The Tinnitus and Hearing Clinic - Noise and Hearing loss
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Why does noise cause a high frequency loss?

Developmental Characteristics of Noise Induced Hearing loss
If you understand the underlying mechanism that causes the noise induced hearing loss, it is easy to understand why the pattern is so distinctive.  The noise source, from a shotgun to a rock concert damages the same part of the cochlea.  How much damage occurs is based on the loudness.  Noise is a wave and the louder the sound, the stronger the wave.  Visualize waves coming into the bay,  The shape of the bay determines where the  waves hit and the larger the waves the greater the force in which they hit the rocks.  This is exactly the same thing that happens with noise exposure.

 The cochlea is a spiral shaped organ.   If look at diagram on the right, you can see that 4kHz is at the apex of the first bend.  If sound is coming in too quick or too hard, then it fails to make the curve and slams into that portion of the cochlea.  The force than dissipates and the surrounding frequency is not harmed as much, hence the notch. Think of a car going around a race track.  On the straight it can handle the speed and power but it must slow down or it will hit the wall, or in our case the 4 kHz hair cells.