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, ceiling, and floor. The reverberation time of such a room is very long, and the longer it is, the more accurate the measurement. A standard sample 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 diffusion 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 chamber 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...