Question on noise cancelling headphones

[Feral]

I've had a look on the forum but didn't see anything relating specifically to this question.

Noise cancellation generates an inverse sound to the background noise to effectively cancel it out. If i appear not to be hearing anything I am assuming i am still hearing sound and potentially at volume. Is there any risk to the health of your eardrums on that basis?

It occured to me during a demo of the Bose cans. As soon as i turned them on i got a sensation of my ears wanting to adjust pressure (a bit like a change in altitude). The demo was for a loud background noise of an aircraft cabin. I'm assuming (and i'll bow to the greater knowledge of the forum members here) that an loud counter signal (not equal due to the closed cups)would need to be generated. Would that equate to listening at high volume? :?

 

[dalethorn]

I used the Bose QC2 for a year, no problems. Recently I did a few weeks of testing of the Logitech UE6000 in Active mode with no effect. I would suggest this much: You may not be able to hear tones below 20 hz or so, but you will feel them if they are present at significant volume, so no worries there. If you have extra-strong freq's around 15 khz or so, you should be able to hear those by turning up the volume some. I got that with one headphone - most worrisome (not Bose or Logitech). If the headphone were able to generate freq's above your hearing range at extreme volumes, that would be worrisome, but again, there should be clues lower down. Stick to a good brand. Whether the waves generated by the headphone are music or 'cancel' waves, it's still regular acoustic waves.

 

[Feral]

Thanks Dalethorn,

I was concerned that i could be listening to loud accoustics in the audible range without realising. I.e. if the output 'cancel' wave was played without me being able to hear the input would that be loud and cause damage to hearing.

Does the fact i can't perceive a sound mean my eardrums are not subject to abuse and that the two waves really do cancel out?

Thanks,

Feral

 

[dalethorn]

Thanks Dalethorn, I was concerned that i could be listening to loud accoustics in the audible range without realising. I.e. if the output 'cancel' wave was played without me being able to hear the input would that be loud and cause damage to hearing. Does the fact i can't perceive a sound mean my eardrums are not subject to abuse and that the two waves really do cancel out? Thanks, Feral

The fact that you don't hear anything means that the noise canceling is matching the sound that you would normally hear from the outside** with equal but opposite phased waves from the noise canceling circuit. This should always be a good thing because cancelation means that nothing bad ever reaches your ears - it's canceled first so no danger. The only danger I would speculate about is something dangerous but inaudible consciously, which would be extreme high frequencies. Coming from outside the headphone, dangerous highs would be at least partially blocked by a closed headphone, so just having the headphone on is helpful. Hopefully the headphone doesn't generate any spurious high frequencies, but one headphone I tested had some very unusual output in the 15 to 17 khz range that I can't explain.

**The sound that gets canceled isn't the sound you hear with headphones off, it's the sound you would hear with headphones on but with the noise canceling turned off. In other words, the amount of noise blocked just by virtue of having a closed headphone sealed to your ears doesn't have to be canceled electronically, since it's already blocked by the earcups and earpads. Which means the microphones that measure the noise that has to be canceled must be located inside of the headphone where your ears are. Otherwise, the design would get much more complicated.

Edit: If you ever heard a "cancel wave" from the noise canceling circuits, or even felt the pressure in your ears like a subsonic wave**, it would mean that the noise canceling circuits are not working correctly.

**I have felt such pressure waves in several headphones, both noise canceling types and regular headphones. There are many causes. Some of the old turntables had serious low frequency (subsonic) rumble and sometimes people recorded from those discs, removed the tics and pops electronically, and released the tracks that way as digital tracks. There are a few tracks I have which have subsonic sounds that aren't part of the music, and one or two of those are uncomfortable to hear. I keep them for comparison and test purposes.