ABSTRACT
As discussed previously in Chapters 5 and 6, the characterization of the absorption properties of materials is essential for acousticians. In room acoustics there are two absorption coefficients that are commonly used: the Sabine absorption coefficient and the random incidence absorption coefficient. The latter is obtained by measuring the absorption coefficient of a specimen at angles ranging from normal to grazing, then integrating to obtain a single averaged value. The method for measuring the Sabine absorption coefficient of a specimen using a reverberation room is given in ASTM C423 [40] and ISO 35-2003 [85]. The process involves exciting the room using either an impulsive source
or steady broadband source at several locations not immediately adjacent to any walls, absorbing surface, or the receiver microphones. The sound source is abruptly terminated (for a steady source) and the sound in the room is left to decay. Several microphones are used to measure the sound pressure level, from which the reverberation time is determined. The reverberation time is defined as the time it takes for the sound pressure level to decay by 60 dB. The Sabine absorption coefficient is proportional to the reverberation time, the details of which are presented in Section 7.3. For a more detailed explanation of measurement of the sound absorption coefficient in diffuse fields the reader is referred to Beranek [45, Chapter 9, Sound in large rooms], Bies and Hansen [47, Chapter 7, Sound in enclosed spaces] or the standards ASTM C423 [40] and ISO 35-2003 [85].
