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

Sound propagation in rigid-frame porous materials is governed by the effective density and the effective bulk modulus of the fluid in the pore space. These quantities, for which analytical expressions have already been derived by other researchers, are frequency-dependent, complex and non-linear. However, because of the complexity of these expressions, it is difficult to obtain physical insight into the acoustic behaviour of the porous materials and to determine the dominant mechanism for sound absorption for a given material at a given frequency. Alternatively there are very simple expressions. In this paper the relationships between the complicated and relatively simple models are studied, and simple non-dimensional expressions for the characteristic 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 is presented, and a simple rule of thumb is given for the required flow resistivity, porosity and tortuosity for a given thickness of porous material.

[1]  V. Tarnow.  Compressibility of air in fibrous materials , 1996 .

[2]  Viggo Tarnow,et al.  Airflow resistivity of models of fibrous acoustic materials , 1996 .

[3]  Joel Koplik,et al.  Theory of dynamic permeability and tortuosity in fluid-saturated porous media , 1987, Journal of Fluid Mechanics.

[4]  Denis Lafarge,et al.  Dynamic compressibility of air in porous structures at audible frequencies , 1997 .

[5]  K. Attenborough.  Acoustical characteristics of rigid fibrous absorbents and granular materials , 1983 .

[6]  E. N. Bazley,et al.  Acoustical properties of fibrous absorbent materials , 1970 .

[7]  J. F. Allard,et al.  Propagation of sound in porous media , 1993 .

[8]  J. Allard,et al.  Sound propagation in air-saturated random packings of beads , 1998 .

[9]  Morgan,et al.  Drag forces of porous-medium acoustics. , 1993, Physical review. B, Condensed matter.

[10]  Maurice A. Biot,et al.  Generalized Theory of Acoustic Propagation in Porous Dissipative Media , 1962 .

[11]  M. Biot.  Theory of Propagation of Elastic Waves in a Fluid-Saturated Porous Solid. II. Higher Frequency Range , 1956 .

[12]  D. L. Johnson,et al.  The equivalence of quasistatic flow in fluid‐saturated porous media and Biot’s slow wave in the limit of zero frequency , 1981 .

[13]  L. R. Quartararo,et al.  Noise and Vibration Control Engineering: Principles and Applications , 1992 .

[14]  M. Biot.  Theory of Propagation of Elastic Waves in a Fluid‐Saturated Porous Solid. I. Low‐Frequency Range , 1956 .

[15]  Viggo Tarnow.  Calculation of the dynamic air flow resistivity of fiber materials , 1997 .