Measuring the acoustic properties of porous materials

The artificial porous materials commonly used in industry are simple in structure, although they vary widely. Among these, materials whose porosity is (mainly or entirely) "open" often exhibit interesting sound absorption properties in airborne acoustics [1] , [2] . By definition, "open" porosity includes only porosity connected to the exterior. For example, glass wools, rock wools, felts, open-cell plastic foams and granular materials are made up of a solid skeleton of relatively homogeneous composition, and open porosity that is fully saturated with ambient air. We deal with the acoustic characterization of these materials only.

An incident sound wave induces relative air movements in the material's "pores". These movements are then effectively dissipated by simple internal friction of the air, whose velocities relative to the structure must cancel out at the air/solid skeleton contact surface or "pore surface" S _p : these are the viscous effect losses. Acoustic phenomena also induce small temperature oscillations in the saturating air, which cancel out on contact with the solid (which remains at room temperature). This results in irreversible heat exchange between the air and the solid: thermal effect losses, which are generally lower than the former. Finally, when the solid itself is set in motion, viscoelastic losses occur, linked to internal friction in the solid matrix undergoing deformation (its vibration may result from inertial and viscous actions –– exerted at the S _p walls by air movements, or more directly, from its contact with a plate, screen, etc.). These structural losses can play an important role, particularly in the vicinity of resonances.

In principle, a distinction can be made between intrinsic acoustic properties, on the one hand, and the overall acoustic properties of the treatment, on the other. Thus, in paragraph 1 , the geometric parameters associated with the microgeometry of the porous structures under consideration are introduced and the intrinsic acoustic properties of the materials are deduced within...