Anatomy, Physiology, and Effects of Noise on Hearing
From pinna to hair cell
Sound passes the pinna and ear canal (which resonate near 2–4 kHz, amplifying mid-to-high frequencies), strikes the tympanic membrane, and travels through the malleus, incus, and stapes to the oval window. The middle ear matches air-borne impedance to the fluid-filled cochlea. The acoustic reflex stiffens the ossicular chain for loud sound, but it is too slow and too small to protect against impulsive noise.
In the cochlea the traveling wave peaks basally for high frequencies. Outer hair cells, roughly three times as numerous as inner hair cells, are the most vulnerable; their loss in the 3–6 kHz basal region produces the classic 4 kHz notch of noise-induced hearing loss. Hair cells do not regenerate in mammals, so sensorineural loss is permanent. Conductive loss (wax, perforation, ossicular disease) affects air conduction and can be bypassed by bone conduction; noise causes sensorineural loss, usually bilateral and symmetric.
Traps
Temporary threshold shift recovers within hours to weeks; permanent threshold shift does not, and one audiogram cannot separate them. Tinnitus, difficulty hearing in background noise, and loudness recruitment often accompany early loss. Acoustic trauma from blasts or gunfire can cause immediate, asymmetric loss.
Rule of thumb: noise-induced loss begins in the higher frequencies (3, 4, 6 kHz) and only later flattens into the speech range, so a normal 500 Hz threshold is not reassuring.