An investigation of the measurement of transmission loss

Abstract

The determination, by means of the two reverberant room method, of the transmission loss of apertures of even very large size is shown to be practical and a new data reduction procedure is presented. The new procedure requires the measurement of sound pressure level differences in both directions and measurements of the Sabine absorption in each room. The use of apertures of large size as effective standard partitions of zero transmission loss is then possible and has allowed the investigation of the measurement technique. Theoretical analysis and measurements on nine different apertures in four different testing facilities have provided the data upon which the following conclusions are based. 1. 1. The ordinary way of determining transmission loss, for example, as stated in ISO DIS 140 may be used with reasonable accuracy for partitions providing a level difference of 10 dB or greater in each direction. However, if the sum of the level differences in each direction is less than 20 dB then consideration must be given to cross-coupling between the rooms during the course of the receiving room calibration procedure. 2. 2. For partitions and apertures which provide level differences of less than 10 dB the transmission loss may be determined by using a new modified procedure based on measurements of level differences in each direction and either (a) initial reverberation times in each room or (b) measurements of the injected acoustic power in each room during the level difference measurements. 3. 3. Experimental data suggest that for satisfactory measurement of transmission loss by means of the two reverberant rooms technique the following conditions should be satisfied: (a) the modal density in all frequency ranges should not be less than half of the reverberation time; (b) the reverberation time should not be less than 550 divided by the measurement band centre frequency; (c) the Sabine absorption should be less than 6% of the total surface area of the receiving room.