Acoustic properties of rigid-frame porous materials - an engineering perspective

Sound propagation in rigid-frame porous materials is governed by the effect ive density and the effective bulk modulus of the fluid in the pore space. These quantities, for which analytical expressions have already been derive d by other researchers. are frequency-dependent, complex and non-linear. Ho wever, because of the complexity of these expressions, it is difficult to o btain physical insight into the acoustic behaviour of the porous materials and to determine the dominant mechanism for sound absorption For a given ma terial at a given frequency. Alternatively there are very simple expression s. In this paper the relationships between the complicated and relatively s imple models are studied, and simple non-dimensional expressions for the ch aracteristic impedance and wavenumber For sound propagation in rigid-frame porous materials are derived using the concepts of acoustic mass, stiffness and damping. An upper bound For thermal losses in a rigid-frame material i s presented, and a simple rule of thumb is given for the required flow resi stivity, porosity and tortuosity for a given thickness of porous material. (C) 2001 Elsevier Science Ltd. Ail rights reserved.