The Acoustic Wattmeter, an Instrument for Measuring Sound Energy Flow

Abstract

An instrument called an “acoustic wattmeter” has been constructed to measure sound energy flow. It consists of a crystal pressure microphone and miniature ribbon velocity microphone mounted close together and connected through separate amplifiers and phase equalizing networks to a thermo-couple-type audiofrequency wattmeter. Provision is made to measure sound pressure and velocity separately as well as the energy flow represented by their product. The normal acoustic impedance and absorption coefficient of a surface may be computed front measurements of sound energy flow into the surface and energy density near the surface. Measurements were made on two representative absorbing materials at normal incidence with the material mounted at the end of a tube, and at random incidence with the material on the floor of a large sound chamber. Referring all results to random incidence, the values of the absorption coefficients obtained by the two methods agree well with each other and are in fair agreement with results obtained by the reverberation room method.