Sound Absorbing Characteristics Of Multi-layer Compound Micro-perforated Absorbers

On the basis of MA Da-You’s sound absorbing theory for the micro-perforated sheet and acoustical modeling theory for porous metals, in this work, the sound absorption mechanism of multi-layer compound micro-perforated absorber and the effects of its structural parameters on overall sound absorption characteristics are studied in detail. The work in this thesis will provide a research reference for the application of the noise reduction technology of micro-perforated absorbent in engineering.Firstly, the basic sound absorption theory of single micro-perforated sheet is briefly reviewed. Many types of two-layer compound micro-perforated absorbing structures are put forward. These compound structures are mainly made up of series or parallel micro-perforated sheets with the different pore sizes, sheet thicknesses, and perforation rates. Besides, several types of series compound sound absorbers including porous metal and micro-perforated plate are developed also. Further, aiming at above-mentioned structures, theoretical acoustic calculations are performed and the corresponding results are presented and analyzed. Finally, the effects of structural parameters with gradient distribution on the sound absorption characteristics of absorbers are given; the concluding theoretical analyses are made also.Secondly, compound MPP absorber involves more variable structural parameters compared with single-layer MPP absorbers with uniform-size holes, which greatly increases the designing difficulty and thus limits their practical application. To fix this problem, this work developed an optimization method for the acoustical optimization of compound MPP absorbers by using ant colony algorithm. The algorithm consists of finding the best variation ranges that provides the maximum mean absorption for a specified frequency range. Ultimately, the correctness of the optimization algorithm is experimentally validated. Results show that ant colony algorithm can be used as an effective technique to optimize compound MPP absorbers.Thirdly, acoustic performance of the two-layer series compound micro-perforated absorption structures was studied by using acoustic module of COMSOL software. Then we compared the acoustic performance with the theoretical results which has been calculated by MATLAB simulation after optimizing the structural parameters using the ant colony algorithm; the results show that a good agreement is obtained. This work will provide a new effective method for the evaluation of absorption coefficient and acoustic impedance of the micro-perforated sheet. It has been found that the ant colony algorithm is an effective optimization one. Also it will lay a foundation for the application and design of micro-perforated composite sound absorption structure in complex physical field.The work in this thesis is supported by the Project of National Natural Science Foundation of China (Grant No.51365046).