What is the audible range for human beings?

$A$ 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.

$(i)$ How do bats make use of ultrasonic waves to catch their prey? Explain.
$(ii)$ $A$ radar signal is reflected by an aeroplane and is received $2 \times 10^{-5} \text{ s}$ after it was sent. If the speed of these waves is $3 \times 10^{8} \text{ m s}^{-1}$,how far is the aeroplane?

$A$ girl is sitting in the middle of a park of dimension $12 \,m \times 12 \,m$. On the left side of it there is a building adjoining the park and on the right side of the park,there is a road adjoining the park. $A$ sound is produced on the road by a cracker. Is it possible for the girl to hear the echo of this sound? Explain your answer.

Mention one practical use of echoes.

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