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.
If you want to find out about how ordinary headphones work first, take a look at our separate articles on loudspeakers and headphones.
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.
There are two ways to reduce the noise in your headphones, one simple and one complex.
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.
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 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®.
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.
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!
Do you believe this? It’s true! We can prove it with a simple sound-recording program called Audacity.
First, I’ve recorded two seconds of a pure tone at 440Hz (in music-speak, that’s the A above middle C). It sounds like this:
On an oscilloscope (a TV-screen used for showing waves), it has an up-and-down wave pattern like this:
Now I’ve used Audacity to invert the sound wave. It sounds exactly the same—like this:
But it looks like this:
Put the two sound waves one above the other and you can see that they are exact opposites:
Now, using Audacity, I’m going to add the inverted sounds for two seconds. And then, straight afterwards, I’m going to add the first wave to itself for two seconds. What’s this going to sound like? A bit like this:
For the first half, we get silence because the two waves cancel out. For the second half, we get a noise that’s twice as loud because the two waves reinforce one another. And that’s how noise cancelling works!
With real noise reduction, it’s never possible to exactly cancel the two sounds out, so there’s always some background noise left over. But it’s still a distinct improvement.
As you might expect, it’s a matter of preference. Passive, noise-isolating headphones tend to be less expensive than active ones, though high-end headphones like those from Etymotic, which have very high-quality audio performance, are still expensive (mine cost me something like $200 or £100 a few years ago, though the price has now dropped). The best thing you can do is try out different headphones and see what suits you. Remember that active noise-cancelling headphones are designed to reduce predictable, steady noises like airplane engine hum, not complex varying sounds like voices, so they’re not so good for cutting out the sound of people’s inane chatter. If that’s the noise that’s bothering you, you need a different solution…
Photo: Earplugs like this are great for blocking out most unwanted noise, but the ultimate solution is to wear headphones on top and play white or pink noise through them. The top plugs are disposable soft foam ones and can be reused a few times before you throw them away. The bottom ones are a heavier duty flanged type that you can use and reuse endlessly.
Students trying to revise while other people play music often fret about getting peace and quiet. Here’s my foolproof solution to noisy neighbors, mad parties, construction noise, and other distractions that stop you working. If you’re bothered by people’s conversations or music, and earplugs or noise-cancelling headphones don’t help you, a really effective solution is to record yourself an MP3 of white noise (steady noise like you’d hear from the wind or the sea) or pink noise (a deeper version of white noise, like an airplane engine) and put that on your music player. You can find plenty of samples on the Internet—and there are also some readymade apps that do the same thing (search your favorite app store for “white noise”). Simply play the noise in your ears at reasonable volume and it should cancel out most things. Because it’s not music or speech, your brain gradually tunes it out and you can concentrate on what you’re doing. The ultimate solution I’ve found for really disruptive construction noise is to put foam earplugs into your ears, put large headphones on top, then play the white or pink noise as well. The combination of earplugs, headphones, and white/pink noise will cancel out virtually any background noise without damaging your hearing. A pretty extreme solution, but it really does work! Two things to remember. First, foam earplugs are viscoelastic, so you have to curl them up tightly to squash them into your ear canal, then hold them there for 30 seconds or so while they relax and seal themselves firmly in place. Second, if you block noise this way, you will also block sounds from things like telephones and (more seriously) smoke detectors and carbon monoxide alarms.
Photo: Apps like White Noise for Android let you play a variety of natural, ambient sounds through your headphones, including waves crashing on the beach and rainstorms. Use an app like this as an alternative to white noise, if you find it too harsh and disruptive.
Many people suppose that Bose, which sells the best-known brand of noise-cancelling headphones, invented the technology—and did so relatively recently. In fact, as Professor Colin Hansen (of the Department of Mechanical Engineering, University of Adelaide) points out in an excellent introductory book on the subject, the technology is much older. Hansen traces it back to experiments with telephones in 1878, and notes that the first patents were issued (separately) to Romanian aircraft engineer Henri Coandă in 1932 and German physicist Paul Lueg the following year (see his US patent number 2043416: Process of Silencing Sound of January 27, 1933, patented in the United States in 1936). Both men developed systems for cancelling out sound waves by adding other waves in antiphase. Many others built on this work, including synthesizer pioneer Harry F. Olson. Until the early 1990s, active noise reduction was little more than a “laboratory party trick”; then, as Hansen notes, the science swiftly became a practical technology, with a growing number of everyday commercial applications—the best known of which are noise-cancelling headphones.