ACOUSTIC LUMPED ELEMENT TECHNIQUES TO MEASURE THE LOW FREQUENCY RESPONSE OF POROUS MATERIALS

The acoustic experimental characterization of the dynamic properties of a porous material is generally carried out through the standard three- and four- microphones techniques. These methods are wave decomposition based and allow to assess the transfer matrix of the specimen. Recently, two innovative acoustic measurement methodologies based on the lumped element approximation have been developed. It has been demonstrated their potentiality to overcome the difficulties of the standard techniques in the low frequency range. The bulk modulus measurement setup relies for a rigid-end back to the material, whereas the complex density measurements require an open-end. Furthermore, for each test two measures are needed: one with the sample, named full, and one where only air fulfil the volume, named empty. The empty test allows to consider any imperfect rigid condition in the bulk modulus measurement, and the effect of the radiation impedance on the open-end in the complex density. In particular, the measures of the viscous and the thermal dynamic properties of a tested sample are obtained from the acoustic pressures picked up by two microphones: one placed in the cavity behind the loudspeaker, and one in the cavity where the sample is placed.