Angie Stein, age 13, of Dayton, Ohio, for her question:
HOW DO WE HEAR?
Hearing and equilibrium are handled by the ear. It is an organ composed of three divisions: external, middle and internal. The greater part of the ear's equipment is enclosed within the temporal bone of the head.
The external ear includes the auricle or pinna (the external flap of the ear) and the external auditory canal, which is about an inch in length. The middle ear, on the inner side of the eardrum, embodies the mechanism for the conduction of sound waves to the internal ear. It is a narrow passage, or cleft, that extends vertically for about 1/2 inch and for about the same distance horizontally.
Traversing the middle ear is a chain of three small, movable bones called the ossicles: the maileus, or hammer handle; the incus, or anvil; and the stapes, or stirrup. The ossicies connect the eardrum acoustically to the middle ear.
The internal ear, or labyrinth, is the part of the temporal bone containing the organs of hearing and equilibrium to which the filaments of the auditory nerve are distributed. It is separated from the middle ear by the oval window.
The internal ear consists of membranous canals housed in a dense portion of the temporal bone and is divided into the cochlea (snail shell), the vestibule and three semicircular canals. All these canals communicate with one another and are filled with a gelatinous fluid called endolymph.
Sound waves are carried through the external auditory canal to the eardrum, causing it to vibrate. These vibrations are communicated by the ossicular chain in the middle ear through the oval window to the fluid in the inner ear. The movement of the endolymph stimulates a set of fine hairlike projections called hair cells in the cochlea. The hair cells transmit signals directly to the auditory nerve, which carries information to the brain.
The overall pattern of response of the hair cells to vibration of the endolymph encodes information about sound in a way that is interpretable by the brain's auditory centers.
The range of hearing, like that of vision, varies in different persons. The maximum range of human hearing includes sound frequencies from about 16 to 28,000 cycles per second. The least noticeable change in tone that can be picked up by the ear varies with pitch and loudness.
A change of vibration frequency (pitch) corresponding to about 0.03 percent of the original frequency can be detected by the most sensitive human ears in the range between 500 and 8,000 vibrations per second. The ear is less sensitive to frequency changes for sounds of low frequency or low intensity.
The sensitivity of the ear to sound intensity (loudness) also varies with frequency. Sensitivity to change in loudness is greatest between 1,000 and 3,000 cycles, where a change of one decibel can be detected and becomes less when sound intensity levels are lowered.