Low frequency acoustic method to measure the complex density of porous materials.

In this paper, a new acoustic lumped element-based method to measure the complex density of porous materials at low frequency is provided. Based on the electroacoustic analogy of wave propagation inside a porous medium, an analytical derivation of the measurement method is given. Measurements can be performed in a short open tube and by placing the tested sample between an acoustic source (i.e., a loudspeaker) and the open-end section by using two microphones. To validate the methodology, different typologies of porous materials, characterized by different values of the complex wave number k ̃ and thickness d, have been tested. The proposed method provides accurate and reliable results when the lumped element hypothesis is satisfied [for abs k ̃ d < 0.5] and the radiation impedance is negligible (for _k R < 0.5, where _k is the wavenumber in air and R is the radius of the tube in which the measurements are carried out). It is shown that, by means of the measurements in two configurations with (full) and without (empty) the tested material, the accurate evaluation of the radiation impedance at the open-end section are not required.