J. S. Bolton
E. R. Green
Ray W. Herrick Labs., School of Mech. Eng., Purdue Univ., West Lafayette, IN 47907
Perforated screens are frequently used to face sound absorbing porous materials since they serve both to protect the porous material and to enhance its effectiveness at low frequencies. Recently, exact theories describing the combination of a perforated screen and fibrous materials that can be modeled as equivalent fluids have been presented for both normal and arbitrary incidence angles. In this paper a similar analysis is presented that is appropriate when the sound absorbing material is an elastic porous material. The perforated screen itself is modeled as an Euler--Bernoulli panel whose stiffness and mass per unit area are modified by its degree of perforation. The boundary conditions describing the interaction of the elastic porous material and the screen require, among other things, that the velocities of both the solid and fluid phases of the porous material are simultaneously equal to the normal velocity of the nonperforated area of the panel. In addition, the volume velocity of the air within the perforations is required to equal the porosity weighted sum of the fluid and solid velocities of the porous material in the regions in which the porous material is adjacent to a perforation. By applying the boundary conditions appropriately, closed-form expressions may be found for the amplitude of each of the infinite number of plane waves scattered and transmitted by periodically arranged perforations. Example calculations that illustrate both the nature of sound absorption by the combination of polyurethane foam and a perforated screen and sound transmission through the same combination will be presented.