Methods of measurement of acoustic noise radiated by an electric machine

The acoustic field of an electric machine is described, and the basic measuring techniques and instrumentation for measuring noise are described. The results of sound-pressure measurements in free-field and semireverberant-field conditions, and measurements of sound pressure above a reflecting plane with various shapes of measuring surface are presented and discussed. From these measurements, the total sound power is calculated. The errors involved in using different numbers of sound-pressure sampling points in the power calculation, and of using approximate calculating procedures, are also investigated. Evidence is presented concerning the accuracy of methods of measurement and calculation.