By: CJ

Noise-cancelling headphones can make a busy airplane, crowded café, or noisy street feel surprisingly quiet. At first, it might seem like the headphones are simply blocking outside sounds, but active noise cancellation works in a more interesting way.

The technology uses microphones, speakers, and some fairly clever audio processing to reduce certain sounds before they reach your ears.

Passive Noise Blocking vs. Active Noise Cancellation

There are actually two different ways headphones can reduce outside noise.

The first is passive noise isolation. This is simply the physical design of the headphones. Ear cups create a barrier around your ears, while earbuds sit inside or around the ear canal. Materials such as foam and silicone can block some outside sounds.

Active noise cancellation, usually called ANC, takes this a step further.

ANC uses microphones to listen to the sounds around you. The headphones then create another sound designed to cancel out some of that unwanted noise.

How Does One Sound Cancel Another?

The basic idea comes from the way sound waves interact.

Sound travels through the air as waves. When two sound waves meet, they can either strengthen each other or reduce each other depending on how they line up.

Noise-cancelling headphones take advantage of this by creating a sound wave that is essentially the opposite of the unwanted sound.

For example, imagine a steady low rumble coming from an airplane engine. The microphones inside the headphones pick up that sound, and the headphones produce an opposing signal. When the two interact, the unwanted noise is reduced.

The headphones aren’t actually deleting the original sound. They’re creating another sound that reduces how strongly it reaches your ears.

Why ANC Works Better on Some Sounds

Noise cancellation is particularly good at dealing with steady, predictable sounds.

The constant hum of an airplane engine is a good example. A train, air conditioner, or bus engine can also produce sounds that ANC handles fairly well.

Sudden noises are much harder.

If someone suddenly drops a book beside you, the headphones have very little time to detect the sound and create a matching response. Voices can also be difficult to completely remove because they change constantly and contain a wide range of frequencies.

This is why you can still hear someone talking even while wearing good noise-cancelling headphones.

The Microphones Do More Than Listen

ANC headphones usually have microphones that monitor the environment. Depending on the design, there may also be microphones positioned inside the ear cups.

External microphones can detect sounds approaching the headphones, while internal microphones can monitor what is actually happening inside the ear cup.

The headphones’ processor uses this information to adjust the cancellation.

All of this happens extremely quickly. The system has to analyze the sound and produce its response fast enough for the cancellation to be useful.

Why You Can Still Hear Some Noise

Noise cancellation isn’t perfect.

Higher-frequency sounds, sudden noises, and nearby conversations can still make their way through. Physical noise can also reach your ears through the headphones themselves.

This is why ANC is usually most noticeable with low, continuous sounds rather than every sound around you disappearing completely.

The combination of passive isolation and active cancellation is what makes modern noise-cancelling headphones so effective.

Does Noise Cancellation Make Music Sound Better?

It can, although that isn’t necessarily its main purpose.

When there is less background noise, you don’t have to compete with outside sounds to hear your music. You may therefore be able to listen at a lower volume while still hearing the details clearly.

ANC can also make podcasts, audiobooks, and calls easier to follow in noisy environments.

For many people, the biggest benefit isn’t complete silence. It’s simply having fewer distractions competing for their attention.