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...[ Read more ]

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.