Give four practical uses of ultrasonic vibrations.

State whether the following statement is true or false:
The minimum distance between a source and the reflector of sound should be $34\, m$.

$A$ person fires a gun standing at a distance of $55 \, m$ from a wall. If the speed of sound is $330 \, m s^{-1}$,find the time taken for an echo to be heard. (in $s$)

The stem of a tuning fork is pressed against a table top. Answer the following questions:
$(i)$ Would the above action produce any audible sound?
$(ii)$ Does the above action cause the table to set into vibrations?
$(iii)$ If the answer above is yes,what type of vibrations are they?
$(iv)$ Under what conditions does the above action lead to resonance?

The rear view mirror of a motorbike starts vibrating violently at a particular speed of the motorbike.
$(a)$ Why does this happen?
$(b)$ What is the name of the phenomenon taking place?
$(c)$ What could be done to stop the violent vibration?

$(a)$ What is meant by intensity of sound?
$(b)$ Mention the conditions for an echo to be heard clearly.
$(c)$ $A$ ball is dropped into a pond from a height of $44.1 \, m$. The splash of sound is heard $3.13 \, s$ after the ball is dropped. Determine the velocity of sound in air.