How do noise cancelling headphones work?
Some of the most visited archive entries on this site are the reviews of noise cancelling headphones
and the info about my Etymotic earplugs
It's been ages since I looked at how people get to this site, but in an effort to provide the reader friendly service visitors have come to expect (ok I'm making this part up), I've decided to start properly answering the questions that have led people to this site. One visitor to this site last month was someone searching for information about "How do noise cancelling headphones work?"?Answer
: They basically work on two different principles: active
or passive cancellation
Passive noise cancellation basically means blocking out sound by the way of insulation. Nothing fancy or electronic about it. There are various ways of blocking out the sound; either making a big cup around your ears or by ensuring that the plugs you put in your ears go deep and have foamy or soft, sound-absorbing material around them: This is what the Etymotic series
of earplugs do.
Actively noise cancelling headphones go about things a different way: in the earpiece of the headphone, pointing away from your ear, lies a microphone. This microphone records the ambient noise that's coming in the direction of your ear. The microphone is hooked up to electronics that either is set up to completely block out external noise or to distinguish between noise that is background noise and noise that should be directed to your ear (e.g. police sirens, voices etc). Whatever it is set to be cancelled out, it uses the waveform physical property of sound: sound (noise as well as music) travels through air as vibrations that can be represented as waves, and waves can be cancelled by providing a wave that is shaped in the opposite way. The electronic circuit in the headphones creates an exactly opposite wave to the "noise" waves, and this effectively cancels out all the ambient noise. This is the way the BOSE headphones work, for example.