$(a)$ 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.

(C) Intensity of sound is defined as the amount of sound energy passing per unit time through a unit area held perpendicular to the direction of propagation of sound.
$(b)$ The conditions for an echo to be heard clearly are:
$(i)$ The time interval between the original sound and the reflected sound must be at least $0.1 \, s$.
$(ii)$ The minimum distance between the source of sound and the obstacle must be approximately $17 \, m$.
$(c)$ Given: Height $h = 44.1 \, m$,initial velocity $u = 0$,acceleration due to gravity $g = 9.8 \, m \, s^{-2}$.
Step $1$: Calculate the time taken by the ball to reach the water surface using $S = ut + \frac{1}{2}gt^2$.
$44.1 = 0 \times t + \frac{1}{2} \times 9.8 \times t^2$
$44.1 = 4.9 \times t^2$
$t^2 = \frac{44.1}{4.9} = 9$
$t = 3 \, s$.
Step $2$: Calculate the time taken by the sound to travel back to the observer.
Total time = $3.13 \, s$.
Time for sound = $3.13 - 3 = 0.13 \, s$.
Step $3$: Calculate the velocity of sound.
$V = \frac{\text{Distance}}{\text{Time}} = \frac{44.1 \, m}{0.13 \, s} \approx 339.23 \, m \, s^{-1}$.