Noise Control Technology Based On Acoustic Artificial Structures And Its Applications In Pump Systems

Pump systems are widely used in various industrial activities,which cause serious noise pollution during working.Therefore,noise control of pump systems have been a research hotspot.Recently,acoustic artificial structures have attracted a great deal of attention in the field of noise reduction and elimination due to the characteristics of ultra-thin,open,and easily adjustable,as well as advanced acoustic manipulation performance,and has application prospects in noise control of pump systems.In this dissertation,by utilizing finite element numerical simulation,theoretical analysis and experimental measurement,the performances and mechanisms of ultra-thin,ventilated broadband sound insulators or absorbers designed based on acoustic artificial structures are investigated,and the applications in practical scenarios such as pump room and pump truck are discussed further.The main research contents are as follows:1.Ultra-thin,broadband sound absorption walls and its applications in noise control of pump systems(1)A type of ultra-thin planar sound absorption wall composed of multi-cavity metasurface unit cells(thickness～1/90 wavelength)is proposed.Based on the excitation of the monopolar mode,this absorption wall can realize the near-perfect sound absorption at 239 Hz with the maximum absorption coefficient and fractional bandwidth of 0.97 and 12.9%,respectively,which mainly arises from the high thermalviscous loss around the circular hole and 4 narrow slits of the unit.By combining 4 unit cells with different parameters(2×2 array),the fractional bandwidth of the compound unit cell can be further enhanced to 18.7% based on the coupling of unit cells.The measured and simulated results match well with each other.(2)A type of sound absorption wall consisting of periodic dual-channel resonant unit cells(thickness～1/23 wavelength)backed with a wall is presented.Due to the impedance matching design and dissipation by the thermal-viscous loss induced inside the two channels of the structure,this absorption wall can achieve near-perfect sound absorption at 141 Hz with a fractional bandwidth of 9.1%.Moreover,by designing a composite unit cell comprising six unit cells with different parameters,the fractional bandwidth has been enhanced to around 40% with an average absorption coefficient of0.86,without increasing the thickness.The measured and simulated results are in good agreement with each other.(3)A type of ultra-thin broadband sound absorbing pump room constructed by composite dual-channel resonant unit cells is proposed for the first time to control the noise created by the multistage pump system for emergency water supply in the frequency range of 1300-2100 Hz.A broadband sound absorption with the average absorption coefficient of 0.86 and the corresponding fractional bandwidth of 42.7% can be achieved in the frequency range of 1355-2090 Hz,which can fully cover the noise frequency band of the pump.It is worth noting that the thickness of the composite dualchannel resonant unit cell is only 42 mm,showing the characteristic of ultra-thin.2.Ventilated,broadband sound insulation structures and its applications in noise control of vehicle mounted pump systems(1)A type of low-reflection bidirectional sound insulation tunnel is demonstrated,which is constructed by a pair of ultrathin lossy acoustic metasurfaces(thickness～1/35wavelength)with the phase gradient dφ/dx=-6.8 rad/m(f=370 Hz)attached symmetrically on both sides of a straight tunnel.Based on the sound manipulation by both acoustic metasurfaces,the tunnel with the bidirectional sound insulation can be obtained in the frequency range of 337-356 Hz,and the minimum value can reach about-27 d B at 343 Hz.Such a phenomenon can be explained by the sound reflections and the acoustic blind area(ABA)created by the acoustic metasurfaces for the left and right incidences.In addition to the characteristics of bidirectional sound insulation,the lowreflection of sound through the tunnel can be realized simultaneously by the thermoviscous energy loss in the coiled channels of the unit cells.The measured results agree well with the simulated ones.(2)A type of broadband,ventilated sound-insulation barrier is proposed,which is composed of a single layer of periodic subwavelength unit cells(thickness～1/28wavelength).Based on the coupling of symmetric and asymmetric eigenmodes of each unit,sound insulation through the open barrier can be obtained around 121.5 Hz with a minimum transmittance of 0.06.Moreover,the distance between two neighboring units H is twice as large as the width of the unit cell,showing high-performance ventilation.Furthermore,a broadband multi-layer ventilated sound-insulation barrier with a fractional bandwidth of 19% is designed.The measured results agreed with the simulated ones.(3)A type of ventilated sound insulation system is designed for the first time based on the noise control requirements of a vehicle mounted pump system.The designed sound insulation system is constructed by acoustic metasurfaces with double phase gradient based on ultra-thin hooklike unit cells.Numerical simulation of the sound insulation performance shows that a broadband sound insulation can be achieved with the transmittances less than 0.1 in the frequency range of 1195-2310 Hz,and the corresponding relative bandwidth is about 63.6%,which can effectively control the working noise of the vehicle mounted pump system while taking into account ventilation.3.Broadband silencing ducts and the experimental verification(1)An ultra-low frequency,broadband silencing duct based on dual-spiral channel resonant unit cells(thickness～1/47 wavelength)is demonstrated.Based on the ultra-low frequency sound absorption performance of the dual-spiral channel resonant unit cell,a silencing duct composed of N unit groups is designed,which achieve highperformance sound absorption at 73 Hz with fractional bandwidth of 17%.On this basis,the silencing duct with ultra-low frequency broadband sound absorption is further designed by combining 5 unit cells with different values of t,the fractional bandwidth is about 60.6%,and the average absorptance can reach about 0.92 in the frequengy range of 63-103 Hz.(2)A broadband silencing duct based on multi-cavity resonant unit cells(thickness～1/12 wavelength)is presented.By combining two unit cells with different parameters,the silencing duct can achieve the near-perfect sound absorption at 283 Hz with the fractional bandwidth of 9.2%,which arises from the mutual coupling of MMR II of both unit cells.Moreover,by increasing the number of unit cells,the broadband near-perfect sound absorption is realized,and the fractional bandwidths can reach about25% and 46% for the cases of 6 and 13 unit cells,respectively.(3)The sound absorption performances of the silencing duct composed of 2 and 6multi-cavity resonant unit cells have been measured experimentally,and the results are consistent with the simulated ones,which verifying the sound absorption performance of the silencing duct designed.