurDif - 40%
2. Reverb - 25%
3. I-ACk - 20%
4. BRat - 10%
5. ITDiG - 5%
6. Loudness - 0%


1. Ludwig, Art, Room Acoustics, etc.,

2. Beranek, Leo, Concert and Opera Halls - How They Sound, 1996 by Acoustical Society of America

3. Everest, F. Alton, Master Handbook of Acoustics - Fourth Edition 2001 by The McGraw-Hill Companies, Inc.

4. Rettinger, Michael, Handbook of Architectural Acoustics and Noise Control 1988 by Blue Ridge Summit

5. Ando, Yoichi, Concert Hall Acoustics, 1985

6. Haan, C. H. and Fricke, F. R., Surface diffusivity as a measure of the acoustic quality of concert halls, 1993

7. Hafter, Ervin, Auditory Perception Lab,

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