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Noise cancelling headphones and hearing damage

Music1969

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#1
Hi all

Hoping to have some input by qualified people, not only electrical engineers looking at things from electrical engineering perspective.

But of course any discussion is welcome.

I’ll use an extreme example for my question:

If we have headphones that do nothing in terms of passive isolation but have perfect active noise cancellation (ANC), then standing next to a dangerously loud noise source, we may perceive no noise at all, but is there still physical / mechanical permanent damage to our hearing happening with ANC?
 

Don Hills

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#2
No. Since the net result at the eardrum is no pressure waves, there is no damage.
I have had the experience of standing next to a large Atlas-Copco "silent" air compressor (as used to power jackhammers) while wearing good ANC phones, and hearing only a low hum while my chest was vibrated vigorously by the SPL.
Some people report feeling an uncomfortable "pressure" in their ears with ANC phones active. I have experienced anechoic chambers and the feeling is very similar.
 

pozz

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#3
The example is sort of impossible. If you call it perfect noise isolation then there can be no damage because the pressure generated by a percussive blast or sustained noise will be completely cancelled. So we have to throw that out.

ANCs have dB ratings like any other kind of isolation, which means that they'll counteract noise up to a point beyond which you'll start to suffer.
 

solderdude

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#5
If we have headphones that do nothing in terms of passive isolation but have perfect active noise cancellation (ANC), then standing next to a dangerously loud noise source, we may perceive no noise at all, but is there still physical / mechanical permanent damage to our hearing happening with ANC?
Hearing damage is caused by a too high SPL level (and also time depending) entering the ear canal and hitting the ear drum and destroying the sensitive haircells in the Cochlea..
When you hear nothing the SPL is either below the audible treshold or in a frequency above your upper audible limit (age dependent mostly)
When there is a dangerous SPL at very low frequencies your body will detect that (ears are insensitive for obvious reasons)

So with a perfect (only possible to 1 or 2 kHz) NC circuit and providing there is no 150dB SPL above the audible limit there won't be any hearing damage possible.
When one has reached the limit of the 'correction signal' (afterall the headphone amps inside are very low power) you will start hearing things again when the SPL outside exceeds that of the correction.

In such case ear plugs + NC would be the way to go.
 
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