People experience three types of hearing changes after exposure to noise: temporary threshold shift, permanent threshold shift, and acoustic impairment.
The effects of noise on hearing are varied.
When the noise intensity is low, it can affect normal communication, but generally it will not cause temporary or permanent damage to the auditory system.When the intensity of noise is high, it will cause temporaryHearing lossThis kind of hearing loss will last for a while after the noise ceases.
If the noise intensity exceeds a certain critical level, it will damage the internal structure of the cochlea and cause permanent hearing loss, and sometimes even damage the peripheral auditory structures such as the tympanic membrane and the ossicular chain. In addition, it will also cause damage to the central structure of the auditory system. Certain influence.
Temporary threshold shift
Temporary threshold shift refers to a temporary decrease in hearing acuity after exposure to strong noise, which is typically manifested as decreased hearing acuity, ear fullness (a subjective sensation caused by high-frequency hearing loss, and has nothing to do with middle ear pressure) and tinnitus.
The duration of temporary threshold shift symptoms is very short, less than 1 hour, or it may be several hours or days.
The extent and duration of the temporary threshold shift depends on the intensity and duration of the exposed noise.
The lowest absolute sound pressure level that causes a temporary threshold shift is expressed as 75dB in A weighting, which is 75dB A.
Stimulation frequency is also one of the influencing factors of temporary threshold shift.Pure tone 2000~6000Hz range is most likely to cause temporary threshold shift, and signals below or above this range are relatively less likely to cause temporary threshold shift.
There are many other physiological changes that accompany the temporary threshold shift. Because the outer hair cells cannot maintain normal physiological functions, they will cause metabolic changes.The swelling of outer hair cells is also related to metabolic processes.When the hair cell swells, the cell will rotate so that the static cilia face the direction of the covering membrane.When the cell swelling disappears and the direction of the static cilia returns to the normal position, the hearing returns to normal.
Permanent threshold shift
When the temporary threshold shift cannot be fully recovered, it is called permanent threshold shift.The most common situation is that each temporary threshold shift is left with a slight permanent threshold shift, and multiple such temporary threshold shifts result in a permanent threshold shift.The basic influencing factors of permanent threshold shift are the same as those of temporary threshold shift, which are also intensity, duration and frequency spectrum.
After noise exposure, swollen hair cells can rupture, causing permanent hearing loss.In addition, hair cell distortion, fusion of static cilia, or static cilia can no longer effectively transfer energy to the hair cells may occur.Further damage will lead to degeneration of the auditory nerve fibers and alter the central auditory system.
Acoustic damage occurs during a strong noise exposure, such as an explosion.Even if the exposure time is short, but the intensity is too high, it may cause permanent damage to the cochlea.In addition to directly damaging the inner hair cells of the cochlea, the tympanic membrane may also rupture, and the ossicular chain may be interrupted.Hearing loss in this situation will immediately attract attention.
Although noise-induced hearing loss generally presents a slope-descent type, as the degree of 8000Hz hearing loss gradually increases, the hearing loss will disappear, and when the cochlea is damaged, low-frequency hearing will also decrease.The most common hearing curve for long-term noise exposure is bilaterally symmetrical, but there may be asymmetry.
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