The Acoustic Wattmeter, an Instrument for Measuring Sound Energy Flow

An acoustic wattmeter has been constructed to measure sound energy flow. It consists of a crystal pressure microphone and a miniature ribbon velocity microphone mounted close together and connected through separate amplifiers and phase equalizing networks to a thermocouple-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 mobility and absorption coefficient of a surface may be computed from measurements of sound energy flow into the surface and sound energy density at the surface. Measurements were made on samples of hairfelt and Type B Acousti-Celotex 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-time method.