What Your Noise-Cancelling Headphones Can and Can’t Do

This Amazon commercial bugs the heck out of me. In it, a young boy jumps on a bed, raucously playing an electric guitar while another kid plays drums. Meanwhile, a father relaxes blissfully undisturbed because he’s wearing noise-cancelling headphones. This scene drives me bonkers because, as a headphones expert, I know the truth: That’s not how noise-cancelling headphones work.

It’s a popular misconception that noise-cancelling headphones can block out any sound around you. People purchase them in the hopes of dimming the din of kids at play, loud-talking officemates, the barking dog next door, and airplane engines. The trouble is, active noise cancellation is really effective on only one of those things. (Spoiler: It’s the airplane engine.) The reason has to do with the physics of sound and how noise-cancelling headphones work.

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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 cancelled out.

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Active noise-cancelling 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-cancelling 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-cancelling 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-cancelling 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.

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Take a look at the chart below. The Sony MDR-7506 is a pair of passive studio headphones that typically costs under $100. Above 1 kHz, these headphones block about as much sound as all the active noise-cancelling headphones. That said, even the best passive over-ear headphones can’t completely block higher-frequency sounds, especially if those sounds are very loud. But they can muffle most day-to-day sounds to the point where they won’t break your focus.

The most reliable way we’ve found to block higher-frequency sounds is to use earbuds that are designed to completely and deeply seal the ear canal. Many audiophile-style earphones that use over-ear cable routing are designed to slip farther into your ear, so they also do well at blocking higher-frequency sounds. Another solution is to add noise-isolating foam tips to your existing earbuds; these tips sometimes come with earbuds or are available from third-party suppliers such as Comply.

The benefit of using passive noise-isolating headphones and earbuds, aside from often saving money, is that you won’t experience eardrum suck, an uncomfortable sensation that some people encounter with ANC headphones. The feeling can range from subtle (as if your ears need to pop) to intense (like a full-on headache). Some people don’t experience it, and some are able to ignore it or adapt to it; but for others, it’s a dealbreaker that prevents them from using ANC headphones.

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 true 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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