The application of laser Doppler velocimetry to the measurement of underwater acoustic pressure fields

Underwater acoustic pressure fields are widely employed by the medical and marine communities in a variety of applications. Ranging from communication through pulse echo and other diagnostic techniques to methods of inducing cavitation in the case of shock-wave lithotripsy, acoustic fields vary considerably in frequency, magnitude, duration and linearity. The conventional instrument for recording measurements from within an acoustic field is the hydrophone, which provides a measurement of the pressure variation at a point within the field. Hydrophones are restricted in that they only provide a measurement at a single point and whilst it is possible to establish arrays of multiple devices, these are expensive and additionally perturbing. This work successfully demonstrates the ability of the beam from a Laser Doppler Velocimeter (LDV) to record quantifiable measurements from an acoustic field in a non-perturbing manner. The theoretical interaction of an LDV beam with an acoustic field is discussed and supporting experimental evidence is presented. The technique is developed by the application of a scanning LDV system, which takes repeated measurements from an array of predetermined positions in two dimensions. This facilitates the whole field mapping of a range of characteristics from an acoustic field, both theoretically and experimentally. Experimental results are also presented enabling the influence of impeding obstacles within the field to be observed. It is concluded that the novel application of Laser Doppler velocimetry for underwater acoustic measurement described in this thesis represents a significant development. It is anticipated that this work will be of interest to a wide-ranging audience including the acoustic calibration community, transducer manufacturers and clinical users. Recommendations are made for additional work, which would enable further refinement and development of the technique.