Page 220 - The Master Handbook Of Acoustics
P. 220
195
ABSORPTION OF SOUND
and if we carry it further than indicated in Fig. 9-15A at each odd mul-
tiple of quarter wavelengths. Absorption minima occur at even multi-
ples of quarter wavelengths.
These effects are explained by reflections of the sound from the
solid wall. At the wall surface, pressure
will be highest, but air particle velocity
1.0
will be zero because the sound waves can-
not supply enough energy to shake the 0.8 4"
wall. At a quarter wavelength from the
wall, however, pressure is zero, and air 0.6 3"
particle velocity is maximum. By placing 2"
the porous material, such as a drape, a Absorption coefficient 0.4
quarter wavelength from the wall, it will
have maximum absorbing effect because 0.2
the particle velocity is maximum. At half
wavelength, particle velocity is at a mini- 0
125 250 500 1 kHz 2 kHz 4 kHz
mum, hence absorption is minimum.
Frequency - Hz
In Fig. 9-15B, the spacing of the drape
from the wall is held constant at 12 inches FIGURE 9-12
as the absorption is measured at different Absorption coefficients of Sonex contoured foam of
4
frequencies. The same variation of absorp- various thicknesses.
tion is observed, maximum when the spac-
ing is at odd quarter wavelengths and
1.0
minimum at even quarter wavelengths
from the wall. At this particular spacing of
12 inches, a wavelength of spacing occurs 0.8
at 1,130/1 1,130 Hz, a quarter wave-
length at 276 Hz, and a half wavelength at 0.6
565 Hz, etc. Absorption coefficient 18 oz/sq yd
Figure 9-16 shows actual reverberation- 0.4 14 oz/sq yd
chamber measurements of the absorption 10 oz/sq yd
of 19 oz/sq yd velour. The solid graph is 0.2
presumably for a drape well removed from 0
all walls. The other graphs, very close 125 250 500 1 kHz 2 kHz 4 kHz
together, are for the same material spaced Frequency - Hz
10 cm (about 4″) and 20 cm (about 8″) from FIGURE 9-13
the wall. The 10-cm distance is one wave- The sound absorption of velour hung straight for
18
length at 3,444 Hz, the 20-cm distance is at three different weights of fabric. (After Beranek. )