DETECTION OF ULTRASONIC WAVES
Quartz crystal method :
This method is based on the principle of Piezoelectric effect. When one pair of opposite faces of a quartz crystal is exposed to the ultrasonic waves, the other pair of opposite faces develop opposite charges. These charges are amplified and detected using an electronic circuit.
Thermal detectors :
A platinum wire is placed in the region to be tested for ultrasonic waves. At nodes, due to alternate compressions and rarefactions, alternate heating and cooling is produced. Change in temperature at the node brings about changes in the electrical resistance of the platinum wire. This is detected by means of a Wheatstone’s bridge. No change in temperature occurs at the antinode.
Kundt’s tube method :
If the wavelength of the ultrasonic waves is greater than a few millimeters, a Kundt’s tube can be used to form a stationary wave pattern with well defined nodes and antinodes. In the air or gas medium, lycopodium powder collects in the form of heaps at the nodes. In liquid medium, powdered coke is used to detect the position of nodes.
