Measurement of the Transmission Loss of a Finite-Depth Aperture

Abstract

To provide experimental verification of an approximate solution for the diffraction of sound waves by a circular aperture in a plane rigid wall of finite thickness, a series of experiments was conducted on the transmission of reverberant sound through steel pipes inserted individually into a thick wall between two reverberation chambers. The usually recommended methods of evaluating the transmission loss proved inadequate because long, narrow apertures have sharp resonant peaks. By electronically averaging and equalizing the outputs of four microphones in each chamber and using a slowly sweeping sine wave with a small super-imposed warble, it was possible to record continuous plots of the aperture transmission loss with high resolution of frequency and amplitude. Experimental accuracy of 1 dB was possible over most of the frequency range of 300–12 000 cps, diverging to 2 dB at the frequency extremes, and for apertures with sharp resonances, diverging to about 4 dB at the peaks. The accuracy was limited by reading the “average” of the recorded fluctuating sound level, which was most difficult at resonant peaks. For larger-diameter apertures, which gave data relatively uninfluenced by flanking sound, the agreement with theoretical results was within 1 dB over most of the frequency range.