Generally, there are 4 methods to detect ultrasonic waves, which are named as follows:

1) Piezoelectric sensor

2) Sensitive flame method

3) Thermal sensor method

4) Kundt's tube method

Thermal sensor method:

• A platinum thermoelectric sensor connected to a balanced meter bridge acts as the sensor for this method.
• When ultrasonic waves pass through the medium, the pressure changes are adiabatic in nature, i.e., at the nodes (areas of compression), the air medium heats up and (in the rarefied areas), the medium cools down.
• These temperature changes are detected by the sensitive platinum thermoelectric sensor, and thus its resistance changes.
• The change in resistance causes the bridge to become unbalanced.
• Thus, the unbalance of the bridge acts as an indicator for the presence of ultrasonic waves.

Piezoelectric sensor:

• The direct and inverse piezoelectric effects presented by crystals like quartz are used to detect ultrasonic waves.
• We know that, in the direct piezoelectric effect, charges are generated in the crystal due to mechanical vibrations.
• When ultrasonic waves strike a quartz crystal, they become charged on the opposite surfaces of the crystal.
• The charge developed in this way is very weak, and therefore it is suitably amplified before confirming the presence of ultrasonic waves.

Sensitive flame method:

• This is a very simple method in which a flame is brought into the path of ultrasonic waves for detection.
• When the flame is placed in the path of ultrasonic waves, it flickers at the node position, while it remains stable at the antinode position.
• This behavior of the flame confirms the presence of ultrasonic waves.

Kundt's tube method:

• Kundt's tube is a narrow long hollow cylindrical tube, one end of which is open and the other end has a moving piston.
• Initially, lycopodium powder is evenly sprinkled inside the tube.
• Then ultrasonic waves are passed through the tube, and the piston is adjusted to create a standing wave pattern.
• At this point, nodes and antinodes are formed by the standing waves, with lycopodium powder accumulating at the nodes and being thrown off at the antinodes.
• At this point, the distance (d) between successive nodes is measured from the pile of lycopodium powder, and the wavelength is calculated (d = λ / 2).
• From the value of the wavelength, the presence of ultrasonic waves can be identified.

Light meter:

• This is used for measuring the amount of light.
• In photography, it is designed either to measure the light reflected from the object or to measure the light falling on the object.