Page 228 - The Master Handbook Of Acoustics

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ABSORPTION OF SOUND

Table 9-2 Sound absorption by people (Sabins per person).
Frequency, Hz
125 250 500 1 kHz 2 kHz 4 kHz

College students informally dressed — 2.5 2.9 5.0 5.2 5.0
seated in tablet arm chairs 6
Audience seated, depending on 2.5– 3.5– 4.0– 4.5– 5.0– 4.5–
spacing and upholstery of seats 22 4.0 5.0 5.5 6.5 7.0 7.0

in sound-pressure level affects those most important first reflections
from the sidewalls. All of this apparently results from interference.

Absorption of Sound in Air

For frequencies 1 kHz and above and for very large auditoriums, the
absorption of sound by the air in the space becomes important. For
example, a church seating 2,000 has a volume of about 500,000 cubic
feet.

Frequency (Hz) Absorption (sabins per 1,000 cu ft)
1,000 0.9

2,000 2.3
4,000 7.2

Notice that for 50 percent relative humidity the absorption is 7.2
sabins per 1,000 cubic feet or a total of (500) (7.2) 3,600 sabins at 4
kHz. This is equivalent to 3,600 square feet of perfect absorber.
This could be 20 percent to 25 percent of the total absorption in the
space, and there is nothing that can be done about it other than to take
it into consideration and taking consolation in at least knowing why
the treble reverberation time falls off so much!

Low-Frequency Absorption by Resonance

The concept of wall reflection, graphically portrayed for drapes in Fig.
9-15, applies as well to bass traps. This phrase is applied to many