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:
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? 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.)