THESIS
2022
1 online resource (xxvi, 126 pages) : illustrations (some color)
Abstract
This thesis focuses on the design and implementation of a passive underwater
acoustic metamaterial absorber, and a tunable active panel metamaterial. The former
relies on the effective acoustic parameters of the structure to manipulate the sound
waves at the sub-wavelength scale, whereas the latter aims at exceptionally effective
acoustic properties tuning at low frequencies, a regime where the passive absorbers are
constrained by natural law to require bulky and thick samples. Both types of
metamaterials have broad applications.
For the passive underwater absorber, we have designed and fabricated a tungsten-polyurethane
composite with a low longitudinal sound speed that is not only
impedance-matched to water but also can be combined with the Fabry-Pérot resonator
integration scheme to...[
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This thesis focuses on the design and implementation of a passive underwater
acoustic metamaterial absorber, and a tunable active panel metamaterial. The former
relies on the effective acoustic parameters of the structure to manipulate the sound
waves at the sub-wavelength scale, whereas the latter aims at exceptionally effective
acoustic properties tuning at low frequencies, a regime where the passive absorbers are
constrained by natural law to require bulky and thick samples. Both types of
metamaterials have broad applications.
For the passive underwater absorber, we have designed and fabricated a tungsten-polyurethane
composite with a low longitudinal sound speed that is not only
impedance-matched to water but also can be combined with the Fabry-Pérot resonator
integration scheme to realize an ultra-thin underwater acoustic absorption metamaterial.
This design can attain high absorption over the range of 2-20 kHz, which is
demonstrated by both simulations as well as by experiment carried out in a large water
pool. Most notably, by changing the composite’s parameters (Young’s modulus and density), we have realized the close approach to the minimum thickness of the whole
absorber as dictated by the causality principle. The average thickness of our absorber is
only 8.9 mm, belonging to the category of deep sub-wavelength.
We have also designed an active panel metamaterial that can effectively modulate
the boundary impedance, to achieve either total absorption of the incident wave, or
alteration of the reflected wave’s phase in the range of 50-120 Hz. The active panel is
actuated by a piezoelectric actuator with an optimized displacement amplifier; its
functionalities are verified by both simulations and experiments, with excellent
agreement between the two. In contrast to the active noise cancellation (ANC)
technology commonly used in sound extinction via the destructive interference
mechanism, our design does not need a feedback loop; only an initial adjustment is
necessary. In addition, we present the application of an active panel in room acoustics.
The results show the possibility for its large-scale use.
The two main topics presented in this thesis have extensive application prospects in
different acoustic fields. They also provide new ideas and schemes for low-frequency
and broadband acoustic wave manipulations.
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