Improved measurement of turbine-generator sound levels in semi-reverberant spaces with directional microphones

Abstract

The sound field in a turbine-generator hall approximates that of a reverberant field. Accurate measurements of the direct machine sound levels cannot be made except very close to the machines (i.e., in the nearfield) because of the reverberant sound and other equipment noises. The potential of directional microphones to reduce the influence of these noises on the sound-level measurements of turbine-generators was examined from a theoretical and experimental point of view. In comparison with an omnidirectional microphone, the theory shows that a directional microphone essentially “extends” the direct field of a source in a reverberant environment to greater distances. Thus, at a given location from a machine, a more accurate measurement can be made with a directional than with an omnidirectional microphone. The magnitude of the improvement is a function of the source directivity, the microphone directivity, the turbine hall room absorption and the distance from the source. Because of its directivity, a directional microphone could also be used to improve measurements of the relative noise contributions of several closely spaced sources. Comparative field measurements in turbine-generator halls with omnidirectional and directional microphones are presented. Comparisons between the theory and the measurements are presented and the resulting implications are discussed.