Investigations Of The Properties Of The Negative Effective Parameters Acoustic Metamaterials

Acoustic metamaterials(AM)is a kind of artificially designed materials,which possess the characteristic to anomalously regulate the propagation of acoustic waves.Being different to the wavelength-scale phononic or the photonic crystals,the unit cells of the AMs are on the subwavelength level,i.e.,the dimensions of the unit cells are much smaller than their operating wavelengths.Because of this feature,the structures of the AMs cannot be distinguished as the acoustic waves propagate in AMs.Hence,the AMs can be regarded as homogeneous effective media,which can be described with effective mass density(EMD)and effective bulk modulus(EBM).Many intriguing acoustic effects that do not exist in nature materials have been realized via the AMs,such as negative refraction,acoustic cloaking,slab focusing,extraordinary acoustic transmission,inversed Doppler Effect,and so on.In this dissertation,based on the fundamental principles of the metamaterials and the concept of acoustic local resonance,the locally resonant structure model with negative EMD was proposed through the methods of simulations and experimental measurements.And then the AM samples based on the model mentioned above that have negative EMD were successively constructed in air medium and water medium,respectively.Furthermore,the designed micro-structure units with negative EMD were coupled with other micro-structure units with negative modulus,and the AMs with double negative acoustic parameters were thus obtained in air and water medium.The unique acoustic characteristics of the designed AMs were also proved with experiments.The main work of this dissertation can be summed up as follows:1.The proposition of hollow tube acoustic meta-atom with negative effective mass densityA hollow tube structure metameta-atom based on local resonance model was proposed,and the AM consisting of double-layer hollow tubes was prepared.Theoretical analysis indicates that the resonant frequency of the hollow tube structures is closely related to the tube's length.The shorter the tube is,the higher the resonant frequency becomes.Experimental measurements show that the larger the diameter of the hollow tube is,the more involved the air medium is,and the stronger the resonance becomes.By using the parameter extraction method based on homogeneous medium theory,the EMD of the proposed AM with tube diameter of9.6 mm was extracted from its transmission and reflection coefficients.The results show that the EMD is negative in the frequency range of 1520-1930 Hz.And the calculated results obtained with the COMSOL Multiphysics simulation software agree well with the experimental data,which proves the validity of the model of the structure.2.The realization of double-negative parameters via hollow tube and split hollow sphereAM with double negative parameters is realized by coupling the hollow tube metameta-atom and the hollow sphere metameta-atom,where the former has negative effective mass density and the latter has negative effective modulus.The diameter of the hollow tube is set as9.6mm.The external diameter,wall thickness,and orifice diameter of the hollow sphere are25mm,0.5mm,and 6mm respectively.The hollow tube and the hollow sphere are arranged in a double-layer hollow tube and double-layer hollow sphere structure.Owing to the resonance between the tubes and the spheres,both of the EMD and EBM are negative values in a range of 1612-1654 Hz.And a sub-wavelength resolution of achieved by the AM with double negative parameters can be proved with a square sample with areas of 85×85 mm~2.3.Meta-atom cluster acoustic metamaterial with broadband negative effective mass densityHollow tube meta-atom clusters consisting of hollow tube meta-atoms with different tube lengths that resonated in a broadband frequency region were proposed and fabricated.It is demonstrated that interaction among different resonances resulting from the hollow tubes with different sizes is weak.By taking a polyethylene hollow tube with a diameter of 5 mm and the wall thickness of 1mm,a series of sample metamaterials composed of the hollow tubes with lengths of 98 mm,68 mm,48 mm,and 29 mm were prepared.The transmission and reflection measurements indicate that the resonant frequency of the hollow tube increases with the reduction of the tube's length.Based on a hollow tube meta-atom cluster composed of seven kind of hollow tubes with different lengths(the lengths of the tubes are 98 mm,68 mm,51mm,48 mm,41 mm,32.5 mm and 21mm),a double-layer AM was prepared.The transmission and reflection curves of the meta-atom cluster metamaterial demonstrate that the resonant frequency remains the same when the different hollow tube meta-atoms resonate individually or when they are gathered and resonate together,which further proves that there is no strong coupling effect among the hollow tubes during the resonance.The aforementioned meta-atom cluster metamaterial sample achieves negative effective mass density in a broadband range from1560 Hz to 5580 Hz,and it also achieves negative effective refraction in the corresponding frequency band.4.Acoustic metamaterial with negative mass density in waterTaking the hollow tube as a structural unit,the AM with negative EMD in water medium was obtained.On one hand,the resonant mechanism of hollow tubes in water medium is as same as that in air medium,and negative EMD can be realized by the hollow tubes in both medium as the directions of the medium's acceleration and the sound field's driving force are opposite around the resonant frequency.On the other hand,the resonant frequencies of the hollow tubes in the two media are closely related to the tube's lengths,and the resonant frequency can be adjusted to ultrasound band from the band of audible sound by carefully designing the geometry sizes of the hollow tubes.Based on the aforementioned features,the AM with hollow tube structures can be extended from air medium to water medium.Using hollow tubes with a diameter of 2.6 mm and a length of 9 mm for double-layer hollow tube AM,negative EMD can be achieved in the range of 35.31-35.94 kHz in water medium,and negative refraction is achieved in the corresponding band at the same time.The simulation results further prove the correctness of the achievements of negative EMD and negative refraction in water medium.5.Ultrasound acoustic metamaterials with double-negative parametersBy coupling the hollow tube meta-atom and the split hollow sphere meta-atom into a structural unit,a perforated hollow tube meta-molecular structure was designed,and the AM with double negative parameters in water medium was prepared on the basis of the meta-molecular structure.In water medium,two kinds of resonances,hollow tube resonance and hollow sphere resonance can be generated by the perforated hollow tube in water medium in the resonant band,which is able to accomplish negative EMD and negative elastic modulus at the same time.The transmission and reflection curves obtained through experiments and simulations indicate that the perforated hollow tube AM possesses the transmission and reflection features.Furthermore,with parameters extraction method,negative EMD and EBM are obtained in the range of 36.66-36.84 kHz.The experiment demonstrates that the designed AM with double negative parameters can achieve slab focusing effect in the band where double negative acoustic parameters are reached.