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

What is reverberation?

Draw a curve showing density or pressure variations with respect to distance for a disturbance produced by sound. Mark the position of compression and rarefaction on this curve. Also,define wavelengths and time-period using this curve.

$(a)$ $A$ boy strikes one end of a long pipe with a stone. Another boy who keeps his ear close to the other end of the pipe hears two sounds in a short interval of time. Explain why.
$(b)$ List two uses of ultrasonics.

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

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 $, m$)