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Finite amplitude propagation in acoustic beams
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Finite amplitude propagation in acoustic beams
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Finite amplitude propagation in acoustic beams
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Dissertation
Finite amplitude propagation in acoustic beams
1986
Overview
The nonlinear propagation of an initially sinusoidal acoustic wave is described by modelling the field with a Gaussian beam. The theoretical model takes account of the effects of nonlinear distortion, absorption, dispersion and diffraction, although the application is mainly limited to the field on the acoustic axis of a transducer. The field of a circular disc radiator is explicitly considered, for both the unfocused and the focused case, and the numerical solution of the basic equations is described. The application of previously derived, approximate solutions is demonstrated and these solutions are assessed for their accuracy, particularly when used to predict the amplitudes of the harmonic components. Experimental verification of the theoretical predictions has been obtained for propagation in water in the frequency range 1 to 100 MHz and also for propagation in a tissue-mimmicking gel. A particular feature of the measurement system, which uses a broad-band hydrophone, is the ability to record many harmonic components. It is demonstrated that nonlinear distortion occurs in media with acoustic properties similar to those of tissue and that the characteristic behaviour of the field is significantly different to the behaviour of the corresponding field in water. Consequently it is difficult to predict the behaviour of the field in tissue from measurements made in water, but nevertheless a procedure has been developed to make such predictions. As an application of the work described here, a new method for the absolute calibration of hydrophones over a wide range of frequencies has been developed as well as a technique for intercomparing the sensitivities of hydrophones at a number of frequencies simultaneously.
