Examination of the effect of source location on sound power measurement at low frequencies

Abstract

An analysis of the relationship between source location and measured sound power output of discrete frequency sources in reverberation rooms is presented, based on data obtained during the qualification of the 94 m3 reverberation room at the Centre for Building Studies. Total measurement uncertainty is evaluated at several source positions for both a bare chamber and for the final configuration with a rotating diffuser and low‐frequency absorption. A high total measurement uncertainty at low frequencies proved to be the major obstacle to qualification as has been observed in numerous other studies (see for example the Sept.–Oct. 1976 issue of Noise Control Engineering). The modifications to the room and averaging over some positions reduced the measurement uncertainty to acceptable limits. It was observed that source position averaging was ineffective for the bare chamber configuration as pressure/frequency responses were highly correlated for different source positions. The addition of diffusing elements and low‐frequency absorption increase modal overlap and result in less correlated pressure/frequency responses between source positions and hence improve the averaging effectiveness. [Work supported by NSERC.]