The mechanism of hearing in the ear. In these two illustrations above, one natural a diagrammatic, the dashed red lines show the route along which sound vibrations are from the external auditory meatus (ear canal) to the organ of Corti in the cochlea.
When sound waves reach the ear they pass down the ear canal (auditory canal) until they arrive at the eardrum. The sound waves vibrate the eardrum and the vibrations are transmitted to the handle of the small bone in the middle ear known an the hammer (malleus). From the head of the hammer the vibrations pass to the anvil (incus) and then to the stirrup (stapes). The "sole plate" of this last bone fits across the small aperture in the wall of the bony labyrinth known as the oval window (fenestra vestibuli). The vibrations of the stapes thus pass through this aperture, along the fluid in the scala vestibuli of the cochlea , through the helicotrema and down the scala tympani to be dissipated through the fenestra tympani.
As the vibrations pass along the perilymph of the scala vestibuli they are transmitted to the endolymph on the cochlear duct and thus to the basilar membrane. High-pitched sounds cause resonance of the basilar membrane at the bottom of the cochlea, whereas sounds of lower pitch cause resonance in places correspondingly nearer to the helicotrema. The vibrations of a part of the basilar membrane shake the hair cells in the adjacent parts of the organ of Corti; this causes them t emit nerve impulses which pass along the cochlear division of the auditory nerve to the brain.
The fibers of the cochlear nerve are connected to the cortex of the auditory center in the superior temporal gyrus. Each fiber of the nerve serves only a short length of the organ of Corti, and is provided with its own particular area in the auditory cortex. Consequently a pure note, which causes vibration of only a few hair cells, is responsible for the excitation of only a small, though exclusive area of cortex. It is this association of each part of the organ of Corti with a particular area of the auditory cortex which makes possible the recognition of sounds of different pitch.
The areas of temporal cortex adjacent to the superior temporal gyrus are concerned with the memory of sounds and their association with the other senses and the emotions.
Range of hearing
Human hearing extends from a frequency of about 20 cycles per second (Hz) to 20,000 cycles. Some animals are able to generate sounds far beyond this range: the chart shows the frequencies generated by bats, porpoises, and grasshoppers, and for comparison the frequency-production ranges of birds and dogs. Musical instruments have two kinds of frequency-range: the range of notes that can be played (shown as a solid line) and the range of overtones that go to make up the characteristic sounds of the instrument (broken line). The ranges shown are those of the violin, the saxophone family (from bass to soprano), and the harp. For reference, the note middle C is marked in yellow.