The Manipulation of Sound with Acoustic Metamaterials

Abstract

The original work presented in this thesis pertains to the design and characterisation of resonant-cavity-based acoustic metamaterials, with a focus on airborne sound. There are five separate experimental chapters, each with a unique approach to the design of periodic structures that can support and manipulate air-bound acoustic surface waves via diffractive coupling between resonant-cavities. The first two chapters concern measurement of the acoustic transmission though various kinds of periodic slit-arrays, whilst the latter three chapters utilise a near-field imaging technique to directly record and characterise the dispersion of trapped acoustic surface waves.

The first experimental chapter investigates the effect that thermodynamic boundary layers have on the Fabry-Perot-like cavity resonances that are so often utilised in acoustic metamaterial design. At audio frequencies, these boundary layers have a decay length that is typically more than two orders of magnitude smaller than the width of the resonating slit-cavities, hence it may na¨ıvely be assumed that their effect can be ignored. However, by studying in detail the effect that reducing slit-cavity width has on the frequency of the measured cavity-resonance, for both a single slit cavity and a slit-cavity array, it is found that these boundary layer effects become significant on a far larger scale than their characteristic thickness. This is manifested in the form of a reduction in the resonant frequency as the slit-width is narrowed. Significant attenuation of the resonance and a 5% reduction in the effective speed of sound through the cavity is measured when the boundary layers form only 5% of the total width of each slit. Hence, it is both shown that the prevalent loss free treatment of acoustic slit-cavities is unrealistic, and that one may control the effective speed of sound through the slit-cavities with a simple change in slit-width.

The second chapter explores the effect of ‘compound’ grating structure on trapped acoustic surface waves, a compound grating having a basis comprised of more than one resonating element. The angle dependent acoustic transmission spectra of four types of aluminium slit-array are recorded, and for the compound gratings, it is found that sharp dips appear in the spectra that result from the excitation of a ‘phase-resonance’. This occurs as new degrees-of-freedom available to the acoustic near-field allow the fields of adjacent cavities within a unit-cell to be both out-of-phase and strongly enhanced. By mapping the transmission spectra as a function of in-plane wavevector, the dispersions of the modes supported by each sample are determined. Hence, the origin of the phase-resonant features may be described as acoustic surface waves that have been band-folded back into the radiative regime via diffraction from higher in-plane wavevectors than possible on a simple grating. One of the samples is then optimised via numerical methods that account for thermodynamic boundary layer attenuation, resulting in the excitation of a sharp, deep transmission minimum in a broad maximum that may be useful in the design of an acoustic filter.

The third chapter introduces the near-field imaging technique that can be utilised to directly characterise acoustic surface waves, via spatial fast Fourier transform algorithms of high-resolution pressure field maps. The acoustic response of a square-lattice open-ended hole array is thus characterised. It is found that over a narrow frequency band, the lattice symmetry causes the acoustic surface power flow to be channelled into specific, predictable directions, forming ‘beams’ with a well defined width.