Sound waves travel and affect the gas molecules that compose the air by putting them into motion. Sound waves traveling through the air consist of regions where the air molecules are closer together (compressed) and regions where those molecules are further apart (rarified). The amplitude of sound is proportional to the differences in air molecule densities between the compressed and rarified air.
Amplitude is expressed as a log scale in decibels. The pitch of the sound is determined by the frequency of the sound waves measured as the number of waves per second (
hertz: Hz).
To detect sound, the properties of the sound wave must be transmitted to a solid structure. That transition occurs at the ear drum where sound waves cause the tympanic membrane to vibrate at the frequency of the sound waves. That vibration is passed along the middle ear across three bones: from the
malleus (hammer), to the
incus (anvil), and finally, onto the stapes (stirrup). The stapes is in contact with the cochlea at the oval window. Once the vibration reaches the stapes, it is passed to the
oval window, which vibrates the fluid of the cochlea. There are two membranes within the cochlea (basilar and vestibular) that separate the
cochlea into three fluid filled compartments (scala vestibuli, scala tympani, and scala media). The tympani and vestibuli are filled with perilymph while the media is filled with endolymph. Vibration is passed from the oval window to the scala vestibuli onto the scala tympani. This will cause the basilar and
vestibular membranes to vibrate as well. The organ of Corti, which contains the hair cells that detect sound, is located on the top of the basilar membrane. The stereocilia of the hair cells are embedded within the tectorial membrane. When the basilar membrane vibrates, the stereocilia that are attached to the tectorial membrane are sheared. This movement bends the stereocilia, causing a change in membrane potential within the hair cell.
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