Three-Minute Tech: How noise-canceling headphones work

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[In our Three-Minute Tech series, we tell you everything you really need to know about a technology in three minutes or less.]

When you’re in a noisy environment—on an airplane, bus, or train, say—and you want to listen to music, you have three choices. You can turn up the volume so the headphones audio drowns out the background noise, you can use headphones that block some of that noise, or you can use noise-canceling headphones, which allow you to maintain a safe level of volume, while getting rid of much of the ambient sound. Obvious, the latter two options are much better for your hearing.

I’m a headphone fan, and use a number of different headphones, depending on what I’m listening to, and where. Noise-canceling headphones are one type that I enjoy for specific types of listening. Here’s an overview of how noise-canceling headphones works.

[For more on the different kinds of headphones, see our infographic of headphones types.]

Blocking sound: Two ways to do it

There are two ways to reduce or eliminate background noise. The first involves passive noise isolation—physically blocking that noise by keeping it from ever entering your ear canals. Some noise-isolating headphones use large, around-the-ear or on-ear pads to keep out noise; others use small earpieces with silicone or foam eartips that actually fit in your ear canals like earplugs. The latter, which include models we call in-ear-canal headphones (or canalphones) and canalbuds, are among the most popular types of headphones today, and in some cases they can reduce noise more effectively than a good pair of “noise-canceling” headphones. (You can even get custom in-ear monitors with earpieces specifically made for your particular ears for better comfort.) Noise-isolating headphones let you listen at lower volumes than non-isolating headphones while eliminating a fair amount of background noise.

Audio-Technica’s ATH-ANC7 is a good example of noise-canceling headphones.

The second type of noise reduction is noise cancelation, more accurately called active noise cancelation or reduction because it uses special circuitry to detect background noise and actively remove it from the audio signal.

To more effectively reduce external noise, most active-noise-reduction headphones combine these approaches. The most common type of noise-canceling headphones use a closed, full-size design that covers or completely surrounds your ears to block noise—especially high-frequency sounds, which have very short wavelengths. The active noise-canceling technology focuses on midrange and lower frequencies, such as engine noises.

Active noise reduction

Active noise reduction has been around for a long time. It was originally developed for use in aviation, so pilots could talk to each other and effectively communicate by radio. When you see a helicopter pilot in a movie, he or she is undoubtedly wearing noise-canceling headphones to communicate with a control tower and possibly with passengers. You can find the same type of headphones in noisy factories, on the tarmac at airports, and at race tracks. These headphones are designed either to reduce noise while they receive and send wireless audio—in cases where someone is communicating by radio, for example—or simply to eliminate as much of the dangerous volume of engines or machinery as possible.

The noise-canceling technology pioneered in these commercial products is also available in consumer-grade headphones, letting you protect your hearing while listening to music or audiobooks at lower volumes.

At the left, two normal waves (bottom) and their combined waveform (top). At the right, the two bottom waves are mirror images of each other, and the combined waveform on top is null, or silent.

The technology works using tiny microphones on the outside of the headphones that monitor ambient sounds, along with special circuitry inside the headphones that turns those sound waves upside down and feeds the resulting inverted waves through the headphones to your ears. This results in destructive interference—when you take two sound waves that are out of phase (each a mirror image of the other), they cancel each other out, resulting in silence. In other words, that external noise is, well, canceled.

Pros and cons of noise-canceling headphones

While noise-canceling headphones are great for listening to music in noisy environments, or even for just blocking out noise, with no audio playing, there are tradeoffs.

On the plus side, they offer:

  • Improved listening experience in noisy environments
  • Safer listening volumes
  • Less fatigue over extended listening periods, such as on airline flights

But on the other hand:

  • More expensive than comparable-quality standard headphones
  • Audio quality inferior to that of standard headphones in the same price range
  • The active circuitry is much more effective at canceling constant noise than intermittent noise; it’s also better at canceling midrange and lower frequencies than higher frequencies
  • Require batteries, either disposable or rechargeable (a charge usually provides around 20 to 25 hours of use; on some models, once the battery is dead, you can’t use the headphones for any audio, even without the noise-cancelation circuitry)
  • Can be less comfortable than standard headphones because of the focus on getting a good noise seal—some fit more tightly and their earpads can feel hot after extended listening
  • Active circuitry can produce an audible hiss, and on some models, generates a sense of air pressure on your eardrums, which some people find uncomfortable

This story, "Three-Minute Tech: How noise-canceling headphones work" was originally published by TechHive.

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