State whether the following statement is true or false: Sound waves are transverse,whereas light waves are longitudinal in nature.

$(a)$ $A$ 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?

State three conditions necessary for sound to be heard.

Sound made in front of a tall building $18 \, m$ away is repeated. Name the phenomenon and briefly explain it.

What is a crest?

The stem of a tuning fork is pressed against a table top. Answer the following questions:
$(i)$ Would the above action produce any audible sound?
$(ii)$ Does the above action cause the table to set into vibrations?
$(iii)$ If the answer above is yes,what type of vibrations are they?
$(iv)$ Under what conditions does the above action lead to resonance?