1. Theory

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

• infrasound with a frequency between 0 and 16 Hz;

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

• ultrasound with a frequency between 16 kHz and 1 GHz;

• hypersonics with a frequency greater than 1 GHz.

Ultrasound waves are elastic waves that, when propagating in an elastic medium, possess all the general properties of periodic progressive waves, such as propagation, attenuation, and reflection (Figure 1 ).