Ultrasonic Waves –

The wave having frequency above 20KHz are called as ultrasonic waves. They can produce a highly focussed and directed beam.

Magnetostriction Effect –

It is used for producing ultrasonic waves and is explained as follows –

When a rod of ferromagnetic material is placed in steady magnetic field parallel to its length, the rod undergoes a small change in length. This phenomenon is called magnetostriction  effect.

The change in length is proportional to intensity of magnetization if the magnetic field is well above magnetic saturation. If magnetic filed is below magnetic saturation, the change in length is proportional to B2.

$\therefore \dfrac {\partial l}{\partial x}=kB^{2}$ where, k=material constant & $\dfrac {\partial l}{\partial x}$ = change in length / unit length of rod.

k is +ve, if rod expands on magnetization.

k is –ve, if rod constants on magnetization.