The acoustic fields of the Wolf electrohydraulic lithotripter.

Electric sparks are used as the sources for both intra- and extracorporeal shock wave lithotripters. Upon ignition, a pressure pulse, headed by a shock, is generated that propagates as a spherically diverging wave. Simultaneously, a bubble is created that, in the case of the Wolf Model 2137.50 Electrohydraulic Lithotripter, expands to a radius of approximately 5 mm and collapses spontaneously after approximately 1 msec. Upon rebound, the bubble generates a second pressure pulse that is almost equal in amplitude and acoustic energy to the first shock wave. Measured pressures are almost entirely positive and decrease in amplitude with the reciprocal of the distance from the source. For the Wolf lithotripter at its maximum output setting, the pressure amplitude at a distance of 3 cm from the spark is typically 3 MPa.