Low Frequency Attenuation of Sound with Acoustic Metamaterials

The work presented within this thesis pertains to the use of acoustic metamaterials for low frequency absorption and attenuation of sound. Acoustic metamaterials are structures composed of periodic and sub-wavelength locally resonant unit cells and are typically an order of magnitude or smaller than the associated wavelength of the frequency they are designed to manipulate. All naturally occurring materials have both positive mass density and bulk modulus; acoustic metamaterials have the ability to artificially manipulate these properties to have negative effective quantities dependant on the frequency of the incident acoustic wave. Within this thesis, a general analytical method based upon the linear superposition of terms derived with the transfer matrix method are used to derive simple analytical expressions for complex acoustical systems. Good agreement is found when in comparison to results produced using the transfer matrix method and numerically. One limitation with the effective property models presented is the inability to capture the evanescent coupling between Helmholtz resonators. The mechanism to achieve perfect absorption for one port systems has been explored with the development of single frequency and broadband frequency perfect absorbing acoustic metamaterial unit cells. Through the use of Helmholtz resonators with porous inclusions within their cavities, one port perfect absorbers have been developed that obtain perfect absorption at 290 Hz using a single Helmholtz resonator with a sample thickness of 1/28 the wavelength, and over a broadband frequency range between 275 and 625 Hz using a system of three Helmholtz resonators with a sample thickness of 1/10 the wavelength. In a two port system, Helmholtz resonators with porous inclusions within the cavity have been utilised to achieve perfect absorption at a single frequency and over a broadband frequency range. The single frequency perfect absorber has a sample thickness of 1/16 the wavelength at a frequency of 300 Hz. The broadband perfect ...