This thesis describes the problem of finite amplitude acoustic waves in fluids when diffraction effects are also important. To study this problem the example of a circular planar piston transmitter is used. A brief review of the current literature on the finite amplitude field of a circular piston is given. This precedes a set of experimental results which provide support for the current theories. The main concern is with the development of a numerical model which describes the finite amplitude acoustic field within the near-field or fresnel region. The model, although one dimensional, is able to predict the generation of harmonics within the whole of the near-field region. However, only the harmonic amplitudes on the acoustic axis of the piston are taken as an example. Prediction of harmonic growth, harmonic phase and waveform shape is undertaken. It is found that the growth of the harmonics within the near-field is not monotonic, an oscillation in both amplitude and phase is observed with increasing distance. The results give rise to waveform shapes which are asymmetric with respect to their compression and rarefaction phases. Results from the model are compared with experimental measurements. Throughout reference is continually made to the acoustic field within the near-field for the case of infinitesimal waves. This subject is discussed in an appendix, giving a review of the large amount of literature on the subject. Also, a new closed-form expression is developed for the linear field in this region.
|Date of Award||1983|