Mechanisms and symptoms of noise induced hearing loss (the sciency bits)

  So today I decided to get stuck right into the technicalities of noise induced hearing loss, namely, what are the mechanisms of hearing loss and how they relate to their symptoms. Don't get me wrong, while I managed to find some really interesting information, it took some serious academic trawling through long paragraphs and even longer journals in the quagmire known as PubMed..egh.
I mean, can't they just word their titles like normal ppl?
Ok. *deep breaths* 
Enough ranting.

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       The initial effect of high intensity sounds is an involuntary muscle contraction in the middle ear called the acoustic reflex produced by the body to protect itself. This reflex begins to be activated by sound intensities greater than 80-85 dB. This reflex consists of the contraction of small muscles attached to the malleus and the stapes which have the effect of reducing middle ear bone movement and hence reducing the amount of vibration passed along to the cochlear, sparing the delicate hair cells. 

 This causes what is known as ‘temporary threshold shift’, which explains the stuffiness and ringing sensation (tinnitus) goes away after a day or so. Tinnitus is a common byproduct of having been 'acoustically overstimulated' and should be taken seriously as a sign to ‘rest’ your ears.

 A temporary threshold shift of 25 dB means that a sound has to be 25 dB louder to be perceived at the same volume than before the threshold shift occurs. The degree and recovery from a temporary threshold shift is affected by the intensity of the stimulus, the duration of the stimulus and individual susceptibility. If such overstimulation is repeated, usually over years, then the threshold shift will become irreversible, that is a permanent threshold shift, or noise induced hearing loss. This is caused by the death of overstressed cochlear hair cells and is termed 'sensorineural hearing loss' because this type of hearing loss involves a loss of sensory capability (in contrast to conductive hearing loss which is an impairment in the transmission of sound energy to the perfectly functional cochlear hair cells), so sound energy is getting into the cochlear just fine, but the deficient hair cells are unable to pick up all of the signal.

Noise induced hearing loss initially occurs at 3000-4000 Hz, this is the upper range of conversation and the person might have problems differentiating consonants like ‘s’ and ‘t’. With continued overexposure, hearing loss will spread to both higher and lower frequencies.

Here's a video I found that really helped my understanding of how noise induced hearing loss is really like.

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Simulated Sensorineural Hearing Loss:



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The difficulty in identifying noise induced hearing loss is the fact that the symptoms occur slowly and gradually over many years and significant hearing loss only becomes evident until significant, permanent damage has been done. 

Also the gradual loss of hearing and the increasing dependence on visual cues like lip reading often goes unnoticed so these sufferers have no idea that they’re having hearing problems since normal conversation is unimpaired.

Permanent damage to the cochlear hair cells is caused by acoustic overstimulation for a too long a period (a fancy way of saying, too loud for too long). This is caused, partly by the overproduction of reactive oxygen species and free radicals in the hair cells from acoustic overstimulation (Interested readers can consult this link http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/R/ROS.html). These reactive oxygen species and free radicals break down cellular components and damage DNA, ultimately resulting in death of the cell. Cochlear hair cells do not regenerate so once killed, frequency sensitivity in the range where the cells were responsible for is lost.              

    

As you can see from the above pictures, there is significant abnormality in cochlear architecture associated with the damaged hair cells.

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 ‘I hear all right, but everyone’s mumbling’ is a common complaint for unwitting sufferers who have trouble differentiating sounds especially in crowded locations. For musicians, such impairment in pitch discrimination can be quite deleterious to for example, the perfection of a performance.
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For a taste (or a hear? Sorry, bad pun) of how such hearing impairments affects music perception:

Stimulated hearing loss - Music

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Congratulations, you've come to the end of a very long and wordy post.

Until next time...
The crucial question: How much is too much? 
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