[Sylos]

The trick with this material is that it pretty much reflects sound waves. On the way back, these reflected sound waves then crash into the upcoming soundwaves and they cancel each other out.

Headphones with active/acoustic noise cancelling use the same trick, except that they pick up the upcoming sound waves with a microphone and then use a speaker to generate those "reflected" soundwaves.

Actually-reflected soundwaves cannot be as strong as the upcoming soundwaves, so they're never going to fully cancel out the noise. Those generated soundwaves can.

One point is that there's most likely less latency for a soundwave to get reflected vs. picked up by a microphone and then generated by a speaker. However, to my knowledge our human senses have even more latency than both of those, so I don't think that matters.

[swinglock]

Don't extrapolate our slow reaction time to mean more than just that! :) Low latency still matters. Think about latency when scrolling/dragging on a touch screen for example. There's a large difference in feel depending on latency, even if the latency is lower than your reaction time.

[petra]

The latency in the active noise-cancelling systems is a few milliseconds, mostly due to processing time.

That's why they are less capable in filtering non-repetitive noises like speech, especially their higher frequencies.

[hatsunearu]

This is a very bad meme that's been propagated so much across the internet.

1. The latency is active noise cancelling systems is very low; much less than the frequency of most sounds.

2. Noise cancelling is ineffective at higher frequencies because the space between the speaker driver and the ear becomes large enough that the destructive interference becomes imperfect.

3. The feedback has to come from somewhere, and once the frequencies become high enough, the phase shift caused by the physical distance between the driver and microphone start to affect the feedback loop.

4. Because of these issues at higher frequencies, they just lower the feedback gain at higher frequencies. This effectively makes the active noise cancellation system be not sensitive to higher frequencies.

5. This is very much akin to frequency compensation in other topics like buck converters and op amps--the additional phase shift in the feedforward path that often increases as frequency goes up becomes problematic to compensate, so they just make the feedback loop insensitive to that region.

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the quick way to debunk this is two tests:

1. Play some bandlimited noise (https://www.youtube.com/watch?v=5qV5j9wD5e8) really loudly and I guarantee you any shitty ANC headphone will cancel this out just fine. Noise by definition is non-repetitive.

2. Play a high frequency sinusoid (https://www.youtube.com/watch?v=TRKB5kWs7KE) really loudly and it will go straight through any ANC headphone. A sinusoid is as repetitive as it gets.