The Effect of Room Absorption on the Sound Power Output of the Ilg Reference Sound Source in a Reverberant Room

Abstract

Previous work by others indicated differences in the sound power output of the Ilg depending upon the measurement techniques used. An experiment was performed in which the sound power output of the Ilg reference sound source (RSS) was determined from measurements of the space-time average of the pressure and the reverberation time as the room absorption was progressively increased by adding up to 14 panels (2ft×4ft×3 in.) of Owens Corning type 703 fiberglass. The results compared favorably with free-field measurements over a reflecting plane for low values of absorption and high frequencies but diverged for high values of absorption and low frequencies. Differences due to absorption were traced to using the Norris-Eyring equation for calculating absorption from the reverberation time. It is shown that use of a modified Sabine equation will yield the correct relationship between the absorption and reverberation time. Differences at low frequency disappear if the spatial pressure in the center of the room is corrected to the spatial pressure throughout the room as suggested by Waterhouse [J. Acoust. Soc. Amer. 27, 247 (1955)]. It is concluded that the sound power output of the RSS is independent of room absorption.