1. Theory

The sound spectrum is generally divided into four intervals defined according to the wave emitted:

• infrasound with frequencies between 0 and 16 Hz ;

• audible sounds with frequencies between 16 Hz and 16 kHz ;

• ultrasonic frequencies between 16 kHz and 200 MHz ;

• hypersound frequencies above 100 GHz ;

Ultrasound is an elastic wave with all the general properties of sound waves, such as the deformation of the medium in which it propagates (figure 1 ).