Phase gradient method of measuring the acoustic impedance of materials

A free-field method using a point source has been developed to measure the normal specific impedance of porous materials. The phase gradient is measured as a function of distance along the normal to the materials passing through the point source. The phase gradient is obtained from a two-microphone probe. The phase and amplitude of the complex reflection coefficient are used as constants adjusted for the best fit to experimental results using a standard optimization technique. Results are obtained on layers of fiberglass and polyurethane foam in the frequency range between 250 and 400 Hz. Good agreement is found when the measured results are compared to the predictions of Delany and Bazley [Appl. Acoust. 3, 105–116 (1970)]. The method has the advantages of requiring a minimum of instrumentation and is particularly adapted to measurements at low frequencies.