十万个为什么·科学启蒙30篇（4）
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How Noise-Cancelling Headphones Eliminate Unwanted Sound
为什么耳机降噪能抵消噪声
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Noise-cancelling headphones use built-in microphones to detect incoming ambient sound waves.
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A tiny processor then generates an 'anti-noise' wave with the exact opposite phase.
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When the original noise and anti-noise meet, they cancel each other through destructive interference.
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This works best for steady, low-frequency sounds like airplane engines or AC hums.
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Higher-pitched noises like voices are harder to cancel because their wavelengths are shorter.
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Some models combine this electronic method with physical sound insulation using earcup padding.
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Battery power is needed to run the microphones, processor, and speaker drivers continuously.
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The result is quieter surroundings without needing to raise music volume dangerously.
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Passive noise isolation alone blocks sound physically, but active cancellation adds smart tech.
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Both methods together give users clearer audio and better hearing protection over time.