Acoustical measurement of the trapping constant of porous materials

A simple method of measuring the trapping constant of porous materials saturated by air is presented. The trapping constant is evaluated from acoustical measurements of the imaginary part of the compressibility of the saturating air at low frequencies. Measurements performed on random packings of spheres are compared with predictions obtained from numerical simulations. Conversely, the connection between the behavior of the compressibility and a well defined physical parameter provides new information about the efficiency of porous sound absorbing materials at low frequencies.