What is $SONAR$?

$A$ 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.

$(a)$ Which of the following frequencies are audible to human ears: $2 \, Hz$,$5 \, Hz$,$20 \, Hz$,$200 \, Hz$,$2000 \, Hz$?
$(b)$ How are moths of certain families able to escape capture?

Infrasound can be heard by

$(i)$ Define the time period of a wave.
$(ii)$ Give the relation among speed of sound $V$,wavelength $\lambda$ and its frequency $\nu$.

If the frequency of a tuning fork is $400 \, Hz$ and the speed of sound in air is $340 \, m s^{-1}$,find how far sound travels when the tuning fork makes $16$ vibrations. (in $, m$)