Page 212 - The Master Handbook Of Acoustics

187
ABSORPTION OF SOUND

Table 9-1 Mountings commonly used in sound-absorption measurements.

New mounting Old mounting
designation* designation**

A Material directly on hard surface #4
B Material cemented to plasterboard #1
C-20 Material with perforated, expanded or other
open facing furred out 20 mm (3/4″) #5
1
C-40 Ditto, furred out 40 mm (1 2″) #8
3
D-20 Material furred out 20 mm ( 4″) #2
E-405 Material spaced 405 mm (16″) from hard surface #7

*ASTM designation: E 795-83.
**Mountings formerly listed by Acoustical and Board Products Manufacturers Association, ABPMA (for-
merly the Acoustical and Insulating Materials Association, AIMA).
The cooperation of Riverbank Acoustical Laboratory in providing information on current practice is grate-
fully acknowledged.

The mounting 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.
Tables of absorption coefficients should always identify the standard
mounting or include a description of the way the material was
mounted during the measurements, or the coefficients are of little
value. Mounting A with no air space between the sound absorbing
material and the wall is widely used. Another one commonly used is
mounting E-405, which is at least an approximation to the varying
spaces encountered in suspended ceilings (Fig. 9-6).

Mid/High-Frequency Absorption by Porosity

The key word in this discussion of porous sound absorbers is inter-
stices. It is simply the space between two things. If a sound wave
strikes a wad of cotton batting, the sound energy sets the cotton fibers
vibrating. The fiber amplitude will never be as great as the air particle
amplitude of the sound wave because of frictional resistance. Some
sound energy is changed to frictional heat as fibers are set in motion,
restricted as this motion is. The sound penetrates more and more into
the interstices of the cotton, losing more and more energy as more and