Ans)An Ultrasonic Interferometer is a easy and direct device to determine the ultrasonic velocity in liquids with a high degree of accuracy.
The principle used in the measurement of velocity (V) is based on the accurate determination of the wavelength (λ) in the medium. Ultrasonic waves of called as frequency (f) are produced by a quartz plate fixed at the bottom of the cell. These waves are reflected by a movable metallic plate kept parallel to the quartz plate. If the separation among these two plates is exactly a total multiple of the sound wavelength, standing waves are formed in the medium. This acoustic resonance gives rise to an electrical reaction on the generator driving the quartz plate and the anode current of the generator becomes maximum.
If the distance is now increased or decreased and the variation is exactly one half wavelength (λ/2) or multiple of it, anode current again becomes maximum. If the separation among quartz plate and metallic plate is changed by d among two successive maximum anode current, then d = λ/2...(1) From the knowledge of wavelength (λ), the velocity (V) can be obtained by the relation : V = λ × f =2df....(2)