what is sound

sound can be understood as a form of energy which gives us the sensation of hearing . 
sound requires a medium to travel ,without medium sound cannot propagates.

propagation of sounds in air:-
sound travels in air making continuous crest and trough alternalively or compression and rarefaction.
example:-- sound coming from a loud speaker to our ear.

propagation of sounds in liquids:-
as we know sound is a form of energy , in liquids it propagates with the help of wave energy.

example:- a wave which can be felt in liquids when we drop a stone in a pond.

propagation of sounds in solids:-

any solid is made up of particles , when we hit a solid means we are providing energy due to which the particles of solid starts vibrating and the sound energy travells from one part of the solid to other.
example:-when we hit on the one side of the rod with our ear on the other side we can easily hear the sound on other side.

speed of sound is maximum in solids ,mediocre in liquids and least in air.