Research On Broadband Single-layer Micro-perforated Panel Absorber

Micro-perforated panel (MPP) absorber was first proposed by professor Maa D-Y in1975, and in the meanwhile its basic theoretical model was built which was called Maa’s theoretical model. Since Maa’s pioneering works, MPP absorber has been successfully applied in a lot of fields, such as buildings, ships, airplanes, mufflers and so on, for it is sturdy, lightweight, erosion-resisting and environmentally friendly compared with the traditional porous sound-absorbing materials and ordinary perforated panel absorbers, which makes it offers an outstanding alternative to the traditional porous materials and is regarded as promising as a basis for the next-generation of sound-absorbing materials.However, as a resonant sound absorbing structure, an obvious disadvantage of traditional single-layer MPP absorbers is that they are effective only in a narrow sound absorption band around their respective resonance frequencies, usually1-2octaves, which preventing them becoming general sound absorbers for a practical application. In addition, due to the limitations for installation space, many noise control issues require small-size noise-reducing structures. Thus, how to broaden the sound absorption bandwidth of single-layer MPP absorbers without increasing the thickness becomes the main focus of current research.In this paper, in order to develop small-size noise-reducing structures, the author has started a relevant research on how to widen the sound absorption bandwidth of single-layer MPP absorbers and has made many useful results.The main contents of study in this paper are as follows:Firstly, sound absorption characteristics of an MPP absorber is determined by its structural parameters such as the perforation diameter, the panel thickness, the perforation ratio and the depth of the air cavity. The influence rule of the variation of the structural parameters on the absorption performance has been studied through numerical simulation with MATLAB software. On the basis of full understanding of the influence rule of the structural parameters on the sound absorption performance, from the perspective of hybrid design of micro-perforated panel absorption structure based on the structure parameters and absorption characteristic, a software platform is developed by the object-oriented programing language C++. It is different from the previous design method by taking restrictions of both structure parameters and absorption characteristic parameters into consideration and balancing the restriction relationship between maximum absorption coefficient and absorption bandwidth of micro-perforated panel absorption structure according to the practical application, aiming at obtaining more satisfactory absorption curve. This platform will offer a set of optimal structure parameters that meet requirements of hybrid design.Secondly, micro-perforated panel absorbers with ultra-micro perforations, namely ultra micro-perforated panel absorbers (ultra MPP absorbers), are experimentally studied in this paper. Although Maa D-Y pointed out that the absorption bandwidth limits of a single-layer MPP absorber can be obtained by reducing the perforation diameter to less than100um, however, it’s difficult for traditional processing technology such as machining punching and needling skills to fabricate such small perforations, therefore MPPs with ultra-micro perforations have seldom been reported yet. This paper conducts an exploratory study of the processing technologies of ultra MPPs, and their trial production based on micro-electronic mechanical systems (MEMS) is carried out. Actually, the machining error exists in any processing technologies. For the traditional MPP absorbers with large perforations, the machining error range that is as large as several micrometers may has little or no effects on the sound absorption performance, but it could not be neglected for ultra MPPs. The theoretical analysis model of MPP absorbers on considering the effect of machining error is set up, and the effects of machining error on the absorption performance of ultra MPP absorbers and the accuracy of Maa’s theory for them are discussed. Results show that the measurements and the predictions calculated by the theoretical analysis model on considering the effect of machining error compare well with the calculations based on Maa’s theory with the maximum error being less than6%, which indicates that Maa’s theory is still available on considering the effect of machining error and the machining error within a certain range does not affect the sound absorption performance of ultra MPP absorbers. In addition, the limitation of Maa’s theory for machining error is discussed through numerical simulation, results reveal that it depends on the hole size of MPPs with ultra-micro orifices. Finally, the measurement of the normal sound absorption coefficients is carried out in impedance tube using standing wave ratio method and all the conclusions drawn above are experimentally validated.Thirdly, multi-size micro-perforated panel absorbers are theoretically and experimentally investigated. Ultra MPP absorber are of great potential to achieve the absorption bandwidth limits of single-layer MPP absorbers and with small size which make it strongly attractive to narrow space, however, its absorption band is shifted to high frequency as the perforation diameter decreases which will result a performance degradation in low frequency range. Moreover, ultra MPP absorbers have relatively high production cost compared with ordinary MPP absorbers with large size perforations. Therefore ultra MPP bsorbers are not suitable for reducing the low-frequency noise. Multi-size MPP absorber has the same absorption effect as multi-layer MPP absorbers provided that its structural parameters are proper designed and with thinner thickness. Since without using ultra micro-holes, multi-size MPP absorber is low-cost. All these advantages make it especially suitable for applications in the narrow space. However, due to the lack of theoretical model, it can not realize the needed design of multi-size MPP absorbers through theoretical analysis. Based on the flow continuity of air on both sides of MPP, the calculation of sound absorption coefficients of multi-size MPP absorbers is derived under normal incidence condition, and the effects of holes arrangements and the partition panels in the cavity on the sound absorption characteristics are discussed. Ultimately, the validity of the theoretical model is verified through experiments.Fourthly, multi-size MPP absorbers introduce more variable structure parameters compared to single-layer MPP absorbers with uniform-size holes, which greatly increases the design difficulty and thus limits their practical application. To overcome this problem, this work has applied the optimization design of multi-size MPP absorbers using multi-population generic algorithm (MPGA). It consists of finding the best combination of the structure parameters of a multi-size MPP absorber within given variation ranges that provides the maximum mean absorption for a specified frequency range. Ultimately, the correctness of the MPGA is experimentally validated. Results show that MPGA can be used as a straightforward, fast and effective technique to optimize multi-size MPP absorbers.