Noise-canceling headphones are a great invention. Instead of walking down the street of a hectic city, being overwhelmed by the sounds of the million-footed beast, you can shut out much of din of traffic and conversation while listening to your favorite tunes. Another common use of noise-canceling headphones is plane trips; the constant sounds of an airplane can be fatiguing, and noise-canceling headphones – even with no music playing – can make for more restful traveling.
It may seem like these headphones use voodoo to silence background noise, but the technique is actually a pretty simple application of physics. They combine both passive and active noise cancellation.
To start with, each of the ear cups is well insulated, blocking out much of the noise around you. This passive noise cancellation blocks out many of the higher frequencies. In fact, you may find that good noise-canceling headphones block enough sound even before you turn on the active noise cancellation.
For this latter feature, each of the ear cups contains a tiny microphone that picks up the external sounds. The headphones process these sounds by flipping the sound waves upside down. When you play two sound waves at the same time – one inverted – they cancel each other out.
A great example of effective noise cancellation is the Grateful Dead’s Wall of Sound. Back in 1974, sound man and chemist extraordinaire Owsley Stanley came up with a setup for the band that was distortion-free, and also served as monitors, so the band could hear themselves play without having blowback monitors on the stage in front of them.
The Wall of Sound was the largest sound system ever built, and packed a lot of power: it weighed 75 tons, contained some 600 speakers, and put out more than 26,000 watts of sound. But there was a problem: since the microphones were facing backwards, toward all those speakers, wouldn’t they create a feedback loop?
Stanley set up microphones in pairs, out of phase, which worked exactly as active noise cancellation does. The singer sang into the top microphone, and the bottom microphone picked up the background sounds. This second microphone’s waves were flipped, canceling out the background.
As you can see in the first image above (from Wikipedia), there is still a remnant of the noise after the cancellation; this is unavoidable. As such, Grateful Dead recordings of the time – notably those officially released of the Grateful Dead Movie Soundtrack – have a bit of a hiss in the vocals.
The same is true with noise-canceling headphones today. If you stop playing music, you’ll hear a slight hiss in the background. Noise-canceling headphones may offer great sound, but they do have this limitation: anything you listen to, with active noise cancellation turned on, will be affected by this hiss. Because of this, no noise-canceling headphone will sound as good as regular headphones of the same audio quality.
So noise-canceling headphones are a trade-off. The sound quality of these headphones isn’t as good as standard headphones at the same price. But they’re great in noisy environments – planes, trains, busy city streets – where you really want to get rid of the background noise. If you use them in quiet areas, make sure to turn off the active noise cancellation; your music will sound better. (Some noise-canceling headphones only work with power; others will work without power, just like regular headphones.)
(If you’re curious, I have a pair of Audio-Technica ATH-ANC7B noise-cancelling headphones. (Amazon.com, Amazon UK) They’re a lot cheaper than Bose headphones, and are well rated. I don’t use them often, but when I do use them, they work very well, and have very good sound.)