The ear is injured by noise in 2 different ways, depending on the type of exposure. The organ of Corti becomes detached from the basilar membrane, deteriorates, and is replaced by scar tissue. Noise-induced hearing loss, in contrast to acoustic trauma, develops slowly over years, is caused by any exposure regularly exceeding a daily average of 90 dB A , and proceeds in 3 stages. In the first stage, sensory cells within the cochlea are killed by excessive exposure. In the second stage, after weeks to years of excessive exposure, hearing loss can be detected audiometrically.
Early loss occurs in the high-frequency range, around the highest C note played on a piano.
With continued exposure, the loss spreads to the lower pitches necessary for understanding speech. At this point, the third stage, the patient usually becomes aware of the problem and may seek medical attention. There is additional loss of sensory cells in the organ of Corti on either side of the central lesions. Noise exposure in the workplace has been known for centuries to produce hearing loss. The US Department of Labor promulgated regulations in the 1970s and 1980s to protect the hearing of millions of Americans working in noisy environments.