Study On The Low Sound Absorption Technology Of Mechanical Impedance Plate Combined With Micro-perforated Panel

Micro-perforated plate structure has the character of cleaning,environmental friendliness,pollution-free and has wide applicability in the field of engineering.However,the low-frequency sound absorption of micro-perforated plate structure limits the application scope of the structure.Hence improving the sound absorption performance of micro-perforated plate at low frequency has become the focus for researchers.There is one absorption peak in single layer mechanical impedance plate combined with micro-perforated plate structure at low frequency,which improves the low frequency sound absorption to a certain extent.But there are still some problems,such as the single peak of sound absorption and the narrow frequency band of sound absorption.In order to solve this problem,two pathways can be followed to improve the low-frequency sound absorption performance of composite structures.They include increasing the number of sound absorption peaks and broadening the absorption band..Firstly,the basic knowledge of sound absorption mechanism of micro-perforated plates and mechanical impedance plates are summarized.On this basis,it is proposed to couple the parallel mechanical impedance on the back of the single layer mechanical impedance plate,which combines with the micro-perforated plate,forming a composite sound absorption structure.The main idea is to form multiple resonance peaks through parallel mechanical impedance,so as to improve the sound absorption performance.The theoretical calculation of composite sound absorption structure is carried out.The composite sound absorption structure consists of two parts:mechanical impedance plates and micro-perforated panel.The lumped impedance method is used to analyze the mechanical impedance.The equivalent circuit of the whole mechanical impedance is obtained according to the characteristics of the parallel mechanical impedance system.The transformation of mechanical impedance to acoustic impedance can be easily accomplished by equivalent circuit diagrams,and the transfer matrix of the mechanical impedance is derived.The transfer matrix of the mechanical impedance plate is connected in series with the transfer matrix of the micro-perforated plate and cavity.The sound absorption coefficient of composite structure is deduced.Using the previous theoretical analysis and selecting the appropriate structural parameters,the frequency of the mechanical impedance is set at low frequency.The experimental study is carried out and the sound absorptioncoefficient of the composite structure measured by using a standing wave tube.Experimental results show that the calculation results are in good agreement with the theoretical results,which proves the established model is reasonable.Compared with single layer mechanical impedance plate,which has only one sound absorption peak,the composite structure has three sound absorption peaks at low frequency,which greatly improves the low frequency sound absorption performance.After analyzing the quality factor of sound absorption structure,it is realized that increasing the damping of mechanical impedance structure can reduce the quality factor and broaden the frequency band of sound absorption.The viscoelastic properties of mechanical impedance are generally provided by dense materials.For sound absorption at low frequency,the elastic coefficients of materials need to be reduced.However,the damping will be decreased and the sound absorption bands will be narrowed with the decrease of elastic coefficients.In this paper,rubber pipe is used to replace the dense material.The elasticity coefficient of rubber pipe is lower than that of dense material and it has a sound absorption effect at the low frequency.The small holes are machined on the rubber pipe to form a porous elastic pipe,which provides viscoelasticity for the mechanical impedance.Experimental research on the proposed structure is carried out.By the comparison of sound absorption properties between porous rubber pipe with small holes and rubber pipe without small holes,it is found that the resonance frequency shifted to the low frequency slightly for the reduced coefficient of elasticity after machining small elastic holes.The experimental results also show that the sound absorption coefficient has been improved and its sound absorption band broadened.When the incident sound arrive at the mechanical impedance plate,the mechanical impedance plate drives the porous elastic pipe to vibrate together.The cavity air is compressed during the vibration process of the porous elastic pipe and the air flows through the holes in the side wall of the elastic tube.It will produce damping and the vibration energy will be dissipated so as to improve the sound absorption performance and broaden the absorption band.