Before you can understand sound, you need to understand how your ears work.
Sound travels through the air as waves. The visible part of the ear, the pinna, collects and funnels these sound waves into your ear canal. The sound waves travel through your ear canal and hit your eardrum (tympanic membrane). This causes the ossicles (three small bones called the malleus, the incus and the stapes) to vibrate. The vibrating ossicles transmit the sound waves to the cochlea. The cochlea contains small hair cells (cilia) that convert sound waves into signals. These signals are then transmitted over nerves to the temporal lobe of your brain. Your brain then deciphers the signals. And this is how you hear sound!
In physics, sound waves travel through solid, liquid, or gas mediums as longitudinal waves (see classifying waves section). Sound waves move through each of these mediums by vibrating the particles in the matter in the direction of the wave’s movement. A sound wave consists of alternating compressions and rarefactions, or regions of high pressure and low pressure, moving at a certain speed.
(A) Air at equilibrium, in the absence of a sound wave. (B) Compressions and rarefactions that constitute a sound wave. (C) A transverse representation of the wave, showing amplitude, A, and wavelength, λ.
The speed of sound depends on the type of medium it is traveling through.
The molecules in solids are packed very tightly. Liquids are not packed as tightly. And gases are very loosely packed. This enables sound to travel much faster through a solid than a liquid or a gas since the sound waves can more readily transfer vibrations from one particle to another in a densely packed medium. Sound travels about four times faster in water than it does in air and about thirteen times faster in wood than air.