Page 243 - Master Handbook of Acoustics

2
window would have an absorption coefficient of 1.0 and by definition; a 1-ft open window provides
1 sabin of sound absorption. Ten square feet of open window would thus give 10 sabins of
2
absorption. As another example, suppose that carpet has an absorption coefficient of 0.55; 20 ft of
carpet would therefore provide 11 sabins of absorption.
2
Either sabins or metric sabins can be used. A metric sabin is the absorption from a 1-m open
2
2
window. Since 1 m equals 10.76 ft , 1 metric sabin equals 10.76 ft sabins. Or, 1 sabin = 0.093
metric sabin.
When calculating a room’s total absorption, all the materials in the room, according to their area,
will contribute to the total absorption:

where S , S , S … =
1
3
2
2
surface areas, ft or m 2
α , α , α … =
3
2
1
respective absorption coefficients
Moreover, the average absorption coefficient can be calculated by dividing the total absorption by
the total surface area:

When absorptive material is placed over a surface, one must take into account the loss of
absorption provided by the original surface. The net increase in absorption over an area is the
absorption coefficient of the new material minus the coefficient of the original material.

The absorption coefficient of a material varies with frequency. Coefficients are typically
published at the six standard frequencies of 125; 250; 500; 1,000; 2,000; and 4,000 Hz. In some
cases, a material’s absorption is given as a single-number rating known as the noise reduction
coefficient (NRC). The NRC is the average of the coefficients for 250; 500; 1,000; and 2,000 Hz (125

and 4,000 Hz are not used). It is important to remember that the NRC is an average value, and also
only accounts for absorption at middle frequencies. NRC is therefore most useful for speech
applications. When considering wider-band music, individual coefficients, at a wider range of
frequencies, should be used.
In some cases, the sound absorption average (SAA) is used to specify absorption. Like the NRC,
the SAA is an arithmetic average, but the SAA uses absorption coefficients from twelve 1/3-octave

bands from 200 Hz to 2.5 kHz. These coefficients are averaged to obtain the SAA value. Finally, the
ISO 11654 standard specifies a single-number weighted sound absorption coefficient for materials
using the ISO 354 testing standard.

Reverberation Chamber Method

The reverberation chamber method can be used to determine the absorption coefficients of absorbing

3
materials; it measures the average value. This chamber is a large room (perhaps 9,000 ft ) with highly
reflective walls, floor, and ceiling. The reverberation time of the room is very long, and the longer it