Page 227 - The Master Handbook Of Acoustics

202 CHAPTER NINE

1.0

0.8

Absorption coefficient 0.6 C

0.4
D

A
0.2 B

0
125 250 500 1 kHz 2 kHz 4 kHz
Frequency - Hz
FIGURE 9-19

1
Carpet absorption coefficients from a commonly used table. (A) 8″ pile height. (B)
1 3 5
4″ pile height. (C) 16″ combined pile and foam. (D) 16″ combined pile and foam.
Compare these graphs with those of Figs. 9-16 and 9-17. 6

For 1 kHz and higher, the absorption offered by college students in
informal attire in the Spartan furnishings of a classroom falls at the
lower edge of the range of a more average audience. The low-frequency
absorption of the students, however, is considerably lower than that of
the more formally dressed people.
Sound propagated across rows of people, as in an auditorium or
music hall, is subjected to an unusual type of attenuation. In addition
to the normal decrease in sound with distance from the stage, there is
an additional dip of up to 15 or 20 dB around 150 Hz and spreading
7
over the 100- to 400-Hz region. In fact, this is not strictly an audience
effect because it prevails even when the seats are empty. A similar dip