This thesis is concerned with the design and construction of an electromagnetic
acoustic transducer (EMAT) and the characterisation of its acoustic field both
conventionally, using a hydrophone and with high resolution laser illuminated
schlieren techniques. During the early 80s the introduction of the EMA T along with
the other types of shock wave source used for lithotripsy, revolutionised the treatment
of stone disease. The process of shock wave induced destruction of calculi and the use
of shock waves in other areas of medicine will be discussed, along with the causes and
effects of stone disease in man. For the first time high temporal and spatial resolution
schlierenimages of the shock waves and there interaction with simulation kidney
stones have been recorded. The technique provides a clearer picture of the
fragmentation process and may assist research into the suitability of shock wave
treatment in other areas of medicine currently under investigation.
Schlieren studies of the acoustic field have shown the complex structure of not only
the EMA T shock wave, but also that associated with cavitation in the field. The
primary source of cavitation is due to the rupture and subsequent collapse of bubbles
generated in the water by the strong rarefaction phase of the shock wave. The images
give evidence for the interaction of these 'primary' cavitation shocks with bubbles in
the field, the collapse of some of these bubbles giving rise to additional or 'secondary'
cavitation shocks. An optical lensing effect introduced by the shock has also been
investigated. Objects seen through or immersed in the field of an EMAT shock wave
such as cavitation, appear highly distorted, due to the strong positive and negative
lensing effects associated with the changing refractive index of the compression and
rarefaction cycles of the shock wave.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.