Two-sensor power measurements in lossy ducts.

Theory and measurements of the use of two adjacent pressure sensors to measure acoustic power flow of a simple harmonic sound wave in a duct are presented. This theory differs from the usual intensity-times-area formulation of this problem by including the phase shift between pressure gradient and velocity, which is caused by viscous drag on the gas at the duct wall. For high standing-wave ratios, the power obtained by this method differs significantly from the product of mid-duct intensity and duct area. These measurements confirm the method to an accuracy of 5%, even at high amplitudes where the acoustic flow is turbulent and the theory might not necessarily be valid.