Page 246 - Master Handbook of Acoustics

FIGURE 12-3 A graph for interpreting the standing-wave ratio in terms of the absorption coefficient.
The standing-wave ratio can be found by dividing any pressure maximum by its adjacent pressure
minimum (see Fig. 12-2).

The advantages of the standing-wave tube method are offset by the disadvantage that the
absorption coefficient so determined is accurate only for normal incidence. In practice, sound
impinges on the surface of a material at all angles. Figure 12-4 is a graph for approximately obtaining

the random-incidence absorption coefficient from the normal-incidence absorption coefficient as
measured by the standing-wave method. The random-incidence coefficients are always higher than
normal-incidence coefficients.