$(i)$ What does the acronym $SONAR$ stand for?
$(ii)$ What is the audible range of sound for human beings?
$(iii)$ How do the bats search and catch their prey in dark night?

State two practical uses or applications of echoes.

$A$ 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.

$(i)$ What causes the reverberation of thunder sound?
$(ii)$ $A$ $SONAR$ device on a submarine sends a signal and receives an echo $5 \, s$ later. Calculate the speed of sound in water if the distance of the object from the submarine is $3625 \, m$.

The speed of sound is $310 \, m s^{-1}$. $A$ person fires a gun and the echo is heard after $1.5 \, s$. Calculate the distance of the person from the cliff from which the echo takes place. (in $, m$)

If the density of a medium through which sound is propagating is minimum,then this point is called: