Research On Absorber For Low-frequency Sound Based On Acoustic Metamaterials

Noise pollution has become one of the most serious environmental problems in today's world,which has seriously affected people's normal work and rest,and invisibly hurts people's health.To this end,many efforts have been made to reduce or eliminate the adverse effects of noise.However,as the long wavelength of low-frequency noise,it is endowed with strong penetrating ability.Therefore,how to eliminate low-frequency noise has been being a challenging problem for a long time.Traditional materials,i.e.,acoustic porous materials,show very low efficiency for attenuating the low-frequency sound as which satisfies the linear response theory.In other words,to effeciently absorb low-frequency sound waves,porous materials with large thickness are needed.To dissipate low-frequency sound with sub-wavelength-thickness materials,some measures should be taken to strengthen the sound energy density,such as resonance.The emergence of acoustic metamaterials provides a new idea for the construction of resonant absorbers,and shows great application prospects.Simultaneously,it also has become a challenging and important frontier topic.Under this background,this paper focuses on the study of sub-wavelength sound absorbers with acoustic metamaterials,covering acoustic single-port and dual-port absorbers.The contents are as follows.In the 1st chapter,the origin and applications of electromagnetic metamaterials and acoustic metamaterials are described.The history of negative-parameter,near-zero-parameter and double-positive-parameter acoustic metamaterials are reviewed.In the 2nd chapter,a single-band perfect absorber and a multi-band near-perfect absorber are proposed.We first have constructed a lossy resonant plate based on an acoustic split-ring resonator coated with acoustic porous materials and devised a deep sub-wavelength(?/37)acoustic perfect absorber theoretically.In addition,the graphical method is used to prove that the perfect absorption(PA)is due to the critical coupling condition of the system.Further,broadband acoustic absorption is achieved by coupling LRP with the backed cavity in front of the rigid wall which shows sound absorption coefficients>80%in the frequency range from 99.1 Hz to 294.8 Hz.Then,based on a unique Double-channel Mie resonator(DMR),a near-perfect multi-band absorber is constructed theoretically and experimentally.In addition,the loss factor and leakage factor of the system are retrieved by the coupled-mode theory(CMT).The results show that the near-perfect absorption peaks are generated by the system satisfying the near-critical coupling conditions.Therefore,two different theoretical methods are used to prove that the PA is due to critical coupling.In the 3rd chapter,based on the causality priciple,a physical model for achieving near-perfect broadband absorption is established.Then,based on the coiled space resonator(CSR)attached with an ultrathin-layer porous material,a broadband sub-wavelength near-perfect absorber is constructed theoretically and experimentally,which shows>95%absorption coefficient in the frequency range from 228 Hz to 319 Hz(wavelength from 12.6 times to 9.0 times material thickness).Further study shows that the absorber shows good robustness to wide incident angles.In addition,in this chapter,the coupled mode theory is further studied.Under the guidance of the theory,an ultra-sparse perfect absorber is constructed,and the theoretical basis for designing multi-band perfect absorbers and broadband near-perfect absorbers is provided.In the 4th chapter,based on transfer matrix theory(TMM),an asymmetric absorber based on lossy Bragg stack(LBS)is proposed,namely,the sound energy is perfectly absorbed when sound waves are incident from one end,while most of the sound energy is reflected by the system when incident from the other side.It is also found that the asymmetric absorptions of the system are result from the asymmetry in reflection caused by the introduced loss factor but does not break the reciprocity of the system.Through further research,the asymmetric absorptions are established in a wide incidence angle and demonstrate good robustness to the defects introduced by the system.In the 5th chapter,two different theoretical methods are proposed to construct anamoulous absorbers,including asymmetric absorbers and symmetric near-perfect absorbers.Firstly,we have constructed an asymmetric absorptive system theoretically and experimentally based on slightly detuned traditional Helmholtz resonator(THR)pairs.The results show that the generation of asymmetric absorptions are attributed to the existence of equivalent virtual soft boundary.Moreover,we have extended the asymetric absorptions to multi-band and broadband.Then,a new physical mechanism is proposed to construct artificial acoustic soft boundary from the view of impedance,and a multi-order Helmholtz resonator(MHR)is proposed to construct asymmetric and symmetric absorbers.It should be emphasized that the methods constituting soft boundaries and the physical mechanism of forming asymmetric absorbers are different.From the point of view of mode coupling,the former one is coupled by two dark modes with different resonance frequencies,while the latter one is obtained by coupling bright and dark modes with the same resonance frequency.In the last chapter,the paper is summarized and the future development is prospected.