How Do Noise Cancelling Headphones Work: A Simple Explanation of the Physics of Sound

Chances are good that if you strap on a pair of headphones, you’re hoping to block out the sound of the outside world. But that’s not always true, especially if you’re exercising in public—in which case you need to hear the approaching sound of anything that might do you bodily harm. Otherwise, it’s safe to assume that when the headphones go on, you’d much rather hear your music, movie or game than the sound of roaring traffic, a noisy air conditioner or, worst of all, a roaring airplane engine.

A good pair of on- or over-ear headphones, or earphones with great-fitting ear tips, will block out some unwanted sound. That’s what’s known as “passive sound isolation,” which basically means that the material of the headphones is forming a somewhat soundproof barrier between your eardrums and the sound of the outside world.

But if you really want to nip noise in the bud, you need headphones with active noise cancellation technology. Phiaton’s BT 100 NC neck band-style earphones and BT 120 NC sweat and water-resistant earphones work by measuring the sound of the world around you via tiny microphones, then mixing the opposite of whatever noise might be a distraction into the sound coming from your headphones.

But wait. Isn’t the opposite of noise just…silence?

Not quite. It helps if you remember that sound reaches our ears in the form of waves. And if you could see those waves, you would see the sound of, say, a perfect A note played from a keyboard rising toward a high point, called the crest, then falling toward a low point, called the trough, then rising and falling and rising and falling again, with all the crests and troughs perfectly spaced.

The opposite of that wave would be one in which the crests and troughs are swapped. In other words, when one wave is rising, its opposite is falling. And when the first wave is passing through its highest point, the other is reaching its lowest.

Of course, the sounds in the world around us aren’t perfect waves. They look more like this:

That wiggly line represents a whole host of sounds played at once. Step back from your screen and squint, and the larger pattern of rising and falling represents low frequency sounds. In other words: bass. The little jiggles that you see along the way represent higher frequency sounds. Put them together, and that pile of squiggles represents a tiny fraction of a second’s worth of all the bass, treble, midrange, etc. that you would hear at any point in time. The taller the squiggle, the louder the sound when it reaches your eardrums.

For active noise cancelling headphones to block that sound from reaching your ears and interfering with your music or movies, tiny sound processors built into the headphones have to measure that sound and create its opposite, which would look like this:

If you played that second sound wave by itself, it would sound pretty much identical to the first. Put the two waves together, though—the original sound and its opposite—and what you end up with is something like this:

You may be thinking that looks even noisier but remember: the crest of one wave perfectly matched with the trough of the other cancels it out, like positive and negative numbers.

In a perfect world where all that sampling, sound processing and tone generation happens instantaneously, the result would be perfect silence. In the real world, those processors take an infinitesimal fraction of a second to work their magic. It’s still enough time that the tiniest of wiggles in the wave, representing the highest of frequencies, might not line up perfectly. That’s why active noise cancelling headphones work best on low frequency sounds. It’s also why they do a better job of canceling constant droning noise rather than quick, instantaneous sounds.

But hey, chances are also good that even if you’re interested in blocking out as much noise as possible, those quick, instantaneous sounds are ones that you might want to let sneak through.

By Dennis Burger 

July 14, 2018

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