Page 180 - Master Handbook of Acoustics

Applying all the absorbent in a room on one or two surfaces does not result in a diffuse condition, nor
is the absorbent used most effectively. Consider the results of an experiment showing the effect of
distributing the absorbent. The experimental room is approximately a 10-ft cube and the room was
tiled (certainly not ideal for a recording or listening room, but acceptable for this experiment). For

test 1, reverberation time for the bare room was measured and found to be 1.65 seconds at 2 kHz. For
2
test 2, a common commercial absorber was applied to 65% of one wall (65 ft ), and the reverberation
time at the same frequency was found to be about 1.02 seconds. For test 3, the same area of absorber
was divided into four sections, one piece mounted on each of four of the room’s six surfaces (one on
each of three walls and one piece on the floor). This decreased the reverberation time to about 0.55
second.

The area of the absorber was identical between tests 2 and 3; the only difference was that in test 3
it was in four pieces. By the simple expedient of dividing the absorbent and distributing it, the
reverberation time was cut almost in half. Inserting the values of reverberation time of 1.02 and 0.55
second and the volume and area of the room into the Sabine equation (see Chap. 11), we find that the
average absorption coefficient of the room increased from 0.08 to 0.15 and the number of absorption

units from 48 to 89 sabins. This extra absorption is due to an edge effect related to diffraction of
sound that makes a given sample appear to be much larger acoustically. Stated another way, the
2
2
sound-absorbing efficiency of 65 ft of absorbing material is only about half that of four 16-ft pieces
distributed about the room, and the edges of the four pieces total about twice that of the single 65-ft 2
piece. Therefore, one advantage of distributing the absorbent in a room is that its sound-absorbing
efficiency is greatly increased, at least at certain frequencies. The preceding statements are true for 2
kHz, but at 700 Hz and 8 kHz, the difference between one large piece and four distributed pieces is
small.
Another significant result of distributing the absorbent is that it contributes to diffusion of sound.

Patches of absorbent with reflective walls showing between the patches have the effect of altering
wavefronts, which improves diffusion. For example, sound-absorbing modules placed along a wall
distribute the absorbing material and simultaneously contribute to the diffusion of sound.

Concave Surfaces

A concave surface such as that in Fig. 9-6A tends to focus sound energy and consequently should be
avoided because focusing is the opposite of the diffusion we normally seek. The radius of curvature
determines the focal distance; the flatter the concave surface, the greater the distance at which sound
is concentrated. Such surfaces often cause problems in microphone placement. Concave surfaces
might produce some awe-inspiring effects in a whispering gallery, but they are to be avoided in

listening rooms and small studios.