Page 207 - The Master Handbook Of Acoustics

182 CHAPTER NINE

Reverberation Chamber Method

The reverberation chamber method of determining the absorption
coefficients of absorbing materials automatically measures the average
value. This chamber is a large room with highly reflective walls, ceil-
ing, and floor. The reverberation time of such a room is very long, and
the longer it is, the more accurate the measurement. A standard sam-
ple of the material to be tested, 8 9 ft in size, is laid on the floor and
the reverberation time measured. Comparing this time with the known
reverberation time of the empty room yields the number of absorption
units the sample adds to the room. From this the absorption attributed
to each square foot of material is determined, giving the equivalent of
the absorption coefficient.
This description is a highly simplified view of the reverberation
chamber method. The construction of the chamber is very important to
ensure many modal frequencies and to equalize mode spacing as
much as possible. The position of the sound source and the number
and position of the measuring microphones must be carefully worked
out. It is common to use large rotating vanes to ensure adequate diffu-
sion of sound. All absorption coefficients supplied by manufacturers
for use in architectural acoustic calculations are measured by the
reverberation chamber method.
If the open window is the perfect absorber, what happens if cham-
ber measurements show absorption coefficients greater than 1? This is
a regular occurrence. The diffraction of sound from the edges of the
standard sample makes the sample appear, acoustically, of greater area
than it really is. There is no standard method of making adjustments
for this artifact. Some manufacturers publish the actual measured val-
ues if greater than unity; others arbitrarily adjust the values down to
unity or to 0.99.
Sound absorption coefficients vary with frequency. It is standard
practice to publish coefficients and make calculations for the follow-
ing six frequencies: 125, 250, 500, 1,000, 2,000, and 4,000 Hz.

Impedance Tube Method

The Kundt tube has been applied to the measurement of the absorption
coefficient of materials. Used in this way it is commonly a standing-