Underwater Acoustic Waves on Structured and Unstructured Plates

Abstract

This thesis details original experimental work on the observation of the underwater acoustic response of flat plates and acoustic metamaterial-like arrays of holes. There are five main experimental chapters that examine the excitation, detection and analysis of the radiative and evanescent acoustic fields of structured and unstructured surfaces. There are two main types of acoustic excitation and detection methods employed to detect and characterise the transmitted and propagating surface fields of these systems. The first two and the final experimental chapters examine the radiative excitation of plate modes and the remaining two experimental chapters investigate trapped surface modes of structured hole arrays. Using computational Fourier analysis of the acoustic fields, the frequency components and wavevector components are extracted from the spatially scanned time-resolved data. These results are used to characterise the fields by plotting the dispersion relation and identifying the acoustic modes. The first experimental chapter investigates the Lamb mode responses of mild steel, aluminium alloy and acrylic plates. By comparing the experimentally extracted dispersion relation to an analytic model of layered media, these plates are elastically characterised. Experimentation is performed using ultrasound pulsed through these plates by a transducer and detected using a hydrophone setup with a usable range of $50$ to $500$~kHz. These results are successfully used to extract values of the elastic modulus and Poisson's ratio for each sample. Whilst results vary, they are reasonably close to that of the industry estimates. This method of non-destructively testing material properties is advantageous compared to the standard methods that render samples unusable after testing. Plate modes are further investigated in the second experimental chapter which examines the dispersion when an aluminium alloy plate is acoustically excited using a symmetrically aligned source. The experimental results are compared against Finite Element Model (FEM) computational data. The results show a region of transmission where the acoustic fields are focussed. Modelled results of the fields inside the plate agree with the shape of the fields in frequency and wavevector domains. Focussing of the fields is attributed to the region where the symmetric Lamb mode $S_1$ becomes the $S_{1\text{b}}$ mode. This region, labelled the Zero or Negative Group Velocity (ZGV or NGV) region, has gained recent interest as a potential application of concentrating sound power. Additionally, a beat pattern in the transmitted fields of the centred plate is observed and an explanation of it is analytically derived. This focussing phenomenon has applications in enhanced acoustic transmission and absorption. In the third experimental chapter, lines of holes with differing symmetries are examined. Four samples are scanned using near-field excitation and detection of the surface modes of lines of holes. Single lines of holes, double lines of holes, with mirror and glide symmetry, and three lines of holes, with a pseudo-glide symmetry, are shown to support ``trapped" Acoustic Surface Waves (ASWs). These modes are characterised using Fourier analysis of time-domain data to plot the two-dimensional frequency dependent field maps and the evanescent dispersion relation. These experimental results are compared against Finite Element Method (FEM) calculated dispersion. The results show a $2.53$~kHz difference between the asymptotic frequency of the mode calculated using a perfectly rigid structure and that of an elastic hole array. This is expanded upon in the fourth experimental chapter, in which the acoustic near-fields of two-dimensional square arrays of holes in aluminium alloy plates are investigated. ASWs are excited and detected over two different thickness plates and characterised using Fourier analysis of the time-domain data. In addition, in-plane acoustic beaming is observed over very narrow frequency ranges. These types of surface mode supporting structures offer a method of controlling the direction and amplitude of sound. The final results chapter details an array of holes in a pressure-release foam. This sample is excited using a far field source and then the normalised transmission is calculated using a two-dimensionally scanned plane. Spatially scanned frequency dependent results show the propagating fields above the cut-off frequency of the array. Using the spatial plot of the transmitted fields the acoustic cut-off response is plotted. These results are compared to FEM calculated transmission results. In addition, plotting the spatially mapped FEM calculated fields inside the holes shows the Fabry-Perot like resonances inside the holes of the array and the Bessel-function shape of the modes. Comparisons between the experimental and computational results show that the Fabry-Perot-like modes are not visible in the experimental data. In addition, the spatial modulation of the transmitted and reflected diffracting fields is derived. Surfaces that allow the flow of fluid through the structures whilst blocking certain frequencies of sound from propagating through them are already finding commercial applications in air and will have similar sound blocking applications underwater. The works presented in this thesis has important implications and applications in underwater acoustics that range from the first observations of fundamental and widely used concepts to a method of supporting specific acoustic modes over structured surfaces. Although many of these ideas have been relatively well known in acoustics, they were not observed underwater. In addition, water-solid acoustics proved to be a source of much complexity and interest that this work explores using computational models and analytic theory. The work presented in this thesis has important implications for and applications in underwater acoustics. These range from initial observations on fundamental and widely-used concepts to a method of supporting specific acoustic modes over structured surfaces. Although many of these ideas are already relatively well-known in the general field of acoustics, they have not yet been observed underwater. Water-solid acoustics proves to be a source of much complexity and interest which this work explores using computational models and analytic theory.