Ultrasonic waves can be detected by various methods as listed below:
(a)Using radiometer: Ultrasonic waves can be detected using Radiometer. In this method ultrasonic beam is made to fall on a thin mica fan suspended by a thin wire carrying a small mirror from one end of a light rod. Due to pressure exerted by ultrasonic waves the fan gets detected along with the mirror. The deflection can be noted by a lamp and scale arrangement. A beam of light is made incident on the mirror and reflected beam falls back on the origin of scale attached to lamp. When mirror shows deflection by angle 0, then reflected beam on the scale defects by angle 20. Since 20 can be noted from scale, hence deflection of mirror can be found. The deflection is directly proportional to the intensity of ultrasonic waves. Hence we can calculate the intensity of ultrasonic waves with this method.
(b) Kundt’s tube method: Kundt’s tube filled with lycopodium power can also be used for detecting ultrasonic waves whose wavelength is of the order of a few millimeters. When ultrasonic waves pass through tube then stationary waves are formed due to super position of incident and reflected waves. Heaps are formed at the position of nodes. The distance between adjacent nodes is calculated, which is equal to half the wavelength of ultrasonic waves. Hence with this method wavelength of ultrasonic waves can be calculated.
(c) Thermal method for detection of ultrasonic waves: When ultrasonic waves pass through a medium, then alternative compressions and rarefactors are formed. At compression, particles of medium are brought closer and collisions between them increases. As a result of this the temperature of medium increases at compressions. On the other hand, the temperature of medium decreases at rarefaction due to the fact that particles of medium go move away from each other and frequency of collisions is decreased. Thus if we introduce a platinum resistance thermometer in the path of ultrasonic waves in a medium, and move the thermometer along the direction of propagation of waves then temperature reading of the thermometer changes alternatively confirming the presence of ultrasonic waves in the medium. Some times stationary waves are formed in the medium due to superposition of incident and reflected ultrasonic waves. In such a case nodes and antinodes are formed in the medium. At nodes the pressure varies alternatively resulting in cooling and heating effect. Thus a platinum resistance introduced at nodes will store in temperature, while no change in temperature is recorded at antinodes. Thus if we move thermometer in the medium, then resistance of platinum wire of thermometer will change alternatively confirming the ultrasonic waves in medium.
These are the methods to detect ultrasonic waves. If you know another, you can discuss in the comments section.