THESIS
2019
xxvii, 132 pages : illustrations (some color) ; 30 cm
Abstract
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...[
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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.
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