Study On Some Applications Of Acoustic Metamaterials

Due to the wide applications of acoustic waves,the artificially structured acoustic material is an important research field,which includes the study of sonic crystals and acoustic metamaterials.This thesis is mainly focused on studying,both in theory and experiment,bio-inspired directional microphone based on acoustic metamaterials and acoustic unidirectional transmission based on time-varying medium.Inspired by a kind of crickets,we propose,design and prove an acoustic signal amplifier based on coiling-up space structure and Helmholtz resonance cavity which can be designed to work in a very low frequency region.In analogy to the bush cricket' s adoption of air channels in its own body to amplify acoustic signals and for the integration of the system we adopt a kind of coiling-up space structure to construct our amplifier.Coiling-up space structure(periodic zigzag structure)which is a kind of metamaterials with high refractive index combined with acoustic cylindrical cavity,forms a new system which realizes acoustic waves strongly located in the cavity for the resonance frequency region,i.e.the effect of acoustic amplification.Resonance frequency and bandwidth of this new designed resonance system can be adjusted through changing geometric parameters of the front-neck metamaterial and loss in the following air cavity.Since this is a resonance system similar to the classic Helmholtz cavity,it is a sub-wavelength acoustic amplifier.We have showed that for acoustic signals near 165Hz amplification rates in intensity is up to 10 times both in theory and experiment,while the structure size is only 1/30 of incident wavelength.We provide a method for roughly estimating the resonance frequency by considering the equivalent parameters of metamaterials in front-neck region.This simplified theory shows that,the resonance frequency of the structure we designed can be modulated in accordance with the effective parameters of the coiling-up space metamaterial and effectively controlled.Combining an acoustic transducer with the acoustic amplifying structure we have designed constitutes one detection unit of a directional microphone system.Compared with the bare probe array,as results of the amplification effect,signal-to-noise ratio and accuracy are improved in the frequency we desired.We envision that the metamarerial directional microphone can open up new possibilities to impact many aspects,including,but not limited to sonar systems,acoustic communication and navigation systems,noise source identification,non-destructive damage detection and medical imaging.An acoustic asymmetric transmission device exhibiting unidirectional transmission property for acoustic waves is extremely desirable in many practical scenarios.Such a unique property may be realized in various configurations utilizing acoustic Zeeman effects in moving media as well as frequency-conversion in passive nonlinear acoustic systems and in active acoustic systems.Here we demonstrate a new acoustic frequency conversion process in a time-varying system,consisting of a rotating blade and the surrounding air.The scattered acoustic waves from this time-varying system experience frequency shifts,which are linearly dependent on the blade's rotating frequency.Such scattering mechanism can be well described theoretically by an acoustic linear time-varying perturbation theory.Combining such time-varying scattering effects with highly efficient acoustic filtering,we successfully develop a tunable acoustic unidirectional device with 20 dB power transmission contrast ratio between two counter propagation directions at audible frequencies.It is worth noting that our concept of using an acoustic time-varying medium can be equally applied to frequency ranges other than audible frequencies.We believe our concept of linear time-varying acoustic scattering and the design of such unidirectional transmission device may find applications in relevant fields such as non-destructive testing of turbulent flow pipe,acoustic imaging,audible signal processing,and etc.Moreover,the underlying mechanism of this time-varying acoustic scattering is linked to acoustic dynamic phase modulation process.In other words,by taking advantage of dynamic phase modulation,novel physical phenomena such as gauge magnetic potential in condensed matter might be simulated by a carefully designed macroscopic acoustic time-varying system.