Presence of ultrasonic waves can be detected by using any one of the following methods:
1. Quartz crystal method
This method of detecting ultrasonic waves is based on the principle of piezoelectric effect. When one pair of opposite faces of the quartz crystal is exposed to ultrasonic waves, in the other pair of opposite faces, charges get developed as shown in figure. These changes are amplified and detected using suitable electronic circuits.
This method of detecting ultrasonic waves is based on the principle of piezoelectric effect. When one pair of opposite faces of the quartz crystal is exposed to ultrasonic waves, in the other pair of opposite faces, charges get developed as shown in figure. These changes are amplified and detected using suitable electronic circuits.
2. Thermal detection method
Principle
When the ultrasonic waves are propagated through a medium, the temperature of the medium changes due to alternate compressions and rarefactions. In the case of stationery waves, there is a change in the temperature at nodes and no change in temperature at antinodes. Based on this principle, by sensing the change in temperature using suitable components, the ultrasonic waves can be detected.
When the ultrasonic waves are propagated through a medium, the temperature of the medium changes due to alternate compressions and rarefactions. In the case of stationery waves, there is a change in the temperature at nodes and no change in temperature at antinodes. Based on this principle, by sensing the change in temperature using suitable components, the ultrasonic waves can be detected.
It is the most commonly used method to detect ultrasonic waves. In this method, a fine platinum wire included in Callender and Griffith bridge circuit arrangement, as shown in figure, is used to detect the ultrasonic waves.
This circuit is placed in the region where the presence of ultrasonic waves is to be detected. If in the medium, there is a presence of ultrasonic waves, then at nodes due to alternate compressions and rarefactions, alternate heating and cooling effect gets produced in the platinum wire. Thus, a change in temperature brings about a change in electrical resistance of the platinum wire which can be detected (the balanced position of the circuit gets disturbed) with the help of the electrical circuit shown in figure. At antinodes, the temperature remains constant which is indicated by undisturbed balance position of the bridge.
3. Sensitive flame method
This is qualitative method to detect ultrasonic waves. When a narrow sensitive flame is moved in medium of ultrasonic waves, the flame remains steady at the antinodes but flickers at nodes due to the maximum change in pressure. Thus, by observing the behaviour of the flame, the ultrasonic waves presence can be detected.
This is qualitative method to detect ultrasonic waves. When a narrow sensitive flame is moved in medium of ultrasonic waves, the flame remains steady at the antinodes but flickers at nodes due to the maximum change in pressure. Thus, by observing the behaviour of the flame, the ultrasonic waves presence can be detected.
4. Kundt's tube method
This method is used to detect ultrasonic waves of low frequency.
This method is used to detect ultrasonic waves of low frequency.
A kundt's tube apparatus, shown in figure, consists of a long glass tube of more than 1 m and 5 cm in diameter kept horizontally with two supports on a wooden base board.
One end of tube is fitted with an adjustable piston rod with cork. A quartz crystal placed in between the two metal plates is placed at the mouth of the other end of the tube.
 The glass tube is throughly dried by passing through a hot blast of air. A thin layer of dry Lycopodium or cock powder is spread along the entire length of the glass tube. When stationary ultrasonic waves generated by using piezoelectric method are passed in the air contained in the long glass tube, the Lycopodium powder gets collected by the form of heaps at the nodes and is blown off at the antinodes. Thus, the formation of heaps at nodes confirm the nature of the transmitted sound wave to be ultrasonic waves.
One end of tube is fitted with an adjustable piston rod with cork. A quartz crystal placed in between the two metal plates is placed at the mouth of the other end of the tube.
 The glass tube is throughly dried by passing through a hot blast of air. A thin layer of dry Lycopodium or cock powder is spread along the entire length of the glass tube. When stationary ultrasonic waves generated by using piezoelectric method are passed in the air contained in the long glass tube, the Lycopodium powder gets collected by the form of heaps at the nodes and is blown off at the antinodes. Thus, the formation of heaps at nodes confirm the nature of the transmitted sound wave to be ultrasonic waves.
The average distance between two consecutive heaps gives the value of half of the wavelength. Thus using the relation v=fλ=2fd, the velocity of the ultrasonic waves in the medium can also be calculated.
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