The emergence of acoustic metamaterials makes it possible to realize diverse exotic functionalities that cannot be achieved using naturally existing materials. Metamaterials typically refer to the artificial composites, formed by sub-wavelength micro-structures. Inspired by the benefits from the distributed inhomogeneity in acoustic metamaterials, the inhomogeneous-distributed resonators are investigated to realize reflected wave manipulation, sound absorption, and duct noise attenuation in the thesis.

Various metasurfaces with the capability of acoustic wave manipulation have been reported. However, most of them suffer from a number of issues, such as frequency dependent, bulky, structural complexity. This thesis reports two resonance-based metasurfaces used for reflected wave...[ Read more ]

The emergence of acoustic metamaterials makes it possible to realize diverse exotic functionalities that cannot be achieved using naturally existing materials. Metamaterials typically refer to the artificial composites, formed by sub-wavelength micro-structures. Inspired by the benefits from the distributed inhomogeneity in acoustic metamaterials, the inhomogeneous-distributed resonators are investigated to realize reflected wave manipulation, sound absorption, and duct noise attenuation in the thesis.

Various metasurfaces with the capability of acoustic wave manipulation have been reported. However, most of them suffer from a number of issues, such as frequency dependent, bulky, structural complexity. This thesis reports two resonance-based metasurfaces used for reflected wave manipulation. One is based on the inhomogeneous impedance properties, which is composed of varying-depth acoustic liners. This metasurface possesses the functionalities of both reflected wave manipulation and sound energy attenuation simultaneously. In addition, it can overcome the single frequency performance limitation and performs well in a broadband frequency range. The other one is a deep subwavelength (λ/30 operation wavelength) metasurface constructed of an inhomogeneous Helmholtz resonator array with different lengths of extended neck.

The sound absorption properties of Helmholtz resonator with an extended neck (HREN) are investigated. The coupling effects of alternating HRENs with different extended necks in a checkerboard absorber, including a dual-band sound absorber constructed by largely dissimilar resonators and a wide bandwidth absorber composed of strongly coupled resonators, are comprehensively studied. To obtain a broadband absorber within a prescribed frequency range, a set of inhomogeneous HRENs are combined, which overlaps the absorption peaks induced by different HRENs. Their geometric parameters are determined based on the analytical prediction model of HREN coupled with the particle swarm optimization (PSO) algorithm.

Attentions have also been paid to noise radiation from duct and sound transmission in a waveguide. On the basis of the optimized HREN-based absorber, sound radiation from a lined circular duct is studied. Two types of sound source are considered: a point source of sound and a three-bladed rotating propeller. Experimental measurements show the lined duct reduces the radiation noise up to 3 dB in the designed frequency range from 700 to 1000 Hz for both cases. Finally, sound transmission through a waveguide loaded with detuned Helmholtz resonators (HRs) is investigated. A forbidden transmission band is formed when a coherent perfect absorption (CPA) is induced by two asymmetric, slightly detuned resonators. Results show the band is insensitive to the separation of resonators. Based on this finding, a compact and broadband CPA-based transmission-forbidden system is proposed.