Page 270 - Master Handbook of Acoustics

human absorption and how to use it in calculations. One method uses the surface area over which an
audience is seated. Or, we may simply consider the number of people present. In any case, the
absorption units (sabins) due to people must be considered at each frequency and then added to the
sabins of the carpet, drapes, and other absorbers in the room at each frequency. Table 12-2 lists the

absorption per person of informally dressed college students in a classroom along with a range of
absorption for more formally dressed people in an auditorium environment.

TABLE 12-2 Sound Absorption by People (Sabins per Person)

For 1 kHz and higher, the absorption offered by college students in informal attire in the
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. The rule of thumb employed by some acousticians simply attributes 5 sabins at 500
Hz, per seated person.
Sound propagated across rows of people, as in an auditorium or concert 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 as much as 15 to 20 dB around 150 Hz and spreading 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 in sound-pressure level affects important first reflections from the side
walls. This apparently results from interference. The angle of incidence also plays a role. When an
audience is seated on a relatively flat floor, the angle of sound incidence is low and there is greater
absorption. With higher angle of incidence (e.g., in stadium seating), there is less absorption.

Sound Absorption in Air

For frequencies of about 2 kHz and above and for large auditoriums, the absorption of sound by the
air in the space becomes important. Air absorption could account for 20 to 25% of the total

absorption in the space, a significant factor. Air absorption can be estimated from:

where m =
3
air attenuation coefficient, sabins/ft or sabins/m 3
V =