Acoustic waves are mechanical waves that need a material
medium to propagate. Usually, they are classified by taking the human
audible frequency as reference. This range could be placed between
20 Hz and 20 kHz. Lower frequencies are referred to as
infrasound and higher frequencies as ultrasound. The ultrasound
waves suffer changes in their properties (velocity, attenuation, frequency
spectrum, . . .) when travelling through a medium. The
study of these variations is used in diagnosis applications to characterize
the medium. In these applications, the frequency of the
waves is in the range of MHz and the power applied is not higher