Abstract
Low-frequency wave absorption in domains of acoustics and mechanics is an old yet not fully closed topic in terms of both theoretical research and practical applications. This thesis intends to present a comprehensive study on this subject by reviewing recent breakthroughs and proposing new metamaterial-based designs aiming for simpler construction, better compactness, and higher flexibility.
I start with revisiting the development of metamaterial research, elucidating its fundamental physical pictures and highlighting new wave control possibilities offered by it, followed by a detailed review on sound absorption structures, for both conventional types and metamaterial-based types, with an emphasis on the recent breakthrough of causality-law-aspired optimal absorber design. Finally, to further explore physical opportunities beyond the causality constraint, two new types of absorbers are presented for acoustic and vibrational absorption, respectively.
A conceptual design of using a tunable active wall to achieve arbitrarily low frequency airborne sound absorption and continuously adjustable acoustic functionalities is proposed based on the impedance match condition and impedance adjustment through amplitude tuning of the phase-matched motion. Both FEA simulation results and demonstration experiments yield convincible results supporting this theoretical conception.
A new type of compact and low-frequency responsive mechanical resonator is conceived to remove primary vibrational energies from the noise source in order to tackle noise reduction problems requiring high attenuation. The idea exploits coupling between translational and rotational motions, leading to a significantly enhanced oscillator mass which we model as an inertia amplification effect. Experimental characterization on fabricated prototypes verifies the analytical description, showcasing gigantic amplification factors and ultralow resonance frequencies within a compact and ultra-lightweight device. By further adopting a structural modification and utilizing mode hybridization between the normal spring-mass resonance and the inertia-amplified resonance, an inertia-amplification-based absorber with total absorption capability and broadened bandwidth is presented analytically and numerically.