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

Abstract

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 [Formula: see text] and thickness [Formula: see text], have been tested. The proposed method provides accurate and reliable results when the lumped element hypothesis is satisfied [for [Formula: see text]] and the radiation impedance is negligible (for [Formula: see text], where [Formula: see text] is the wavenumber in air and [Formula: see text] 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.