Ultrasonic characterization of plastic foams via measurements with static pressure variations

A method for ultrasonic characterization of plastic foams by changing the s tatic pressure of air that saturates the foam has been proposed. The method is based on high frequency asymptotic expressions of the standard Johnson- Allard equivalent fluid model. It is shown, both experimentally and theoret ically, that the real part of squared acoustical refractive index and logar ithm of the transmission coefficient depend linearly on the inverse of the square root of applied static pressure. These linear relations provide a si mple and convenient way to determine experimentally the constitutive parame ters. The method is illustrated with industrial open-cell foams. Advantages , limitations, and perspectives are discussed. (C) 1999 American Institute of Physics. [S0003-6951(99)04721-X].