On The Acoustic Performance Of The Micro-perforated Plates With Variable Cross-section

Safety is the foundation of all activities.Quietness is human beings' persistent pursuit.Both safety production and quiet living require us to take measures to control noise.The measures include sound absorption,insulation and attenuation,where sound absorbing materials all play an important role.40 years ago,Maa developed Micro-perforated plates(MPPs),which are typically made of thin plates penetrating with a number of sub-millimeter size holes.MPPs backed by a solid wall with an air space make an absorber.The absorber,without stuffing any fibrous materials and foams in the back cavity,has good medium and low frequency sound absorption performance.Now,MPPs are widely applied in industrial,architectural and traffic areas since it is lightweight and robust,resistant to fire,humidity and corrosion,and fiber-free without causing health concern.The MPPs are one of the most promising sound absorption constructions for the next-generation use.MPPs resonate with incident sound wave to consume sound energy achieving sound absorption.It has good sound absorption performance,with maximum effective sound absorption bandwidth exceeding three Octave bands,due to matching resistance with the characteristic impedance of air and small reactance.However,classic MPPs often has a narrow sound absorption bandwidth and a small structural strength.In practical applications,methods,such as multi-layer MPPs,or composite with other structures or back cavity segmentation,are often used to improve its overall performance,which is often limited by the mounting space,installation conditions and project cost.MPPs with a variable cross-section on a thick plate were then developed to improve the mechanical strength without deteriorating the acoustic performance.Recently,researchers had done many works related to MPPs with tapered holes but little for MPPs with geometrical discontinuities.This paper first retrospects the theoretical background of MPPs and then proposed an additional length model of MPP impedance end correction.MPPs with stepwise apertures and overlapping micro-gapped strips(OMGS)are stuied thoroughly.The sound absorption mechanism of the best commercially MPP named as Sound Micro is clarified in detail.Finally,the paper proposes a kind of technically feasible variable cross-sectional MPPs with optimal acoustic performance.The main research results are as follows.(1)The paper proposed an additional hole length model for the impedance end correction of MPPs,which can quickly predict the acoustic impedance of MPPs with high accuracy.(2)This paper studied acoustic behaviors inside and outside perforations of MPPs with stepwise apertures.It is demonstated that the thermo-viscous power dissipation occurred in the small hole dominate and that occurred in the large one is negligible.The step change produces additional acoustic impedance correction and empirical impedance correction models are derived.MPPs of straight minute holes on thin plates with appropriate geometrical design can achieve optimal acoustic performance as anticipated.Such MPPs attached with a thick plate with large concentric holes may have similar acoustic performance since additional acoustic impedance caused by large holes is insignificant.Meanwhile,the mechanical strength of the MPPs is enhanced owing to the added thick plate.(3)This paper proposed a new method to fabricate tiny holes just by manipulating sheets without punching or other advanced methods,which may reduce the manufacturing costs and improve the processing efficiency.OMGS is made by overlapping and staggering two layers of plates,each layer of which is constituted by arranging an array of strip-shaped sheets separated by a micro gap.The upper and lower micro slits and the micro apertures sandwiched between constitute a variable cross-section perforation with geometrical discontinuities inside.The intermediate aperture is just a layer of cross profile with zero thickness and strip-shaped end tube,distinguished from square tube or hexagonal tube in the classical MPP.Acoustic behaviors and equivalent geometrical parameters of OMGS are stuied.The energy loss is discovered mainly taking place at the cross profile.Empirical impedance end correction models for the cross profile are proposed.OMGS can achieve a good acoustic performance on very thick plate.(4)This paper explained sound absorption mechanism of Sound Micro and clarified the misunderstanding of the original intention of the plate fabrication.The optimal acoustic performance of MPP is analysed and the impedance characteristics of micro slit are studied.Finally,an engineering feasible MPP with acoustic performance approaching the best limit is proposed by combining the micro slit and variable cross-section.