Research On The Sound Insulation Properties Of Membrane-type Acoustic Metamaterials

Low frequency noise has long been regarded as a pernicious form of environmental pollution mainly due to its ability to be able to penetrate through thick partitions. The effectiveness of traditional acoustic barriers at low frequencies is limited due to the effect of the mass law. In the past few years, the emergence of a new concept in the frontiers of acoustical physics called acoustics metamaterials has provided some new ideas for noise control engineering. Of particular interest is thin, lightweight membrane-type acoustic metamaterials which has been shown to exhibit unique sound insulating performance at low frequencies. Prior research workhas shown that such type of acoustic metamaterials can achieve a significant increase in transmission loss in a narrow frequency band within mass-law controlled region and a broader band of transmission loss increase is possible by using multiple layers of such types of metamaterials with different tuning frequencies. However, to use themembrane-type acoustic metamaterials in practical engineering, there still exist many problems need to be solved, such as finding an accurate prediction model, acoustic behavior manipulation and tuning, mechanisms interpretation, as well as lightweight and broadband design. This thesis is aimed at solving some of the key theoretical and technical issues to promote the applications of membrane-type acoustic metamaterials in noise control engineering. The following three main aspects have been investigated in the thesis:(1) the analytical models for the calculation of transmission loss of membrane-type acoustic metamaterials;(2) sound attenuation and manipulation mechanisms;(3) the design of broadband low-frequency sound insulation apparatus using membrane-type acoustic metamaterials. The main findings and contributions of this thesisare as follows:1. A modal analysis approach is introducedto calculate the transmission loss of membrane-type acoustic metamaterials.This approach can be used to calculate the sound transmission loss of mass-loaded circular/square membranes with small computational costs.2. The effects of different geometric and material parameters on the sound insulationperformance are analyzed in detail, which is served as a foundation for the design of membrane-type acoustic metamaterials in this thesis. The main conclusions include:(1) The effect of the supporting frame is investigated and it shows that the frequency of transmission loss peak does not change under the influence of frame vibration as it is only dependent on the resonant frequency of the periodic element; though by increasing the stiffness of the frame, the amplitude of the peak can be increased and effective frequency range of the transmission loss increase induced by the peak broadened leading to better sound insulation performance.(2) By periodically arranging the local resonators with different masses next to each other, the low frequency performance of the metamaterial can be enhanced, which provides a new idea for the design of such type of metamaterial to achieve low frequency broadband sound insulation.(3) This thesis also investigates double-layered membrane-type acoustic metamaterials and it has shown that the two layers of membrane together with the air in-between act as a mass-spring system. With sound wave impinging upon the membrane, resonance can occur which can increase sound transmission through the structure while reducing sound insulation. This can be used to explain the existence of the transmission loss valleys.(4) It is also found that the geometric shape of the resonators can also affect the performance of the low frequency sound insulation.3. Based on the noise reduction requirements for a specifictransformer substation, this thesis further explores the practical engineering application for membrane-type acoustic metamaterials. Experimental specimen are designed and tested. The analytical results are validated, which motivates further engineering applications.In summary, this thesis mainly discusses the theories and techniques of sound attenuation through membrane-type acoustic metamaterials to achieve low-frequency sound insulation. A method to calculate the sound insulation in terms of transmission loss of the membrane-type acoustic metamaterials is developed which is then used to investigate the sound insulation mechanism and means to manipulate the acoustic characteristics of such metamaterials. Base on that, several methods to design low-frequency, broadband and lightweight sound insulation apparatus are proposed and laboratory experiments are carried out to validate the theoretical models. The outcome of this thesis potentially provides new theoretical foundation and technical support for the optimal design of sound insulation apparatus using membrane-type acoustic metamaterials.