Name the waves of frequency greater than $20,000 \,Hz$. Give one use of these waves.

$(a)$ What is the audible range of the average human ear?
$(b)$ Explain how ultrasound is used to clean spiral tubes and electronic components?

State whether the following statement is true or false:
The audible range of frequencies for human beings is $20 \, Hz - 20 \, kHz$.

$A$ hospital uses an ultrasonic scanner to locate tumours in a tissue. What is the wavelength of sound in a tissue in which the speed of sound is $1.7 \, km \, s^{-1}$? The operating frequency of the scanner is $4.2 \, MHz$ $(1 \, MHz = 10^{6} \, Hz)$.

$(a)$ What is reverberation? Write two ways of reducing reverberation.
$(b)$ Distinguish between tone and note.
$(c)$ With the help of a simple diagram,explain how defects in a metal block can be detected using ultrasound.

An astronaut cannot hear his companion on the surface of the Moon because