Active Noise Cancellation

[manse]

Does anyone have any experience or comments on active noise cancellation as a means of reducing background noise? I'm looking for sources of information and starting points for a possible project. I've seen these headsets from Sony and others that seem to be based on a pretty cool concept, but I've heard complaints about them not working very well. Does anyone have any idea what these units are actually doing?

[MRehorst]

To paraphrase our lubricious former leader, "That depends on what the definition of well is"...

Those headphone put a small microphone close to each ear to sample the acoustic noise there. The microphone is amplified and fed into the earphone, but the phase is inverted, so the noise is cancelled at your ear. This works mainly for low frequencies for a number of reasons- there is a physical distance between the mic and your ear and the earphone. High frequencies with very short wavelengths may actually be reinforced rather than canceled. But that generally isn't a problem because the inverting amplifier's high frequency response is rolled off to keep the whole system stable. So you get noise cancellation at low freqs with less and less effectiveness as the frequency goes up.

I have some Sony in-ear noise cancelling phones. They are great on airplanes and trains- they allow me to listen to movies or music at a reasonably low volume level because I don't have to crank up the volume to drown out the noise from the vehicle.

MR

[jackinnj]

in our airplane -- expensive but they work great and your head doesn't ring at the end of a long flight.

[Nelson Pass]

Actually, the most sophisticated noise cancelling headphones
and such don't use inverting feedback in the usually understood
sense. Rather, they use DSP to analyze repetitive noise and
have an algorithm to produce its phase opposite. This way
you can hear other things which are not long term repetitive.

[manse]

OK thanks everyone for the feedback but I am more interested in building one of these headsets myself. Not necessarily to save money, but to figure this concept out and play with it a little. Would a simple inverting op-amp circuit and a correctly placed mic/speaker set do the job or is there more to it than that(Adaptive DSP logic aside, that may come later...)?

[pinkmouse]

Have a look at this project on Headwize, it might give you some ideas

[Da5id4Vz]

These were the 1st pair I saw, almost 10 years ago, and not too shaby:

http://www.nct-active.com/nbex.htm

NCT is an interesting company. Their web site is work a browse.

The Headwize article is very interesting.

[Entropy]

I've built the Ryckebush design.

It works well. It has its limitations, but it's an AMAZING help on airplanes. It's the difference between being able to hear or not hear your movie or music.

Which reminds me, I have to fix the circuit/clean it up now that security is a bit stricter. (And I need to figure out a better power source than two 9V batteries.)