Airborne and Underwater Response of Acoustic Structures

Abstract

Acoustics is a vast subject that has been utilised in many forms for millennia. Recent work has, amongst other things, explored the control of sound using geometric structure to complement inherent material properties. In this thesis, structured plates and surfaces are exploited to engineer specific acoustic responses. The acoustic transmittance and reflectance of these systems is explored in air and underwater to further understanding and develop structures that possess tailored acoustic properties. Original investigations are presented across six chapters. The first three investigations explore the transmittance of periodically perforated plates in air. The fourth investigation considers a non-resonant mechanism of obtaining complete transmission by varying the fluid environment and the fluid in the apertures of a periodically perforated plate is explored. The fifth investigation considers the transmittance through a slit in an acoustically soft plate underwater. Finally, the surface waves supported on periodically structured surfaces are explored by observing the reflectance of the surface. An acoustic field incident upon a perforated plate is partly transmitted. However, at frequencies dictated by the thickness of the plate, the acoustic field is completely transmitted. Stacking two plates with a small separation creates a resonant cavity between the plates that is the origin of a narrow acoustic stop-band at the frequency of the resonance. By varying the offset of the stacked plates and by varying the gap between the plates the frequency of this acoustic stop band is controlled. Altering the geometry of the plate surface within the gaps allows the gap to behave like an array of Helmholtz resonators, in doing so the frequency of the acoustic stop-band is significantly lowered. Varying the acoustic properties of the fluid contained within the apertures of a periodically perforated plates changes how sound is transmitted through the structure. By careful choice of the fluid environment and aperture media, it is demonstrated numerically that broadband total transmittance can be obtained. Acoustic tunnelling is demonstrated through an acoustically soft-walled slit underwater. The slit exhibits a cut-off frequency below which no propagating waves can exist, in contrast to a rigid-walled slit where propagating waves exist down to zero frequency. Resonant acoustic tunnelling is observed through two closely spaced slits in a series connection, at a frequency below the cut-off frequency of the lowest supported propagating mode. A preliminary study of pseudo surface acoustic waves on periodically structured surfaces observes the excitation of surface waves in reflection. A long pitch grating, added to the surface allows diffractive coupling of incident acoustic radiation to the surface wave. However, the height of the grating above the sample is shown to strongly affect the frequency at which the surface wave is detected. All the structures investigated may be designed to provide a desired response by careful choice of the geometry and materials.