Date of Award
M.S. in Physics
Physics and Astronomy
Omnidirectional and directional acoustic emission enhancements, at low frequencies as well as broad frequency bands, are highly demanded in audio, medical ultrasonics, and underwater acoustics. Emission enhancement and controlling the directivity of an acoustic source is however restricted to the properties of the source. In particular, the size of the source, in comparison with the wavelength of the sound, plays a very dominant role in determining the quality of the emitted acoustic wave. Most problems arise when there is a small acoustic source emitting very low frequency sound with large wavelength. Prior studies have proposed several solutions to this problem from classical solutions, such as employing coupling horns to loudspeaker drivers, to recently proposed metamaterial designs for enhancing or controlling the directivity pattern of an acoustic source. In this thesis, omnidirectional low frequency emission enhancement by using a sub-wavelength metamaterial structure is achieved experimentally. The enhancement phenomenon is later explained by an acoustic version of Fermi's golden rule (FGR) which relates the emitted power to the change in the Density of States (DOS) in acoustic systems. The same structure is then used to enhance the emission of a dipole source in a deep subwavelength scale while preserving the emitted sound wave directivity of the dipole. Lastly, unidirectional sound emission pattern is achieved by enclosing two in phase acoustic sources inside the metastructure with certain source configurations.
Landi, Maryam, "Low-Frequency Emission Enhancement by Resonant Acoustic Metamaterials" (2017). Electronic Theses and Dissertations. 1346.