What Your Noise-Cancelling Headphones Can and Can’t Do for YOU!

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The physics of active noise cancellation

Active noise reduction technology functions primarily by exploiting a principle of physics called phase cancellation. As you probably know, sound travels in waves, moving the air molecules. These waves travel through the air and into your ear canal, where they vibrate your eardrum. However, if a sound wave meets another sound wave that is the exact same in frequency and opposite in amplitude, the two largely negate each other.

Picture the air molecules as a string stretched between two points. If someone were to press down on the string in the direct center, that would disturb the string, causing a ripple. If, as you pressed on the string on one side, someone else were to press on the string from the opposite side at the exact same place with the exact same force, the string would barely move. Although this visual doesn’t precisely communicate how sound waves work, it helps you to picture how a wave, when matched with its opposite in phase, is effectively canceled out.

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Active noise-canceling headphones use tiny microphones on the inside (and sometimes the outside) of the earcups to process the sound headed toward your ears and immediately play the opposite phase of that sound through the headphone drivers. The opposing forces effectively reduce the air-molecule movement, and you get a reduction in perceptible sound. Again, this description is a simplification, but it’s the basic concept that all ANC headphone designs currently go by.

Generally speaking, this type of active noise cancellation is most effective on lower frequencies of sound, between 50 Hz and 1 kHz. (If you’re curious about what 1 kHz sounds like, watch this video.) This is partly because lower frequencies produce longer waveforms that are easier to line up properly. Also, at higher frequencies, if the waveforms don’t line up just right, you’re more likely to encounter feedback. So most active noise-canceling headphones have a noticeable dip in usefulness right at the 1 kHz point. This is why ANC is better suited for reducing low, sustained sounds like those of motors and airplane engines, and it’s why such headphones can’t filter out screaming kids. (We’ve talked with researchers who say there are ANC concepts in the works that would do a better job with higher frequencies, but that technology is likely still a few years away.)

What type of headphone should you buy?

For frequent flyers or folks who want to ignore an annoying air conditioner hum, active noise-canceling headphones are a great option. But what if you want to block out human voices or barking dogs? Well, that’s where passive isolation comes in. Passive isolation is a physical barrier between your ears and the sounds you don’t want to hear. Many of the best—and, usually, the most expensive—noise-canceling headphones are equipped with earcups and earpads designed to block as much mid- and high-frequency noise as possible while remaining comfortable. So you get the best of both worlds: active and passive noise reduction.

However, if you’re looking to block out only human voices and other higher-frequency sounds, you have other options. A pair of less expensive, passive headphones might serve you just as well. Nearly all closed-back, over-ear headphones—especially those designed for recording—do a solid job of attenuating the sounds of kids playing, workmates chatting, and dogs barking.

Other noise-blocking options

If you’ve tried the suggestions above and you still can’t successfully tune out the sounds that distract you, you have other options. First, white noise, rain, or wave sounds are good for masking external noise. Tons of apps are available for that. Just be sure to listen at reasonable volumes to protect your hearing: Experts say that listening at a maximum of 60 percent of your device’s volume for 60 minutes in duration is usually safe. Then take a few minutes for a noise break before resuming.

Of course, there’s always earplugs. If you’re highly prone to distraction yet still want music, you could go for what I call the “nuclear option,” which is to wear earbuds under hearing-protection earmuffs. If you do this, however, keep in mind that you won’t be able to use the controls on truly wireless earbuds, and corded earbuds will leave you with a wire crease in your cheek. Is it comfortable? Eh … if the earbuds don’t stick out too much and the earmuffs have a deep enough earcup, you can get used to it. But I’ll tell you what, it sure is effective. In fact, it may be the only thing I’ve tried that truly blocks out the noise of a pint-sized rock band like the one in that commercial.

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