Question

The sounds having a frequency of less than 20Hz are called ultrasonic.
A. True
B. False

Answer 1

Humans are audible in the frequency range of 20 to 20000 Hz. Longitudinal waves have opposite properties as that of transverse waves. Ultrasonic wave’s frequency comes under mechanical waves. Ultra word means ‘beyond’ and sonic word means ‘sound’. It is exactly the opposite of infrasonic waves.

Complete step-by-step solution:
We know that ultrasonic waves have a higher frequency than 20,000Hz. It starts from 20,000 Hz and above and ultrasonic waves have a property of longitudinal waves. Therefore ultrasonic waves are longitudinal waves. A human can hear a frequency range of 20 to 20,000 Hz only. This is the audible range of frequency which humans can hear. So the maximum frequency humans can hear is 20,000. More than that humans cannot hear.
So we can say that assertion and reason both are correct and the reason is not the explanation for assertion because we know that ultra means beyond and it is beyond 20,000 that is it is beyond the audible range. So we can say that the wave which has frequency beyond the audible frequency must be an ultrasonic wave.
Hence the above, given in the question, the statement is false. Therefore the correct option is (B).

Additional information:
First, we will see what a longitudinal wave is then we will see how it is included in ultrasonic waves.
Longitudinal wave: The wave motion in which the particle of the material medium vibrates in the direction of waves propagating, is called the longitudinal wave. Take an example of sound waves in air, waves produce in a solid due to the longitudinal vibration. The medium required for the propagation of this wave need not be incompressible. In this wave compression and rarefactions are formed. Because of this, it can carry a large amount of energy.
Ultrasonic waves: The wavelength of ultrasonic waves is very small. It is a longitudinal wave with high frequency and hence they can carry a high amount of energy. They can travel with high velocity in water and suffer little loss and absorption. The velocity is constant for a homogeneous medium. It goes under reflection n and refraction, etc.

Note: Do not get confused between ultrasonic waves and ultraviolet radiation. Ultrasonic waves coms under a mechanical wave and ultraviolet radiation come under electromagnetic waves. Mechanical and electromagnetic waves are two types of waves. Transverse and longitudinal are types of mechanical waves.