The first time you try the noise cancelling feature on earphones you’re probably (and understandably) blown away at their effect. This is especially true in a noisy environment – you pop the earphones in, turn on the noise cancelling feature, and the world fades away. Let’s talk about the very simple science of how do noise cancelling earphones work and how it relates back to something I’ve talked about a lot in mixing.
How Do Noise Cancelling Earphones Work
Earphones or headphones with noise cancelling capabilities have tiny microphones built in which pick up outside noise in real time.
To cancel out this noise, they simply invert the phase of that captured signal then feed it that into your earphones.
In other words, the earphones are amplifying an inverted phase instance of the outside noise into your ears.
Because of the properties of phase cancellation, the outside noise and the instance the earphones are feeding into your ears are cancelling one another out, resulting in silence.
This is demonstrated in this graphic, where “Track 1” is the outside noise, and “Track 2” is the signal you’re “hearing”:
The two wave forms cancel one another out. You may still hear a very faint instance of the outside noise, but this is mostly the product of the earphones not delivering a 100% sound-tight fit to completely block it out.
This is a real world example of when phase cancellation is a GOOD thing. Bringing this back to the context of mixing, phase cancellation can result when recording the same source with more than one microphone or input.
Sound travels very quickly, but it’s not instantaneous to the point that small differences in distance of microphone placement to the source won’t result in the wave forms being captured at a difference of a few milliseconds.
This can be enough to cause phase issues, or in other words anytime the two tracks wave forms aren’t perfectly in sync.
While being out of sync at 180 degrees as I mentioned earlier in describing how to noise cancelling earphones work results in literal silence by way of phase cancellation, just the difference of a few milliseconds can result in the tracks not being completely out of sync but also not being perfectly aligned.
Slight phase issues won’t result in complete silence, but they can be enough to muffle or weaken the combined sound of the two tracks.
In the case of noise cancelling earphones, they automatically align the phase inversion perfectly to account for cancellation so the outside original and instance which is playing through the earphones are always 180 degrees out of alignment to cancel one another out.
So the next time someone is curious how do noise cancelling earphones work, you’ll know the very simple scientific explanation which achieves a very useful effect!
