Page 221 - The Master Handbook Of Acoustics

196 CHAPTER NINE

1,722 Hz. The odd multiples of both the
1.0 10-cm and the 20-cm quarter wavelengths
are spotted on the upper part of Fig. 9-16.
0.8 Draped to 1 /2 area when spaced from the wall, and the effect
The absorption of the velour is greater
Absorption coefficient 0.6 Draped to 3 /4 area is greatest in the 250-Hz to 1-kHz region.

At 125 Hz, the 10 and 20 cm spacing adds
Draped to 7 /8 area
practically nothing to the drape absorption
0.4

spacing is 2.26 feet. When referring to
0.2 because at 125 Hz, the quarter wavelength
quarter wavelengths, sine waves are
0
125 250 500 1 kHz 2 kHz 4 kHz inferred. Absorption measurements are
Frequency - Hz invariably made with bands of random
FIGURE 9-14 noise. Hence we must expect the wiggles
of Fig. 9-15B to be averaged out by the use
The effect of draping on the sound absorption of
1
drapes. “Draped to 2 area” means that folds are of such bands.
introduced until the resulting drape area is half that
9
of the straight fabric. (After Mankovsky. )
Carpet as Sound Absorber
Carpet commonly dominates the acoustical picture in spaces as
diverse as living rooms, recording studios, and churches. It is the one
amenity the owner often specifies in advance and the reason is more
often comfort and appearance than acoustic. For example, the owner
of a recording studio with a floor area of 1,000 square feet specified
carpet. He was also interested in a reverberation time of about 0.5 sec-
ond, which requires 1,060 sabins of absorption. At the higher audio
frequencies, this heavy carpet and pad with an absorption coefficient
of around 0.6 gives 600 sabins of absorption at 4 kHz or 57% of the
required absorption for the entire room before the absorption needs of
walls and ceiling are even considered. The acoustical design is almost
frozen before it is started.
There is another, more serious problem. This high absorbance of
carpet is only at the higher audio frequencies. Carpet having an
absorption coefficient of 0.60 at 4 kHz offers only 0.05 at 125 Hz. In
other words, the 1,000 sq ft of carpet introduces 600 sabins at 4 kHz
but only 50 sabins at 125 Hz! This is a major problem encountered in
many acoustical treatment jobs. The unbalanced absorption of carpet