Hertz is a unit of

State any two characteristics of a wave motion.

$A$ vibrating tuning fork is placed over the mouth of a burette filled with water. The tap is opened and the water level gradually falls. It is observed that the sound becomes the loudest for a particular length of air column.
$(i)$ What is the name of the phenomenon taking place when this happens?
$(ii)$ Why does the sound become the loudest?
$(iii)$ How does the frequency of the loud sound compare with that of the tuning fork?
$(iv)$ State the unit for measuring loudness.

$(a)$ What is the cause of the sensation of sound due to the rolling of thunder?
$(b)$ Give the audible range of the human ear.
$(c)$ What are infrasonic and ultrasonic waves?

$A$ man stands $33\, m$ from a high wall and claps his hands. Will he hear an echo? (Given,velocity of sound is $330\, m s^{-1}$)

What is the audible range for human beings?