“Smart” noise control foams: A feasibility study

Abstract

A porous layer can absorb a significant amount of acoustical energy only if its thickness is comparable to the wavelength of the incident sound. Thus, a porous layer inevitably becomes a less effective sound absorber as the frequency is decreased. In this paper it will be shown through theoretical calculations that the low‐frequency performance of a finite‐depth layer of elastic porous material may be enhanced by applying an appropriate force to the solid phase at the front surface of the layer. In particular, it will be shown that at any angle of incidence the solid phase may be forced in such a way to create a perfect impedance match with an incident plane wave, thus causing the sound to be completely absorbed. Complete absorption is not possible when the incident sound field is diffuse; nevertheless, sample calculations will show that considerable enhancement of the passive absorption is still possible. Note that the success of the approach suggested here requires a significant degree of coupling between the motion of the solid and fluid phases of the porous material. Thus it may be expected that partially reticulated, polyurethane foams will be susceptible to this approach owing to the degree of viscous and inertial coupling between their fluid and solid phases. Several methods for actuating the front surface of an elastic porous material and for sensing its surface normal impedance will be discussed.