Sound occurs when a vibrating source causes rapid variations in the average density of air molecules above and below current atmospheric pressure; sound is perceived when these pressure fluctuations make eardrums vibrate. The changes in atmospheric pressure are called sound pressure, and the fluctuations in pressure are sound waves. Hair cells in the cochlea (inner ear) convert vibrations into electrical signals that then go to the brain.
Sound is a mechanical disturbance that propagates through some medium, such as air or water. Even vibrations that are not audible to the human ear, such as those that come from dog whistles or sonar equipment, fall into the classification of sound.
A sound source, whether it is a diaphragm or a guitar, produces vibrations in the surrounding medium (air, liquid or solid). These vibrations propagate away from the source and form a sound wave (sound waves can be longitudinal or transverse). The medium can reflect, refract or attenuate the sound wave. Pressure within the medium, motion of the medium and viscosity of the medium affect the speed and reach of the sound wave.
The outer part of the ear channels sound waves into the ear canal. Outer hair cells in the cochlea amplify soft sounds and dampen loud sounds. Inner hair cells transfer sound information to the auditory nerves for processing.