How the Ear Works
Outer Ear
Return to Hearing Loss Due to Noise
The Anatomy of the ear has three primary anatomical divisions: the outer ear, the middle ear, and the inner ear.
The outer ear, which collects sound energy, consists of the external part (called the pinna or choncha); the auditory canal (the meatus), which is a bayonet-shaped tube about 1 in long that leads inward from the external part; and the eardrum (the tympanic membrane) at the end of the auditory canal. The resonant properties of the auditory canal contribute to the sensitivity of the ear in the frequency range 2000 to 5000 Hz, enhancing sound-pressure levels by as much as 12 dB (Davies and Jones, 1982).
The middle ear is separated from the outer ear by the tympanic membrane. The middle ear includes a chain of three small bones called ossicles (the malleus, the incus, and the stapes). These three ossicles, by their intercon- nections, transmit vibrations from the eardrum to the oval window of the inner ear. The stapes acts something like a piston on the oval window, its action transmitting the changes in sound pressure to the fluid of the inner ear, on the other side of the oval-window membrane. Owing to the large surface area of the tympanic membrane and the lever action of the ossicies, the pressure of the foot of the stapes against the oval window is amplified to about 22 times that which could be effected by applying sound waves directly to the oval window (Guyton, 1969, p. 300). The middle ear also contains two muscles attached to the ossicies. The tensor tympani muscle attaches to the malleus, and the stapedius muscle attaches to the stapes.
In response to loud noises, the stapedius muscle tightens and reduces sound transmission to the inner ear, thus providing protection against intense sounds. This reaction is called the acoustic, or aural, reflex. The reflex occurs when the ear is exposed to a sound that is about 80 dB above threshold level; the reflex appears to be more responsive to broadband sounds than to pure tones and to lower frequencies than to higher frequencies (Kryter, 1985). The reflex can provide as much as 20-dB attenuation. The muscles will remain flexed for up to 15 min in the presence of intense steady-state noise. When the ear is exposed to intense impulse noise, such as gunfire, there is a delay (or latency) of about 35 to 150 milliseconds (ms) before the muscles contract.
The inner ear, or cochlea, is a spiral-shaped affair that resembles a snail. If uncoiled, it would be about 30 mm long, with its widest section (near the oval window) being about 5 or 6 mm. The inner ear is filled with a fluid. The stapes of the middle ear acts on this fluid like a piston, driving it back and forth in response to changes in the sound pressure. These movements of the fluid force into vibration a thin membrane called the basilar membrane, which in turn transmits the vibrations to the organ of Corti. The organ of Corti contains hair cells and nerve endings that are sensitive to very slight changes in pressure. The neural impulses picked up by these nerve endings are transmitted to the brain via the auditory nerve.