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How Noise-canceling Headphones Work: Noise-canceling Headphones: Cheat Sheet
Update time: 2015/04/30
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Stuff You Need to Know:

  • Sound waves are similar to water waves. They have compressions that correspond to crests and rarefactions that correspond to troughs.
  • Noise-canceling headphones come in either active or passive varieties. Passive noise-canceling headphones use high-density foam or other sound-absorbing material to prevent sound waves from reaching the listener's ears.
  • Active noise-canceling headphones add an extra level of noise reduction by actively erasing lower-frequency sound waves. They do this through destructive interference -- creating sound waves that have the same amplitude and frequency as the ambient noise, but are 180 degrees out of phase.
  • Good noise-canceling headphones provide a noise reduction of about 80 decibels, enough to block jet-engine noise inside an aircraft cabin.

(Source: How stuff works?) 

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