- Bliss, D.B., Study of bulk reacting porous sound absorbers and a new boundary condition for thin porous layers,
J. Acoust. Soc. Am. (USA), vol. 71 no. 3
pp. 533 - 45 .
(last updated on 2007/04/06)
The effect of bulk reaction on the sound absorption properties of porous materials is studied theoretically and experimentally. Particular attention is focused on the important case of a layer of porous material attached to a rigid wall. In general, it is found that the surface of the porous material can be accurately represented by its normal incidence impedance only if a nondimensional flow resistance parameter is large, i.e. only under this condition is the bulk reaction effect negligible. A new boundary condition is proposed which accounts for the effect of bulk reaction when the thickness of the sound absorbing layer is smaller than the acoustic wavelength. The boundary condition relates the surface pressure and its second derivatives along the surface to the normal velocity. Sample calculations for a theoretical porous material show that the oblique incidence absorption coefficient can be calculated very accurately using this boundary condition. In practice, the boundary condition requires two experimentally determined factors: the normal incidence impedance and a bulk reaction coefficient. An experiment was conducted to demonstrate the importance of bulk reaction effects and to show that a method proposed to measure the bulk reaction coefficient is feasible
acoustic wave absorption;porous materials;