Page 249 - Master Handbook of Acoustics

TABLE 12-1 Mountings Commonly Used in Sound-Absorption Measurements

The mounting method has a major effect on the absorption characteristics of the material. For
example, the absorption of porous materials is much greater with an airspace between the material

and the wall. The quarter-wavelength (λ/4) rule dictates that a porous absorber for normal incidence
must be at least a quarter-wavelength thick at the frequency of interest. For example, for a frequency
of 1 kHz, the minimum absorber thickness should be about 3.4 in. Tables of absorption coefficients
should always identify the standard mounting or include a description of the way the material was
mounted during the measurements. Otherwise, the coefficients are of little value. Mounting type A
with no airspace between the sound absorbing material and the wall is widely used. Another method
commonly used is mounting type E-400, which is an approximation to the varying spaces encountered

in suspended ceilings, as shown in Fig. 12-6.

FIGURE 12-6 Commonly used standard mountings associated with listings of absorption
coefficients. With mounting A the material is flat against the backing. Mounting E-400 applies to