Enhancing sound absorption using periodic micro-perforated structure with porous layer

To get desired sound absorption,we proposed a novel periodic composite structure comprised of MicroPerforated Plate (MPP),porous material and air cavity.The composite structure is then solved using equivalent circuit model,with equivalent fluid porous model and Maa's theory.Distributed four-pole elements are used to handle structures which are not compact compared to the sound wavelength.The model procedures are validated and confirmed as satisfactory by published results and finite-element results.Analysis conducted on a single layer shows that,compared with traditional MPP,the porous addition here can increase the sound absorption in the low-to-medium frequency range;however,the advantage of porous materials in the high-frequency range is lost.Meanwhile,by parallel arranging the porous materials and controlling their filling ratios,the absorption curve of the composite structure can be tuned.As to the periodic composite structure,it is found that the influence of layer number N is mainly in the low-to-medium frequency range.When N varies,the half-absorption bandwidth increases over 40% (≥380 Hz) compared with a single layer.Compared with the multi-layered MPP,N=2 and N=4 produce an increase of bandwidth by 50% (≥400 Hz) and30% (≥300 Hz) respectively.As N increases,the sound absorption is better but the enhancement weakens as it tends to the limit of the composite structure.These results show the potential enhancements can be made to the traditional MPP,which can benefit the research on wideband noise reduction in the low-to-medium frequency range.