| With the development of launcher technologies towards higher thrust and higher bearing ratio,the noise environment for precise instruments within fairing and its effective load is increasingly harsh.As a result it is especially urgent to control in-cavity low-mid frequency noise.In most countries,passive noise control technologies are mainly based on porousmaterials,while they are characterized by incapacity of peak noise reduction and relatively small transmission loss.Consequently,except of optimizing in-cavity porous material laying scheme and controlling noise total sound pressure level(SPL),reduction of specific-frequency peak noise has also become the key for study on in-cavity noise reduction devices.With porous materials and horn-shaped Helmholtz acoustic resonator as the study object,studying on in-cavity passive noise control were conducted in the paper.The methodology combining theoretical calculation and simulated analysis were applied to explore problems concerning acoustic characteristics of the MF material and horn-shaped Helmholtz acoustic resonator,their sound absorption mechanisms,laws of noise reduction,laying strategies and others,which were then verified with in-cavity noise reduction experiments.Main works and achievements of the study were as follows.(1)A study on the transmission mechanism of acoustic wave in the porous material was conducted upon the basis of the Johnson-Champoux-Allard(JCA)model for porous materials.By combining acoustic transmission theories and experiments for the porous material,a material parameter library required by melamine foam(MF)acoustic simulation analysis was determined;the laws how thickness and five fluid bulk properties affect MF sound absorption performance were analyzed by theoretical calculation;the paper estimated main acoustic parameters influencing the sound absorption performance of the MF material under full frequency range;under a high-frequency range,the material presented good sound absorption performance when the flow resistance rate of the material was at the range of 10000-13000Pa?s/m~2 and the characteristic length of heat availability was relatively small;besides,when the flow resistance and tortuosity factor of the material were relatively high and the two characteristic lengths were relatively small,the material presented good sound absorption performance under low and mid frequency range.(2)A study was conducted on in-cavity acoustic characteristics of aluminum(Al)and Al/MF cylindrical shells.With cylindrical shells as the study object,relevant acoustic experimental devices and corresponding finite element models were constructed.Acoustic cavity responses of AL and Al/MF cylindrical shells were studied within the 100-400 Hz frequency range,suggesting that the 20-40 mm MF material presented 4.16-7.54 dB noise-reduction effect in the low-frequency range.Basic acoustic theories of the cylindrical acoustic cavity were utilized to derive local impedances of Al/MF cylindrical shell and normal frequency and normal mode of acoustic cavity under non-local boundaries.Finite element models for analysis of in-cavity noise of Al and Al/MF cylindrical shell were constructed,and the simulated results were compared with experimental datas to determine the entity unit modeling method for the MF material on the base of non-local impedance theory.Besides,futher study was conducted to explore the laws how different laying thicknesses and laying areas of MF material affect in-cavity noise reduction under the low and mid frequency range of 100-400Hz.(3)Simulation analysis on noise-reduction performances of in-cavity MF material under the low and mid frequency range and its laying strategies as well as acoustic experiments were conducted.Upon the condition that the mass of in-cavity noise-reduction device shall be lower than 2%of the total mass,the noise reduction schemes for the low and mid frequency range were studied with different laying ratios、laying positions and MF material thicknesses but the same specific gravity ratio,suggesting that influences of different laying strategies on in-cavity noise reduction of the cylindrical shell under the low and mid frequency range were mainly manifested in the frequency range with close sound wave lengthλand acoustic cavity length L.According to the in-cavity low-frequency noise reduction scheme,further analysis was conducted on impacts on high-frequency noise reduction,and suggested that the laying ratio was a key factor determining in-cavity noise reduction under both the low-mid and high-frequency range,while different laying manners and different laying thicknesses with same specific gravity ratio showed relatively high impacts under the low-mid frequency range.(4)Study was conducted to explore low-frequency sound absorption performances of the in-cavity Helmholtz acoustic resonator and noise reductions of different laying strategies.The further exploration on sound absorption mechanism of the resonator and laws of resonant frequency variation suggested that neck viscous damping was significant for acoustic energy dissipation of the resonator,and the exponential-type neck section mode was the identified as optimal mode of in-cavity noise reduction.Laws concerning influences of resonator installation position and installation amount on in-cavity noise reduction of the cylindrical shell were studied,and,based on that,noise reduction strategies combining same-and different-resonant frequency resonators were analyzed and it is suggested that the strategy,that firstly installing same-resonant-frequency resonators of an optimal amount at response anti-nodal points of an acoustic cavity and then installing rest different-resonant-frequency resonators could obtain optimal noise reduction effect under the low frequency range. |
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