Noise canceling headphones

Hi,

I don't know what kind of headphones you have, normally the for the low frequency noise the mic placement is not really important but for the high frequency it is.

The mic (or the mics) in this case is used as a noise reference for the phase inversion noise-cancelling circuit.

I think that's nice to experiment directly on your environment to see if you can improve something, you should only keep the same phase otherwise you can even double the noise.

Don't hate me for asking it, but what do you mean exactly with "keeping the same phase"? Maybe you could give me a term I could google for for more info? And how do I change the phase into one that would possibly even double the noise?

My suspicion is that it does come down even to millimeter-changes in distance. So if I was to move the mic 1 millimetre away from it's position this would be lethal for the noise-cancelling function, wouldn't it?

Thanks a bunch for answering!

You can watch a movie about a succesful 4-mics setup here:

http://www.bose.com/controller?url=/shop_online/headphones/noise_cancelling_headphones/quietcomfort_15/index.jsp

go to the "Videos" link and select the second one.

The technology is called "active noise reduction" or "active noise cancellation". Mainly it picks some sound from outside, change its phase (180deg.) and add this to the original audio signal so the result is some attenuation of that outside environment sound. The 4 mic setup makes another step further and compares what is inside your headphone with what is outside. 

Some other examples and explanations: http://www.silentium.com/products-and-solutions/generator/

• http://www.silentium.com/products-and-solutions/generator/
• http://www.noisebuster.net/what-is-anr.html
• http://www.ams.com/eng/Products/Audio/Active-Noise-Cancellation/ANC-Demo here is a nice demo.

Thank you very much for your time and effort. I have found time to read through all of your links and watch the videos, but unfortunately nowhere could I find the exact answer to my question. Most of the information found on this sources I already knew. So still, my "yes" or "no" question remains unanswered. Would the noise-cancelling function of the noise-cancelling headphones get corrupted, if I was to move it's mic for whatever distance away from it's original position?

My suspicion is, that noise-cancelling wouldn't work properly any more since the produced counter-waves would be out of original phase with the actual sound waves taken by the headphones mic and interpreted by the algorithm (algorithm being unaware that now the mic has a different position). At the end I might get even more noise than before...

Thanks anyone with a correct answer and time to post it here!!!!