High intensity sound (such as explosive sound) will immediately cause permanent damage to the cochlea and middle ear. The noise parameters that determine the degree of hearing loss are not consistent with the usual injury risk criteria or exposure measurements. There is no certain rule between the measurement of this instantaneous strong noise and the degree of hearing loss. One excessive strong sound exposure (such as 105 dBA exposure for 6 hours) will immediately cause permanent threshold shift PTS rather than temporary threshold shift TTS.
When strong sound exposure causes cell necrosis in the cochlea, audiogram will show 4000 Hz notch, flat curve or steep decline of high-frequency hearing. The inner and outer hair cells and supporting cells will be involved, and the basement membrane will be damaged because the pressure of the traveling wave in the cochlea exceeds its elastic limit. The internal septum of the cochlea is damaged, which makes the internal and external lymph mixed, thus expanding the scope of damage.
The most common symptoms caused by sound damage are tinnitus and progressive hearing loss. Post injury stress syndrome exacerbates these symptoms. Research shows that peak pressure level of 132dB will cause sound damage. The cochlea suffered the most damage impact when the peak pressure level was 132 ～ 170dB; If the noise is higher, it will lead to perforation of the eardrum and dislocation of the ossicular chain. When the middle ear is damaged, the noise energy reaching the cochlea will be greatly reduced, and the damage to the cochlea will also be significantly reduced. At this time, the 4000Hz notch in the audiogram is not obvious. Therefore, when the noise peak pressure level exceeds 170dB, the degree of cochlear damage decreases, but the middle ear is damaged.
Noise can also cause damage to non auditory organs. Research shows that normal people exposed to noise will cause stomach contraction. More and more research evidence shows that noise exposure will bring adverse effects on the human body, causing cardiovascular disease, sleep disorders, tension, trouble and cognitive effects, which are manifested as learning disabilities in children.
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