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(Active) Noise canceling headphone questions

Discussion in 'Electronic Design' started by Mr. Civility, Oct 7, 2005.

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  1. Mr. Civility

    Mr. Civility Guest

    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:[1]

    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.)
  2. Rich Grise

    Rich Grise Guest

    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!

  3. Ban

    Ban Guest

    these ones use 2AA cells and last 80 hours on alcalines.
    94dB nominal SPL is quite loud.

    ciao Ban,
    Apricale, Italy
  4. Mr. Civility

    Mr. Civility Guest

    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.)
  5. I read in that Mr. Civility
    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.
  6. riscy

    riscy Guest

    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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