The Listening Room
BAS Volume 1 Issue 06, Published March 1973

  Roy Allison gave an in-depth discussion (replete with equations and graphs) of the most-ignored component in everybody's stereo system, the component that interposes itself between your speakers and your ears: the listening room. The analysis of room acoustics involves three topics: resonances, absorption, and the reverberant field.

Resonances are a problem because any chamber with parallel surfaces behaves like an organ pipe: pressure waves ("sound") will bounce back-and-forth, and at certain frequencies each reflected wave will be in phase with the next cycle of sound being produced by the speaker and so will be reinforced. Consequently those frequencies will tend to build up to excessive strength in certain areas of the room. The corresponding irregularity in heard low-frequency response is a significant deficiency in all home music systems. A clear discussion of how the room resonances are related to room dimensions and shape, and notes on ways of minimizing the effect of the resonances on the sound, are contained in the attached article by Mr. Allison. (Copies of the article were obtained from A.R. for the BAS by Al Southwick.)

Absorption is important for two reasons. (1) Low-frequency absorption tends to "damp out" and broaden the resonances, thus smoothing the low-end response of the room. Wood paneling with an air space behind it (typical in residential construction) is one of the few materials with a relatively high and controllable absorption coefficient in the bass range. However, the desirable smoothing of the low-frequency resonances is obtained at the price of subtracting bass energy from the room and either absorbing it or transmitting it to adjacent rooms. So rooms not built of brick or concrete may, if the paneling is too light or the studs too far apart, tend to have a low frequency roll-off. Indeed, measurements have shown that in typical rooms, excessive flexing of floors and walls removes considerable bass energy. A graph of the measured response of 8 home listening rooms in the Boston area exhibited a bass roll-off — whereas comparable measurements of concert halls show a low-end rise. So you needn't feel embarrassed by the felt need to use amplifier or equalizer bass boost in your system.

(2) The absorptive characteristics of the room have a major effect on the tonal balance of the reproduced sound. What is desired is that the room have an overall average absorption coefficient of about 0.15 or 0.20 — in other words that sound striking the average area of floor, ceiling, wall, or object in the room will be 20% absorbed (give or take a few percent), with the other 80% reflected. A room with entirely low-absorption surfaces such as tile, glass, porcelain (e.g. a tiled bathroom) will make the music blare, bright and hard. Conversely a room filled with thick rugs, upholstery, people, and acoustical tile (which provides no useful damping of bass resonances but has very high absorption at middle and high frequencies) will make the music sound confined. Note that human bodies are efficient absorbers; if your system is adjusted to sound great when only you are listening, it will sound dull when you demonstrate it to la friends. The same problem afflicts most showrooms. So an acoustically varied mixture of room boundaries and contents is desirable in order to approach the preferred average absorption value and an absorption curve that is smooth over the audio range. Incidentally, even the air can affect the sound: the higher the relative humidity, the better the high-frequency propagation. The air rolls off the highs and dry air does it drastically — an effect more important in the concert hall than at home since the loss increases with air-path distance. (So for the most natural orchestral sound the acoustic power output of your speakers should slope gradually downward in the treble to simulate the average propagation loss in the concert hall.)

The frequency response of the reverberant field of a listening room depends on how the reverberation time varies with frequency. The reverberation time at any frequency (how long it takes sound reflecting around the room to die down by 60 dB — typically a tenth of a second at home and one or two seconds in Symphony Hall) obviously depends on the room's average absorption coefficient: if the room's surfaces and contents absorb efficiently the sound dies down very quickly. So the variation of the room's absorption with frequency determines the variation of reverberation time with frequency, which in turn affects the tonal balance of the reverberant field. This is important because, hopefully, you do most of your listening in the dominantly reverberant field of your speakers, and so the tonal balance of the music reaching your ears will depend on the reverberant-field response. It is desirable to do your listening in the dominantly reverberant field because there both the sound-pressure level (i.e., loudness) and the spectral balance (subjective frequency response) of the music will remain approximately constant as you move about. By contrast, in the dominantly direct field of a speaker, the sound becomes weaker as you move away and — more important — diffraction and interference effects in all practical speakers  will cause peaks and valleys in the frequency response which will vary with the listener's position. The total power response (integrated over all directions  from any speaker) is much smoother than the direct-field frequency response at any one location with respect to the speaker. This difference is audible, and it can affect listener fatigue. How can you be sure that you are in the reverberant field? There are three factors: distance, dispersion, and absorption.

Don't sit too close to the speakers, or you will surely be in the  dominantly direct field. As you move away from the speakers you will pass from the direct to the reverberant field.

The distance of the transition point depends on the ream's reverberation time, so establish moderately live room acoustics. The more "live" and reverberant the room, the closer to the speaker the transition occurs. In a "dead" room, you may still be in the dominantly direct field at the opposite end of the room.

Use wide-dispersion speakers. The wider the dispersion, the closer to the speaker the direct/reverberant transition occurs. So one reason why wide dispersion speakers sound better is that it's easier to get into the reverberant field. Mr. Allison indicated that with good modern speakers in rooms of average liveness, the dominantly reverberant field begins 4 to 8 feet from the speaker at most frequencies. The more directive the speaker, the farther back you have to go to get out of the dominantly direct field. It may be worth noting that listening in the dominantly reverberant field need not mean the loss of a clear and specific stereo image and its replacement by a vague sense of "bigness" to the sound. In the reverberant field there is still some sound arriving at your ears direct from the speaker; it gets to you first, and the reverberant-field sound arrives milliseconds later. It is the first-arrival sound which primarily determines your sense of stereo imagery, while the reverberant sound primarily determines your sense of the tonal balance in the music.

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