Handbook for Sound Engineers

98 Chapter 5

offering guidance), many excellent porous and resonant
absorbers are available commercially. Fundamental
information about the design of absorbers is included
here for two reasons: there may be those who want to
build their own absorbers, and more importantly, these
absorbers are sometimes inadvertently constructed in the
process of building rooms. This is especially true of
resonant absorbers.

5.2.1 Absorption Testing

Standardized testing of absorption began with Sabine and
continues to be developed and improved upon in the
present day. As mentioned above, the two standardized
methods for measuring absorption are the reverberation
chamber method and the impedance tube method. One
can also measure absorption in the field by using either
the standardized methods or the other techniques
discussed below.

5.2.1.1 Reverberation Chamber Method

The work of Sabine during the late 19th and early 20th

centuries is echoed in the present-day standard methods

for measuring absorption in a reverberation chamber:

ASTM C423 and ISO 354.5,6 In both methods, the

general technique involves placing a sample of the mate-

rial to be tested in a reverberation chamber. This is a

chamber that has no absorption whatsoever. The rate of

sound decay of the room is measured with the sample in

place and compared to the rate of sound decay of the

empty room. The absorption of the sample is then

calculated.

The method of mounting the sample in the test

chamber has an effect on the resulting absorption. Thus,

standardized methods for mounting are provided.6,7 The

most common mounting methods employed are Types

A, B, and E. Type A simply involves placing the test

sample—usually a board-type wall or ceiling

absorber—flat against the predefined test area in the

chamber (typically on the floor). Type B mounting is

Figure 5-1. Comparison of absorption reflection and diffusion.

Acoustical

treatment

Absorption

Reflection

Diffusion

Temporal response Spatial response