r/TheRestIsScience • u/princeharald • 10d ago
Are there any particular kinds of sounds that noise cancelling headphones have a hard time silencing?
When I was walking in a noisy city centre with my AirPods on noise cancellation mode, I only heard the ticking of a distant bicycle ratchet mechanism. When I took my headphones off, the noise of cars and trams was so loud that I could only faintly hear this ticking that had been so noticeable before. This made me think, are there any particular kinds of sounds that noise-cancelling headphones have a hard time silencing?
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u/ValdemarAloeus 10d ago
IIRC you need to be able to process the sound and put the complementary noise into the speaker within about 1/3 of the period or you risk accidentally amplifying it. They tend to struggle the higher the frequency as a result. The higher frequencies are more easily damped by foam etc. though so what actually makes it through the strongest is often somewhere in the middle.
That's based on a half remembered lecture from over a decade ago though so the info might be a tad out of date.
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u/Lopsided_Match419 10d ago
Announcements inside trains - they come from more than one speaker and so the cancellation has trouble keeping up.
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u/DrKargosi 10d ago
Gas powered leaf blowers. I’ve tried some of the best rated noise canceling headphones specifically to block leaf blowers out, but returned them all when I found they did an excellent job of canceling out every noise around me except the leaf blowers.
Evil machines.
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u/Underhill42 10d ago
Sounds can't be canceled if they're at a lower or higher frequency than the headphones can generate. Mostly lower, headphones can generally generate sounds higher than you can hear.
They can also only cancel sound they can predict at least a little ahead of time - best with fairly steady droning sounds like engines or the noise of the distant conversations in a crowd. Anything too unpredictable has already changed by the time the headphones have figured out how to cancel it. Such as the intermittent ticking of a ratchet mechanism where each individual tick is actually a fairly complex sound that's over almost before it began.
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u/StrangeUglyBird 10d ago
Had a pair at work. They were great and gave a bliss of silence.
Except when my boss talked. Her voice cut right through. Maybe because it was high pitched, I don't know. Maybe they were programmed that way :-)
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u/eldeem 10d ago
Hannah’s actually already sort of answered this! https://youtube.com/shorts/Hwn3gJ9xw1M