Active noise cancellation (ANC) is all the rage in high-end audio. It used to be so expensive that only it was only available to industry professionals that absolutely needed it (airplane pilots and military personnel) and audiophiles willing to pay the big bucks. But by the end of 2019, Sony and Apple both offered in-ear wireless EarPods for around $300, and other companies provided even cheaper versions.

The intention behind active noise cancellation is to dramatically reduce the external “noise” from the local environment, so the listener hears only the “signal,” — the source music or spoken audio.

Passive noise cancellation strategies attempt to isolate the signal from the noise, creating an effective barrier against environmental noises and delivering the signal directly to the ear canal. Well-crafted passive methods can be pretty effective. They are also low-tech and can be quite inexpensive, but they’re not as good as ANC techniques, and they tend to be bulkier and more cumbersome. Passive solutions are not as good for people who are active and mobile.

ANC systems use a combination of microphones and speakers. The microphones are located in the outside of the device, away from the speakers. They capture the environmental noise and feed it to processors that analyze the noise patterns and generate a waveform that is the exact opposite of the noisy audio. When the noise wave runs into the anti-noise, they produce a flatline signal: Ah, silence! The combination of noise and anti-noise is known as “destructive interference.”

The listener is left with only the pure audio signal piped directly into the ear. What’s especially great is the listener doesn’t need to crank up the tunes to college with environmental sounds because those sounds are mostly silenced.

So, how does ANC process environmental noise in real-time and generate a new anti-noise wave before the matching noise reaches your ear? The answer is somewhat surprising. It has to do with the distance between the external microphones and the infernal speaker. It takes between 10 and 100 µs (microseconds) to get from the speaker to the processor, and that is a short enough time for the speakers to play the matched anti-noise wave.

People unfamiliar with this technology have worried that AMC technology could harm the user’s hearing. They fear that the auditory canal is still being exposed to the noise signals, and the user simply doesn’t hear it. But that’s not how hearing works. In order to hear something, the vibration waves have to impinge on the ear canal, but with ANC, the flatline waves don’t trigger sound or harm. In fact, ANC is more likely to protect the user’s hearing.