Measuring the Sound Power of a Reference Source using Sound Intensity

Sound intensity is the time average of sound-power flow per unit area in watts/m². It is generally measured using sound pressure at two closely-spaced microphones. It is commonly believed that it is not possible to measure sound intensity in a reverberation room because multiple reflections in the room create a diffuse pressure field which makes such measurements inaccurate. However there has to be a net flow of sound power in the room from speakers (or other sources) which then passes out through the walls, ceiling and floor. Hence net sound power flow (sound intensity) should be measurable. In a previous paper [1]^* it was shown that it is possible to measure the sound power of a reference source accurately in three different reverberation rooms using sound-intensity measurement. Accurate measurements were also made in other work spaces. In this paper the sound-intensity method is compared with the method used by the manufacturer to calibrate a reference source. The calibration procedure uses the far field approximation for intensity in a semi-anechoic room. Results for the sound-intensity method in different work areas were found to be consistently less, by a few percent, than the manufacturer's calibrated value. Possible reasons for this are considered. It is suggested that manufacturers should use sound intensity measurement to calibrate reference sources instead of the far field approximation.