by Chris Woodford. Last updated: February 5, 2019.
Why do people on trains and buses have to play their music so loud? Look at it from their point of view: they're trying to enjoy a nice bit of Beethoven or Schubert (as if!), but all they can hear is the deafening throb of the engine—so they turn the volume up as far as it will go. But don't worry, because there is a solution to this problem for both of you: noise-cancelling headphones. These amazing gadgets block out the background noise, allowing people to listen to their music without unwanted distractions. Since there is no competition between music and noise, they can set their MP3 players to a much lower volume, which is better for the people around them as well. Let's take a look at how noise-cancelling headphones tell the difference between the sounds you want to hear—and the ones you don't.
Photo: My trusty Etymotic ER-6 noise-isolating earbud headphones. These work by passive noise cancelling: each earpiece has a pair of (washable) plastic suction cups that make a tight seal in your ear canal to stop noise getting in and music leaking out. If you prefer, you can pull off the plastic cups and use disposable, soft foam ones instead.
Two kinds of noise reduction
There are two ways to reduce the noise in your headphones, one simple and one complex.
Passive noise reduction (noise isolation)
The simplest kind is called passive noise reduction or noise isolation. The headphones are designed so the earpieces fit snugly into your ears. No sound can escape to bother the people around you and no background noise can get in either. The Etymotic headphones shown in our top picture work this way. They have earbuds with large pieces of soft, viscoelastic foam built around them, much like foam earplugs. You wear them by squeezing the foam so it makes a perfect seal with your ear canal. They also come with plastic reusable earpieces a bit like the ear plugs you can use for swimming.
Photo: For noise isolating earbuds to work properly, they have to make a tight seal with your ear canal to prevent ambient noise from getting in. But different people have different-sized ears, so how can one pair of earbuds fit everyone? Quite a few manufacturers now attempt to solve the problem by supplying a choice of different tips for their noise-isolating earbuds—and these are the ones that come with the Etymotic HF5. Made from different shapes and sizes of plastic and memory foam, with and without flanges, they simply clip onto the ends of the earbuds.
Active noise reduction
A much more advanced way of getting rid of the noise is called active noise reduction, and it's used in the sophisticated noise-cancelling headphones that pilots use. Headphones like this have a small microphone (sometimes called a "reference" microphone) built into their case. The microphone constantly samples the background noise and feeds it to an electronic circuit inside the headphone case. The circuit inverts (reverses) the noise and plays it into the loudspeaker that covers your ear. The idea is that the noise you would normally hear is canceled out by the inverted noise—so all that's left (and all you hear) is near-silence or the music you want to listen to. Headphones that work in this way include the Bose QuietComfort®, which uses a system called Acoustic Noise Cancelling®.
Some forms of active noise reduction go even further by using multiple reference microphones. Some also have a separate ("error") microphone to sample the final sound output and send feedback to the noise-reduction circuit. If the output is too noisy, the circuit can adjust how it's cancelling to try to improve its performance.
Advertisements for active noise-cancelling headphones tend to exaggerate their benefits: you imagine yourself on the plane or the train, drifting off to slumber in a wonderful field of perfect silence. Unfortunately, that's not what happens. They don't remove all noise; they're best for reducing continuous, low-frequency, constant-tone sounds. For general noise reduction, simple earplugs can be much more effective—and they're obviously much cheaper too.
Photo: Some manufacturers combine active and passive noise reduction in the same headphones. These Audio Technica AN3 earbuds use active noise cancelling (when the power is switched on), but you'll notice they have similar suction-type tips to noise-isolating earbuds, so they reduce noise even when they're switched off. One of the big drawbacks of noise-cancelling headphones is that they need batteries to power their "active" electronics; headphones like this can still eliminate background noise even when the batteries run flat (or are switched off). Photo courtesy of Audio Technica published on Flickr under a Creative Commons (BY-ND-2.0) licence.
How active noise reduction works
Suppose you have the noise of a pneumatic drill (jackhammer) driving you mad. You put on your noise-cancelling headphones, switch them on, and the drilling noise virtually disappear. How does that work? We've already seen that the headphones superimpose a reversed version of the drilling noise on top of the original noise, but why doesn't that simply make the noise twice as loud?
Sound is energy traveling through the air in waves. Sound waves don't look like the waves on the sea—indeed, you can't see them at all. If you could see sound traveling, you'd see it squeezing air molecules together in some places and stretching them out in others. In other words, sound travels by making the air pressure change. Now suppose there's a sound wave traveling between a pneumatic drill and your ear. At any given moment, the air between the drill and your ear has areas where the sound is compressed (compressions) and areas where's it's stretched out (rarefactions). Suppose you could exactly reverse the sound made by the drill and superimpose it on top. Now the original compressions would be replaced by rarefactions and vice versa. Two waves that are precisely reversed in this way are said to be in antiphase. Adding an original sound and the same sound in antiphase would, in theory, make the two sounds completely cancel each other out—leaving nothing but silence!
Artwork: How active noise-cancelling works. 1) Ambient noise hits the outer case of the headphones. 2) One or more reference microphones pick up the noise and feed it to an active noise cancellation (ANC) circuit, which is powered by a battery. 3) The circuit inverts the ambient noise waves from the reference microphones and combines them to produce the maximum amount of noise reduction. 4) A speaker driven by the circuit plays the inverted noise on top of the ambient noise. 5) The inverted waves (purple) cancel out much of the original ambient noise (blue). 6) The greatly reduced noise that remains feeds into your ear. 7) An error microphone picks up the reduced noise so it can be measured by the ANC circuit and used to improve overall noise cancellation.