Research On Low-frequency Broadband Tunable Ultra-thin Sound Absorbers

Noise problem is ubiquitous in our life.Traditional porous acoustic materials have good absorption effect for medium and high frequency noise.However,the control of large wavelength and low frequency noise caused by structural vibration is always the most challenging problem in the field of noise control.In recent years,the development of acoustic artificial structure makes the control of low-frequency sound wave feasible,and the design of labyrinth structure and winding structure based on spatial folding theory has also been widely concerned by researchers,and has become an important direction of low-frequency sound absorption and noise reduction research.Because the increase of sound propagation path through the folding of space sound field can significantly reduce the size of structural acoustic design,the realization of ultra-thin sound absorption in low frequency region is possible.Therefore,the labyrinth structure,periodic structure and other artificial structures based on Helmholtz resonance theory have made good progress in low-frequency noise reduction.Based on Helmholtz resonance theory and spatial folding principle,a low frequency broadband ultra-thin absorber is designed.According to the folding principle and resonance theory,the structure designed in this paper makes the sound energy dissipate fully by friction by increasing the propagation path of sound wave,and achieves low-frequency sound absorption by local resonance.Through the method of multi-directional coupling and longitudinal superposition,the sound absorption band is broadened and the sound absorption performance is improved,so as to achieve the purpose of low-frequency and wide-band sound absorption.We theoretically analyzed the absorption of sound waves and the absorption performance of sound absorption structure,analyzed the absorption mechanism of sound absorption structure,obtained the low-frequency sound absorption structure,designed the structure and established the model,obtained the sound absorption curve through numerical calculation and analyzed it.Firstly,the ultra-thin low-frequency sound absorption board with unidirectional spiral structure is designed.Through simulation calculation,the influence rules of the front cover thickness,hole diameter,spiral tube groove width and spiral tube length on the sound absorption performance are analyzed,and the structural parameters are optimized to improve the sound absorption performance.Then,a method is proposed to improve the unidirectional spiral structure into the bidirectional spiral structure,which is divided into symmetric and asymmetric structures,and the coupling mechanism of the structure is analyzed.The sound absorption frequency band of bidirectional symmetric spiral structure is wider than that of unidirectional spiral structure.The two-way asymmetric structure has two absorption peaks with different resonance frequencies.By analyzing the influence of the parameters such as the hole diameter of the front cover plate,the width of the spiral tube groove,the thickness of the front cover plate and the length of the two-way spiral tube on the sound absorption performance,and optimizing the parameters of the asymmetric structure,the optimal design size and the sound absorption bandwidth of the asymmetric structure are obtained The structure is doubled to achieve low-frequency broadband sound absorption,which greatly improves the sound absorption performance.Finally,a parallel scheme of longitudinal superposition of single-layer sound-absorbing board is proposed.The single-layer spiral structure,two-way asymmetric spiral structure and square four-way labyrinth structure are superposed longitudinally.The sound absorption curves are obtained by numerical simulation,and the influence of the number of layers of sound-absorbing board on the sound absorption performance is analyzed.With the increase of the number of layers of sound-absorbing board,the sound absorption band becomes wider,The relative bandwidths of the three structures are 28%,56%and 92%respectively.The total thickness of the absorber is 32mm,and the center frequencies are about 105Hz,130Hz and 160hz respectively.These three structures are still a good sub wavelength low-frequency broadband adjustable absorber.