Two astronauts cannot hear each other on the moon. Why?

Sound of explosions taking place on other planets is not heard by a person on the Earth. Give the reason.

$A$ 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 $s$)

If the frequency of a tuning fork is $400 \, Hz$ and the speed of sound in air is $340 \, m/s$. Find how far sound travels when the tuning fork makes $16$ vibrations. (in $m$)

$(a)$ 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.

Name three animals which can hear ultrasonic vibrations.