Studies On A Tunable Acoustic Metamaterial With Double-negativity And Sound Insulation Against Low-frequency AndWide-band Acoustic Waves

Acoustic metamaterials achieve great attention owing to unprecedented characteristics, which exhibit great application potentials in various fields. In this thesis, the mechanism of a tunable double-negative metamaterial is presented, and a low-frequency and wide-band sound insulator is designed. The thesis covers five parts as follows:In chapter 1, the discovery of acoustic metamaterials is reviewed, as well as the development from single-negative metamaterials to double-negative metamaterials. Besides, researches on tunable metamaterials and acoustic absorption are summarized.In chapter 2, different methods of Bloch theory, fluid impedance theory and effective medium theory are used to study the performances of different types of tubular acoustic metamaterials characterized by the negative density, negative modulus and double negativity. Bloch theory can present the analytic dispersion equations and distributions of acoustic fields along the metamaterials, and the fluid impedance theory can be used to describe the acoustic impedances and transmission coefficients. The equivalent medium parameters are presented by effective medium theory in low frequency ranges assuring the long wavelength approximation.In chapter 3, a tunable acoustic metamaterial with double-negativity composed of periodical membranes and side holes is presented, in which the double-negativity pass band can be changed with an external direct-current voltage. It is found that in the metamaterial with the symmetrical membranes and side holes, the membranes and side holes work independently of each other. The tension and stiffness of the periodically arranged membranes are actively controlled by electromagnets producing additional stresses, and thus, the transmission and phase velocity of the metamaterial can be adjusted by the driving voltage of the electromagnets. It is demonstrated that a tiny direct-current voltage of 6 V can result in a shift of the double-negativity pass band by 40% bandwidth, which exhibits that it is an easily controlled and highly tunable acoustic metamaterial.In chapter 4, an acoustic metamaterial adopting side structures, loops and labyrinths, arranged along a main tube is presented to block sound. By combining the accurately-designed side structures, an extremely wide forbidden band with the lower cut-off frequency down to 80 Hz is produced, and the sound attenuation from 50 to 200 dB can be achieved in the metamaterial, which demonstrates a powerful low-frequency and wide-band sound insulation ability. Moreover, by virtue of the bypass arrangement, the metamaterial is based on an open structure, and thus, air flow is allowed while acoustic waves can be insulated, which exhibits great potentials in the application of noise-proof for buildings.Finally, the summary of the thesis and the plan for future work are presented in chapter 5.