Determination of sound power levels using sound pressure: The uncertainties related with the measurement surface and the number of microphones

Abstract

The ISO 3740 series of standards allows the sound power levels emitted by a machine to be calculated by measuring the sound pressure levels with a limited number of microphones distributed over the surface enveloping the machine. This series of seven standards is designed for various environments and achieves three levels of accuracy. The A-weighted expanded uncertainties associated with these standards vary from 1 dB(A) to 8 dB(A). Till now, they have been obtained experimentally by using inter-laboratory measurements (reproducibility uncertainty). These uncertainties are generally overestimated. Calculation of these uncertainties would make it possible to reduce their current values. It is then necessary to determine the uncertainties related to each parameter affecting the measurement (measuring instrumentation, operating conditions, environmental correction, meteorological conditions, the finite number of microphones, the angle error, the impedance error, etc.). This paper shows the results of the calculation of the uncertainties related to the limited number of microphones, the angle error and the impedance error in the case of a free field above a reflecting plane with a hemispherical measurement surface. These uncertainties are calculated as a function of a descriptor describing the complexity of the acoustic field. The reduction of each individual uncertainty is then possible by changing several parameters of the experiment (number of microphones, distance between the measurement surface and the source, shape of the measurement surface).