The characteristics of a shockwave should be understood. A shock wave can best be described as a highly nonlinear and unharmonic acoustic soundwave that is characterized by an extremely steep change in the pressure amplitude, the so-called shock front. The parameters of a a shock wave are well known, including the pressure maximum, pulse rise time, half width time of the pressure wave, and a negative pressure phase of the shock wave.(Fig. 1 below). The acoustic energy of the shock wave is proportional to the product Pmax x r2 x t where Pmax is the maximum pressure of the shock wave in focus, r is the radius of the said focal area, and t is the half-width time of the shock wave.

 Fig 1. Parameters of focused Shock wave. , Pmax, pressure Maximum. pressure rise time, tw, half width time. Pr, negative peak pressure.

Energy is expressed in joules or millijoules where as pressure is expressed in bar or megapascal. The focal zone of a a lithotripter is, by definition, the volume within which the shock wave pressure is greater than 50% of the maximum pressure. A common property of all shock wave devices is the cigar-shaped, three dimensional form of the focal zone, with a density for intensity in this focal zone is defined as the acoustic energy divided by the area (intensity = energy/area). This is shown diagrammatically below in Fig. 2.

Fig.2.Focal region of Lithostar. P, Pressure along x, y and z axis