Excess sound absorption at normal incidence by two microperforated panel absorbers with different impedance

A microperforated panel (MPP) absorber is known to be one of the most promising alternatives of the next-generation sound absorbers. However, its absorption frequency range is limited to around two octaves because its absorption solely depends on the Helmholtz resonance mechanism. Therefore, the authors have proposed a combination of two different MPP absorbers in parallel, and revealed its potential for achieving a broader absorption frequency range in terms of excess attenuation. In this study, by the hybrid method of BEM and a mode expansion method, the authors investigate the relationship between the excess attenuation caused by the impedance discontinuity at the boundary of the two different MPPs and the sound absorption coefficient derived using the electro acoustical equivalent circuit model. Measurements of the normal incidence absorption coefficients are carried out to validate the equivalent circuit model, and in the experiments, the configuration of the cavity to realize the parallel arrangements for achieving wideband sound absorption is also discussed referring to the experimental results.