Acoustic Meta-atoms: An Experimental Determination of the Monopole and Dipole Scattering Coefficients

Abstract

Acoustic metamaterials are materials engineered to manipulate and structure acoustic fields to a degree with no parallel in natural materials. They are created from small sub-wavelength sized unit building blocks referred to as meta-atoms. These are generally arranged in a periodic array to form what behaves as a continuous metamaterial. There are many potential applications of acoustic metamaterials all with novel properties not seen in other devices; such as cloaking, super-efficient sound absorbers and thin spatially compact acoustic lenses. Meta-atom scattering terms of monopolar and dipolar symmetry have been shown to relate to the effective bulk modulus and effective fluid mass density of a homogenised metamaterial. Where, taken together these parameters define the acoustic wave propagation through a material. Due to their importance in acoustic metamaterials, this thesis describes the development and implementation of a method to experimentally determine the monopole and dipole scattering coefficients of meta-atoms. To do this, an acoustic measurement apparatus is modified and characterised to ensure accurate two-dimensional acoustic field data can be obtained. A method of extracting the monopole and dipole scattering coefficients from recorded acoustic data is then defined. This method is based on fitting the experimental incident and scattered field data of meta-atoms to acoustic multipole expansions. The scattering coefficients of a rigid circular cylinder is then determined using the developed method, and found to agree well with analytical values. Better agreement is seen at frequencies over 1000 Hz, where experimental error from the measurement apparatus is reduced. Meta-atoms previously presented in the literature are also fabricated, and their scattering coefficients determined. These are found to agree with simulated values of monopolar and dipolar resonances also taken from the literature. Where, better agreement is again observed at resonances above 1000 Hz.