Ultrasonic sound refers to sound pressure with a frequency greater than the human audible range (20 Hz to 20 KHz). When an ultrasonic wave propagates through a medium, the molecules in that medium vibrate at a very short distance in a direction parallel to the longitudinal wave. During this vibration, the momentum is transferred between the molecules. This causes the wave to pass through the medium.
Operating principle
The principle used in the measurement of velocity (ν) is based on the exact determination of the wavelength (λ) in the medium. Ultrasonic waves of known frequency (f) are produced by a quartz crystal fixed at the bottom of the cell. These waves are reflected by a moving metal plate held parallel to the quartz crystal. If the separation between these two plates is exactly an integer multiple of the wavelength of the sound, stationary waves are formed in the medium. This acoustic resonance gives rise to an electric reaction on the generator that drives the quartz crystal and the anode current of the generator becomes a maximum.
If the distance is now increased or decreased, and the variation is exactly half the wavelength (λ / 2), or multiple of it, the anode current becomes maximum. From the knowledge of the wavelength (λ) the velocity (v) can be obtained by the relation: Velocity = Wavelength x Frequency
v = λ x f
The Ultrasonic Interferometer consists of the following parts: