PHYSICAL PRINCIPLES of Ultrasound

Ultrasonic imaging is based on ultrasound, which is soundproduced at frequencies beyond those detectable in humanhearing, that is, >20 kHz. In the same way that ultraviolet(UV) light is invisible to the human eye, ultrasound isinaudible to the human ear. Often objects that serve ascarriers for ultrasound waves need to be treated as waveguides.Nonlinear effects become apparent for ultrasoundpropagation when leaving the range of elastic deformationduring the propagation of waves through a medium.Physical material constants form ultrasound parameters,for example the speed of sound or the attenuation ofsound. Very high frequency sound waves are treated byquantum acoustic laws. Historically, ultrasound wasproduced by oscillating platelets, or pipes.

Magnetostrictionand the piezoelectric effect followed, and are still very much relevantmechanisms for medical and industrial ultrasound. In1918, it was found that the use of oscillating crystals couldbe used to stabilize frequencies. The upper frequency forsound in a given solid material is determined by theseparation of neighboring atoms in the host medium. Thisupper frequency limit is met when neighboring atoms,assuming the linear chain model, oscillate with a 1808phase shift, the so-called optical branch of oscillations ina solid.