ultrasonic waves (ultrasound) Sound of frequency greater than 20 kHz, the limit of hearing for humans. Ultrasonic waves are generated using two phenomena. In the first called the piezoelectric effect * , slices of crystals when subjected to an alternating potential difference, across its opposite faces, alternately contract and expand producing sound. Input ac voltage of high frequency generates ultrasonic waves. In magnetostriction effect, the length of a rod of ferromagnetic material changes when subjected to an alternating magnetic field produced by a coil and an oscillator.

Ultrasonic waves find wide variety of applications. In underwater communications these waves are found suitable because of its low absorption in water. In engineering its applications are, flaw detection in metal structures, drilling, soldering and cleaning of surfaces in electronic industry. In medical diagnostics it is an important tool for imaging the human body. Its chief advantage over X rays is that it does not have any harmful effect.