CICADA-INSPIRED SOUND GENERATOR WITH DUAL RESONATORS

Abstract

Male cicada’s superior sound producing ability has been well studied by entomologists and ethologists. The secret behind the loud sound is the dual-resonator structure: the primary resonator is a series of buckled ribs/beams on its tymbal organ, while the secondary resonator is a large air cavity in the abdomen with a pair of openings. However, the understanding of the dual-resonator structure is incomplete, and few endeavors have been reported on developing cicada-inspired novel acoustic devices. To this end, this dissertation research aims to achieve a fundamental understanding of the cicada-inspired sound generating structures, and to apply the knowledge to develop a dual-resonator system with superior sound generating ability.First, a clamped-clamped buckled beam – the fundamental vibration source of the dual-resonator system – is modeled and tested for free vibration responses during the snap-through process between its bistable positions. It is found that the free vibration of the buckled beam is independent from actuation. In terms of the natural frequencies and the vibrational mode shapes, the free vibration is determined only by the geometry and material properties of the buckling beam. The experiment provides a comprehensive insight of the snap-through process and the induced free vibrations. Second, both analytical and experimental methods are used to investigate the buckling beam from an energy perspective, including the force needed for actuating the buckled beam, the work by the actuation force, the elastic energy of the buckling beam, and the sound radiation characteristics. Results show that although the actuation forces depend on its acting locations, the work by the force remains constant, which is equal to the elastic energy difference between the first symmetric buckling mode and the first anti-symmetric buckling mode. Acoustic analysis shows that the sound radiation is mostly generated from the first symmetric vibrational mode. Third, the dual-resonator system consisting of a buckling beam and a Helmholtz resonator is proposed. Considered as an equivalent two-degree-of-freedom vibration system, the dual-resonator system is modeled and studied for optimal sound output. Finally, a dual-resonator system is fabricated with the optimal parameters. Experimental characterization shows superior sound outputs of the dual-resonator system similar to what are observed in male cicadas. This dissertation sheds new lights on the structural-acoustic interaction of buckling beam and Helmholtz resonator that is found in the sound-producing organs of male cicadas and develops a cicada-inspired dual-resonator system for the first time. Findings from this research not only enhance the existing knowledge on male cicadas, but also pave the way for its engineering applications that require highly efficient sound radiation.