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According to experience of mine and of my friends using active noise cancellation headphones, the technology is much more efficient in reducing "periodic" noise, such as humming of jet engine, fridge etc., than reducing "non-periodic irregular" noise, such as human voice.

Is it only a personal perception, or is it a measurable fact with some deeper reason behind it?

I cannot imagine why it should work like this. Intuitively, the speakers must produce the "anti-noise" not regarding the periodicity of the noise.

I understand the difference between characteristics of a periodic sound and the human voice. However I still don't get how it applies to the anti-noise headphones. They cannot simply play the anti-wave against the last remembered wave, they have to play the anti-wave against the new wave. So how does it help if the headphones know what the last wave was?

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  • $\begingroup$ Electronics and dsp forums here will give you electrically based answers. You are right, the micriphone produces a fast DC signal, which, if reversed, can cancel out the noise image. For example they can connect the mic signal in reverse to the speakers to reverse the phase and delete it from the perceived sound... So your best info would be to transfer this to electronics. You'd still need transistors like diodes to keep the mic and speakers from being on the same circuit. Basically, electronics is too slow for instantaneous audio mic voltage reversal. Id like to know an informed answer. $\endgroup$ – com.prehensible Jun 6 '18 at 8:44
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    $\begingroup$ a periodic signal is predictable, pure noise is not $\endgroup$ – hyportnex Jun 6 '18 at 13:13
  • $\begingroup$ @hyportnex See my edit $\endgroup$ – Honza Zidek Jun 7 '18 at 17:14
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It's not just personal perception, it is the case.

The reason is that for a standing noise, once the counter-motion of the headphones is computed it can be played indefinitely and thus cancel the noise. In other words, a hum or in the ideal case a tone, have the same waveform at t=5 seconds than at t=0.1 seconds when it was initially picked up by the microphones.

Other noises such as speech contain sudden changes in amplitude or transients. Clicks or bangs as well ar characterized with very quick changes in their waveform. Thus by the time the microphones pick them up and the electronic measures them and counteract, they are already gone and counter-vibration is of no use.

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  • $\begingroup$ I understand the difference between characteristics of a periodic sound and the human voice. However I still don't get how it applies to the anti-noise headphones. They cannot simply play the anti-wave against the last remembered wave, they have to play the anti-wave against the new wave. So how does it help if you know what the last wave was? $\endgroup$ – Honza Zidek Jun 7 '18 at 17:12
  • $\begingroup$ @HonzaZidek well because a periodic or quasi-periodic wave repeats itself indefinitely so once you recorded one cycle you can inverse the phase and play it in sync with the incoming sound. You can’t do that with a transient. $\endgroup$ – Cape Code Jun 7 '18 at 17:21
  • $\begingroup$ So it means that the anti-noise headphones always play the anti-wave against the last heard noise wave? $\endgroup$ – Honza Zidek Jun 7 '18 at 18:22
  • $\begingroup$ @HonzaZidek it’s a way to put it. It’s a physical system, the delay cannot be 0. $\endgroup$ – Cape Code Jun 7 '18 at 18:39

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