A method for ultrasonic characterization of plastic foams by changing the static 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 theoretically, that the real part of squared acoustical refractive index and logarithm of the transmission coefficient depend linearly on the inverse of the square root of applied static pressure. These linear relations provide a simple and convenient way to determine experimentally the constitutive parameters. The method is illustrated with industrial open-cell foams. Advantages, limitations, and perspectives are discussed.