(Active) Noise canceling headphone questions

I thought I knew how these things worked, but after looking at a few in the local CompUSA store and on-line, I'm puzzled.

For reference, I'm talking about consumer-grade active noise-canceling headphones such as those on this page:

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So, you have a microphone on the outside of the headphone speaker and the speaker points into your ear. The mic signal is fed 180 degrees out of phase to the speaker and voila, some of the external noise is cancelled (limited by multiple sound paths, phase delay differences, use an adaptive filter DSP if you're really gung-ho, yada yada).

What bothers me is that these things seem to generally run on one AAA cell, which doesn't seem to be much if you're planning to power two headphones at any volume for any length of time. (I haven't found longevity specs on the battery for any of the headphones.) This assumes that the input signals and the noise error signals are combined electronically and the outputs (of some chip) drive the speakers. Even with class D audio amp chip efficiencies, I don't see a lot of lifetime.

Is is possible that the error amp is driving a separate winding in the speaker, i.e. that custom multi-winding speakers are being used?

Or perhaps the battery just powers the microphone and its amplifier only and the output is simply coupled into the speaker signal via a transformer or low-impedance active output?

Or is it really just that a single AAA probably *will* power two power amplifiers for an acceptably long enough time, given typical music volumes, headphone efficiencies, etc.?

Can anyone clue me in?

[1] Note that the Panasonic one doesn't use a battery at all and yet claims to be "active" noise-cancelling! (Or perhaps that's just the category that web site put them in.)
Mr. Civility
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Mr. Civility
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At what frequency? ;-)

No, I'm just hassling you - the thing is, it's simply an inversion.


I can try - when you look at the SPL of the noise without the noise- cancellation feature, inside an ordinary pair of headphones, you don't need very much.

When I was in the USAF, we wore "ear defenders" - same form factor as stereo headphones, but with nothing inside but some foam rubber, and they blocked out enough noise that you could comfortably work next to a running jet engine. Stick a mic element on the outside, run it through an inverting amplifier, and I'd be surprised if a milliwatt wasn't almost too much. Of course, if you wanted to talk with anybody, you had to scream your lungs out!

Cheers! Rich

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Rich Grise

these ones use 2AA cells and last 80 hours on alcalines.

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94dB nominal SPL is quite loud.

ciao Ban, Apricale, Italy

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Thanks for the responses.

I missed a big clue. Most of these headphones say they work as normal headphones when the battery is removed. So, obviously the speakers aren't being driven by power-hungry power amps. The inverted noise signal is being slipped into the circuit another way, perhaps with transformer coupling or a low impedance semiconductor device in series with the speaker or extra speakers or extra windings on the speaker coils.

Since this is a design group, how would *you* do it? (Spec: 1 AAA battery, "acceptable" lifetime.)

Mr. Civility
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Mr. Civility

I read in sci.electronics.design that Mr. Civility wrote (in ) about '(Active) Noise canceling headphone questions', on Sun, 9 Oct


Microphone, hearing-aid type audio amplifier, probably transformer-coupled to the transducers in parallel with the signal from whatever is feeding the headphones.

Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
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John Woodgate

This is interference cancellation rather than noise cancellation, since noise modem is totally random. Interference is predictable signal that may be elimiated.

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