Sound absorption characteristics of thin parallel microperforated panel (MPP) for random incidence field

Abstract

In this study, the absorption coefficient of a thin parallel MPP for random incidence field is investigated. A coiled backing air cavity realizes the thin structure, and the parallel configuration is applied to have a wider sound absorption bandwidth than a single MPP. The effects of the oblique sound incidence on the thin parallel MPP are investigated by numerical studies for both single oblique incidence and random incidence. These effects are significant from a practical standpoint, whereas the result of a coiled backing air cavity must be carefully evaluated. This paper focuses on developing, modeling, and evaluating parallel MPP with a coiled backing cavity. It is found that the oblique incidence can result in lower sound absorptions and shift in resonance absorptions compared to the normal incidence angle, especially for high incidence angles (>70°). In addition, the performance of the thin MPP at a normal incidence angle can be maintained up to the incident angle of 70°, where the sound absorption coefficients of 0.8 and above are present. Compared with a straight backing cavity, the use of coiled one does not introduce deviation in sound absorption characteristics except for the cavity modes slightly higher resonance frequency under the same axial length. Moreover, the proposed structure behaves as a locally reacting material as long as the cavity’s longest dimension is smaller than half an operating wavelength for the targeted frequency range. This allows the sound absorption characteristics of normal incidence and diffuse sound field excitation is insignificantly different. Hence, this characteristic can deal with two folded problems in terms of bulky absorber issue and sensitive performance on oblique incidence.