## 2 Characteristics of the Shock Wave and Its Sources  
_Characteristics of the Shock Wave and Its Pro-Hailing Effect on the Living Organism_  
Shock waves used in physiotherapeutic procedures (outpatient treatment) and aesthetic medicine are an example of an acoustic wave [13], whose propagation medium, in this case is the tissue of a living organism, is, de facto, the hydrated environment [2; 13; 14]. An acoustic wave is a disturbance involving the propagation of mechanical energy into an elastic medium (here, water) through its particles, more precisely, as a result of vibrations of these particles. The source of the shock wave is violent phenomena, such as an explosion, exceeding the speed limits, or ultimately collisions [13; 14]. The latter phenomenon is used in the devices of physiotherapists to generate a shock wave. Every person has experienced the effect of shock wave in many occasions while hearing the sound of a balloon bursting with air, a supersonic plane flying over, or a collision of objects [13; 14]. One of the parameters of the acoustic wave propagation medium is acoustic impedance, defined as a measure of the wave resistance created by this medium [13]. Due to the fact that the acoustic wave is a mechanical wave, it is possible to distinguish the phenomena of propagation, diffraction, and absorption depending on the previously mentioned wave impedance of the medium [1; 13]. Shock wave propagation in living organisms is possible, as the tissues consist mainly of water molecules. This makes the treatment using waves of this type a non-invasive therapy--a sought innovation in medical treatments. Depending on the type of shock wave, it can penetrate the tissue up to 3 cm (radial wave EPAT, the case further discussed in paper [15]) and up to 12 cm with a focused wave (lithotripsy procedure) [1]. The wave injected into the tissue penetrates it until it hits a target with a completely different acoustic impedance. In this point, we deal with refraction and absorption as positive side effects of the wave, which confirms the therapeutic nature of the wave [2]. In other words, a collision of a shock wave in the tissue with, for example, a calcareous deposit or a degenerative change, i.e., a solid target with a different impedance than the tissue, results in the disruption of this pathological area. The shock wave effect takes place here. It is worth mentioning the importance of all the means of coupling the waves generated by the apparatus with the operated tissue. The preparations are selected in such a way that they have a similar acoustic impedance to the tissue--then an anechoic propagation of the shock wave from the apparatus to the body is possible. Examples include ultrasound gel or castor oil [1; 2]. However, lubricants should not be used because the air they contain significantly reduces the possibility of wave coupling [2]. Unfortunately, due to the difference in the acoustic impedance of the apparatus applicator (more precisely, the applicator transmitter) and the operated tissue, only 10% of the generated energy is actually transferred to the body [13; 16]. In addition to the above-mentioned conductive medium (with an acoustic impedance similar to that of water), researchers also use preparations when constructing measurement stands, for example; in paper [7], a 5 mm layer of sanitary silicone was used, and in [13], viscoelastic substances were suggested [17]. The shock effect is not the only phenomenon occurring in the shock wave. Below, we discuss all the phenomena occurring in the wave propagation medium using the example of a registered wave. Figure 1 shows an example of a shock wave registered on the test bench presented in further sections of the paper. The presented shock wave signal was registered using a stand with a hydrophone and a pneumatic shock wave generator (operating parameters 5 bar, 1 Hz). Details are provided later in the paper.