What is a crest?

What is an echo? When can we distinctly hear the echo of a sharp sound? Why cannot we hear an echo in small halls?

The pulse rate of a man is $80$ beats in one minute. Calculate its frequency. (in $ Hz$)

$(a)$ $A$ mobile ringing inside a vacuum chamber cannot be heard outside. Why?
$(b)$ Represent a transverse wave graphically.
$(c)$ What is meant by loudness of sound? On what factor does it depend?

$(a)$ What are the basic factors on which the speed of sound in a medium depends?
$(b)$ Why are sound waves called mechanical waves?
$(c)$ How will you differentiate a high-pitch sound from a low-pitch sound with the help of a graph?

Define the following characteristics of sound:
$(a)$
$(i)$ Pitch
$(ii)$ Loudness
$(iii)$ Quality or Timbre
$(b)$ $A$ boy receives his echo $3 \, s$ later. Find the distance of the reflecting surface from the boy. Speed of sound in air is $342 \, m s^{-1}$.