Week 5 Topic 2 - Summary

How do we know that years of exposure to loud noises causes hearing impairment?

Basically – From studies!

Over the last 40 years various studies of large groups of people that have had exposure to noise at work have found that ongoing exposure to noise can cause hearing loss; even some sounds quieter than shouting can, over years, cause some damage to hearing.

It has been found that thirty years exposure will produce in half the exposed workers
-about 5 decibels of high-frequency hearing loss for an average exposure to 85 decibels of noise at work;
-over 10 decibels of high-frequency hearing loss for an average exposure to 90 decibels of noise at work;
-over 20 decibels of high-frequency hearing loss for an average exposure to 95 decibels of noise at work.





What happens to the ear itself?

Noise-induced hearing loss is a form of sensori-neural loss (i.e. due to disorder of the sound sensing mechanism within the cochlea or the nerve supplying it) that gradually worsens over a period of years.
Continuing loud noise causes the degeneration of the outer hair cells, especially in the basal turn of the cochlea (which is responsible for sensing high frequency noises). This damage can range from loss of some stereocilia up to complete death and loss of cells. The pathogenesis is not fully understood but is suspected to be partly vascular and partly biochemical.

This damage to the outer stereocilia means that the variable amplification function of the outer hair cells no longer operates effectively at certain frequencies – usually the higher frequencies. Therefore, the range of intensity at which sound is perceived is narrowed.





How does conductive hearing loss differ from noise-induced hearing loss?

N.B. Because noise damages only the outer hair cells, pure noise-related hearing loss cannot exceed the level of amplification afforded by those hair cells; it cannot exceed 65 decibels.
Also, noise-induced hearing loss starts at the frequency of 4000 hertz.

Noise-induced hearing loss is particularly related to resonance of the ear canal. Whereas conductive hearing loss occurs when there is pathology located between the entry to the ear canal and the cochlea. It prevents the auditory stimulus from reaching the cochlea. Because bone conduction occurs, conductive deafness is never complete; at worst it can cause a 60 decibel deficit.