BASICS OF ACOUSTICS: 1.2 The wave propagation velocity and frequency

1.3 The wave propagation velocity.
The wave propagation velocity is defined as the path traversed by the wave per time unit (second). The velocity of body waves (longitudinal, transverse, surface type) depends only on the elasticity and density of the medium, and wave type:a5where E – elastic modulus in Pa; ρ – density of the medium kg / m3. It should be noted, that for purposes of the ultrasonic monitoring, propagation velocity of bulk and surface waves is considered independent of the frequency, amplitude and direction of propagation.
1.4 Frequency waves (oscillation)
In ultrasonic testing the most widespread is, inversely proportional to the period,  Frequency: = 1/ rate – the number of oscillations that is done by the midst particle per time unit.

Unit of frequency: Hz, kHz, MHz are shown in Table 1.1.


The frequency determines the interaction nature between acoustic waves and matter (Fig. 1.5). High-frequency oscillations with frequency greater than 10 MHz experience has strong attenuation and do not penetrate the tested object to a considerable depth. Low-frequency oscillations with frequency less than 1 MHz are diffracted encircle defects with dimensions smaller than the wavelength. Therefore, in the acoustic control the widest use received ultrasonic vibrations at frequencies from 0.4 MHz to 10 MHz.

Fig. 1.5 – Frequency sound waves classification.