Any harmonic distortion in the headphones will be introduced relative this, as well as being in the higher frequencies where it's less noticable, but still may cause hearing loss.

I've used ANC IEMs instead of proper hearing protection around loud machinery before and although it may have felt quieter, my ears still "rang" afterwards so I was causing permanent hearing damage. People who think ANC "sounds quieter" and use it instead of proper hearing protection (or just turn the music too loud) will definitely suffer hearing loss, though not directly because of the ANC.

I am not an audiologist, but an interesting similar issue happens with our eyes. If you wear tinted lenses with no UV filter, your body will be happy that it looks darker, and open your iris more. Now since your iris is open more, you actually cause more UV damage to your eye without noticing. I remember this being a big story when polarized 3D movie glasses came out and people were keeping them on outdoors.

i dont think it's feasible to cancel out such loud noise using just ANC. A sealed, airtight headset (or those tight earplugs) would have to be used. If you're using ANC for such loud noises, you'd still hear the noise through your bones/face imho. For example, if a car beeped its horn at you while using ANC, i hear it.

ANC is merely for constant low background noise like airplane engine.

You're right though that it doesn't work as well at high volumes. The louder the noise, the more distortion in the microphone/amplifier/driver, so the efficacy of the noise cancelling is reduced. For example in this THD graph of the AirPod Pro's, at 100dB it adds approximately 3dB compare to the 90dBtest.[1] Going much louder and you'll start getting digital clippings as the built in amplifier won't have enough range.

> . For example, if a car beeped its horn at you while using ANC, i hear it.

Car horns are above 100dB, and at the frequency they are, APD only measures about 20dB of attenuation [2], so that only brings it down to 80dB, which is still very loud.

> ANC is merely for constant low background noise like airplane engine. What I find very neat is that even though only 20-30dB of isolation is measured, many noises really do "go away". If you play around with ANC you'll realize that as the background noise gets louder, the white noise hiss from the ear piece gets louder. This mostly the result of error in the system, but this increase in background noise has the effect of masking any external sounds that make it through. I believe it also moves a lot of that energy to higher frequencies where you don't notice it.

[1] https://www.rtings.com/headphones/1-4/graph#1625/4029 [2] https://www.rtings.com/headphones/1-4/graph#1625/4053

not always true. Minor phase mismatch will create higher frequency sound. You wouldn't hear ultrasound, though how bad it is to have an ultrasound source in you ears i don't know.

The phase mismatch can be temporal - circuitry reacting either a bit too fast or too slow - as well as spatial - the sound wave front most probably aren't parallel to the speakers frontal plane.