Boundary element acoustic analysis of hybrid expansion chamber silencers with perforated facing

The substructure boundary element approach is developed to predict and analyze the acoustic attenuation characteristics of hybrid expansion chamber silencers with perforated facing. The silencers are divided into a number of acoustic domains with single medium (air or sound-absorbing material), and treating the sound-absorbing material as an equivalent fluid with complex-valued density and speed of sound (or complex-valued characteristic impedance and wavenumber), and then the boundary element method (BEM) may be applied to each domain leading to a system of equations in terms of acoustic pressure and particle velocity. Using the specific acoustic impedance of perforate, which takes into account the effect of sound-absorbing material, the relationship of acoustic pressures and particle velocities between the inlet and outlet of silencer may be obtained and then transmission loss is determined. For the straight-through perforated tube reactive and dissipative silencers, the predictions of transmission loss agree reasonably well with experimental measurements available in the literature, which demonstrated the applicability and accuracy of the present approach. The BEM is then used to investigate the effect of internal structure on the acoustic attenuation characteristics of hybrid expansion chamber silencers with perforated facing. The numerical results demonstrated that the hybrid expansion chambers may provide higher acoustic attenuation than the reactive expansion chamber in the mid to high frequency range.