What is the audible range for human beings?

Sound travels in air if

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 $342 \, m s^{-1}$.

What is a crest?

$(i)$ Even if a loud explosion were to take place at any place on the moon,it would not be heard at a nearby point. Give reason.
$(ii)$ Explain in brief the dependence of speed of sound on the nature of the material medium and temperature.
$(iii)$ Identify the two factors on which the loudness of sound depends.

What do you understand by the term ultrasonic vibrations?