Study On Elastic Micro-Perforated Plate/Membrane Coupled With Cavity And Its Application In Low Frequency Noise Elimination

The new CODES ON NOISE LEVELS ONBOARD SHIPS passed in 2012 by International Maritime Organization(IMO)demand more stringent noise mitigation requirements for onboard living and entertainment venues and have become the mandatory rules.How to block or eliminate noise becomes a serious task.The technique of reducing noise mainly blocks or absorbs sound waves in the propagation path of noise.Micro-perforated panel(MPP)sound absorbers are good candidate for noise reduction application on ships because of their sturdiness,light weight,corrosion resistance and environmental friendliness.The MPP was proposed as a sound absorber by Professor Ma Dayou in 1975,who had developed a general theory for sound absorption by MPP.It is generally called Ma's theory.At present,the theoretical research on MPPs is mostly based on Ma's theory.In most existing studies,MPPs are considered as rigid structures,and the sound absorption coefficient is the only parameter for evaluating the performance of the entire structure.However,some experimental and numerical calculations have shown that although the boundary conditions are not severely affected by the MPP of a large-sized thin plate structure,the vibration of the plate itself has a relatively strong influence on the acoustic performance of the MPP sound absorber.Therefore,the fluid-solid coupling between the plate and the back cavity should be considered in the study of the MPP absorber.In this thesis,the fluid-solid coupling of the plate-cavity was studied,a set of coupling model was established,and the Ma's theory was integrated into the model.Based on the established model,the corresponding computing platform software was compiled.The MPP sound absorber in cabin,the MPP embedded in double plate sound insulation structure and the breathable and ventilated acoustic metamaterial using micro-perforated film were studied.The main research contents in this thesis include the following aspects:1.The coupling of MPP is added to the in the plate-cavity fluid-solid coupling framework.The basic theory in this thesis includes the coupling of ordinary plates and cavities,the coupling of MPPs and cavities,and the coupling between two cavities.In the derivation of the model,the unknown parameter of the MPP is replaced by the vibration displacement and the vibration velocity.The velocity of the vibration includes two parts,one being the velocity of the MPP itself,and the other being the velocity in the micropore.The velocity of the MPP itself is solved by the governing equation of the plate;the velocity in the micropore is related to the cavity on both sides of the plate,and is calculated by the Markov theoretical model.In doing so,the Markov theoretical model is integrated into the plate-cavity coupling model.Based on the coupling model,a software platform for calculation is developed,which can conveniently combine the structure of the board and the cavity,simplifying the pre-and post-processing flow of the calculation.2.Research on the sound absorption performance of the MPP sound absorber.For the finite-size MPP sound absorber,the thinner the MPP and the larger the plate area,the more obvious the influence of the vibration of the MPP on the sound absorption performance.In the past studies,the normal incidence sound absorption coefficient was often used as the only parameter indicating the performance of MPP absorber.When the MPP sound absorber is used in a large area in the cabin,the MPP is coupled to the back cavity and the front cavity at the same time.Therefore,the actual noise reduction performance is relatively large compared with the sound absorption coefficient under normal incidence.To this end,this paper studies the sound absorption performance of the MPP absorber in consideration of the simultaneous coupling of the MPP and the front and back cavity,and uses the noise reduction NR to evaluate the noise reduction effect.The comparison between the experimental and calculated results proves that the noise reduction NR can predict the noise reduction effect of the MPP absorber in the cabin with good accuracy,and proves the correctness and stability of the coupled model.Based on the coupling model,the parameters of the MPP were studied,which provided promising direction for the optimization of the MPP sound absorber in cabins.3.Study on the sound insulation performance of the micro-perforated plate inserted into the double-layer structure.Due to the influence of ”plate-cavity-plate” resonance,the sound insulation performance of the double-layer plate is significantly reduced at the resonant frequencies.Generally,the performance is improved by the method of sandwiching the porous sound absorbing material,but the effect at low frequencies is not obvious.Due to the health,fire and other hidden dangers of porous sound absorbing materials,it is necessary to find alternative materials.This thesis proposes to insert a parallel micro-perforated plate in the double-layer plate to mitigate the resonance of the sound insulation structure and improve the sound propagation loss at low frequency.The validity and reliability of the coupled computing model are verified by experiments.From this coupled model,the acoustic propagation loss of a typical two-layer microperforated plate structure can be predicted.The calculated results show that the low-frequency sound propagation loss is significantly improved compared to the threelayer board with the same geometric parameters at the resonant frequencies of the double layer structure.In order to optimize the sound insulation performance of the micro-perforated plate structure inserted into the double-layer plate,the position,perforation rate,pore size and thickness of the micro-perforated plate were studied,and an optimized structure was obtained.The optimized MPP embedded in double-layer structure has a thickness of 6 cm,and the sound transmission loss at the resonant frequency is at least 15 dB higher than that of the double-layer and three-layer plates.In the low frequency band below 200 Hz,the total value of TL is 22 dB higher than that of the double-layer structure of the same thickness,and is 20 dB higher than the three-layer structure with 20 cm total thickness.4.Study on the structure of a micro-perforated film using a Decorated Membrane Resonator(DMR)and a Hybrid Membrane Resonator(HMR).DMR and HMR are new acoustic metamaterials that break the mass density law.Both the DMR and the HMR that have been studied are air impermeable.Moreover,the use of sealed gas in the HMR makes it very easy for the gas to change in pressure due to temperature changes,which causes the pretension of the membrane to change and also changes the operating frequency of the HMR.This limits the scope of work of the HMR.Meanwhile,if the large sound-absorbing wall surface needs ventilation,the existing HMR technology is also difficult to apply,or it is impossible to ventilate,or the performance of the absorption wall is significantly lowered,and it is difficult to achieve perfect absorption.In this thesis,the theory and technology of elastic micro-perforation is incorporated into DMR and HMR to make air permeable DMR and HMR.The theoretical calculations and experimental tests show that the HMR made by microporous DMR can still achieve perfect absorption.The surface impedance of the DMR extracted from experimental data shows that the perfect absorption can be achieved when the impedance matches the air impedance,i.e.the acoustic resistance is 1 and the acoustic reactance is 0.When small holes are drilled on the back wall of the cavity of the HMR,an air permeable wall of perfect absorption can be constructed.In the experimental investigation,when holes were drilled on the back wall of the cavity of a HMR at perfect absorption(absorption coefficient 99.99%),the perfect absorption was negatively affected.However,by just slightly adjusting the volume of the cavity,perfect absorption(absorption coefficient > 99.99%)can again be obtained.