There are three different types of shock waves generators used today.
The first is the electrohydraulic generator, which uses the tips of an electrode as a point source. This electrode is placed in the first focal point F1 of a semiellipsoid and high voltage is switched to the tips of the electrode. Between this tips an electrical spark is generated and a shock wave is released right from the beginning by the vaporization of the water between the tips. The spherical shock waves are reflected by a metal ellipsoid and focused into the second focal point F2 which for the therapy is adjusted to the therapeutically volume inside the patients body. This principle is shown in Fig. 1a.
The second generator is the electromagnetic one, which uses an electromagnetic coil and a metal membrane opposite to it. A high current pulse is released through the coil generating a strong varying magnetic field, which induces a high current in the opposite membrane. The electromagnetic forces accelerate the metal membrane away from the coil creating a slow and low-pressure acoustical pulse. To focus the wave an acoustical lens is used. The focal point is defined by the focal length of the lens. The amplitude of the focused acoustical wave is increasing by nonlinearities when the acoustical wave propagates towards the focal point. The rise times of electromagnetic generated shock waves are in the range of a few hundred nano seconds (10-9 s). Another construction using a cylindrical source. The high current pulse forms an cylinder shaped pressure wave which is reflected by a hyperbole shaped metal reflector to achieve focusing. Again the rise time shortens to a few hundred nano seconds while the amplitude is increasing on the way to the focal point. The principle of a flat coil generator is shown in Fig. 1b.
The third generator forms acoustical waves by the piezoelectric effect. A few hundred to some thousand piezoelectric crystals are mounted to a spherical surface. When switching a high voltage pulse to the crystals they immediately contract and expand generating a low pressure pulse in the surrounding water. The system is self focusing by the geometrical shape of the sphere. Again the shock wave is created by nonlinearities and increasing amplitudes during the propagation of the wave to the focal point. The principle is shown in Fig. 1c.
A typical pressure profile of a shock wave in the focus F2 of an electrohydraulic system is shown in Fig. 2. Generally a shock wave can be described as a single pulse with a wide frequency range (up to 20 MHz), high-pressure amplitude (up to 120 MPa), low tensile wave (up to 10 MPa), small pulse width at –6dB and a short rise time. Basic physics could show rise times of the positive pressure amplitude <10 ns. For shock wave devices the measured rise times are in the range of 30 ns as a result of the limited time resolution of the pressure recording hydrophones [2, 3, 4]. An optical hydrophone has a considerably higher time resolution compared to a piezoelectric hydrophone. The rise time of an electrohydraulic generated shock wave measured with an optical hydrophone is below 10 ns.
The positive pressure amplitude is followed by a diffraction-induced tensile wave with a few µs duration. The energy density (up to 1.5 mJ/mm2) and the pulse energy (up to 100 mJ) are determined from the temporal and spatial distribution of the pressure profile. The energy density describes the maximum amount of acoustical energy, which is transmitted through an area of 1 mm2 per pulse. The total pulse energy is the sum of all energy densities across the beam profile multiplied by the area of the beam profile. It describes the total acoustical energy per released shock wave.