If you look on amazon right now under the headphones/audio department, you’ll see headphones advertised as noise reducing, noise blocking, noise cancelling, or sound isolating. Upon first glance, they may all appear to be the same. Either that, or they’re all different and shopping for headphones just got a lot more difficult. However, there’s only one term that is worth the hype.

Noise cancelling is the only term from the many listed above that actually means something. All of the other terms simply refer to headphones that passively obstruct your ears from noise, such as with foam padding and a good shape to fit the side of your head. Noise cancelling is more than that.

Technically known as Active Noise Control (ANC), noise cancelling is simply intentional destructive interference of sound waves. Sound waves are all around us, and the microphones built into noise cancelling headphones can detect these sound waves accurately in real time. Then, the noise cancelling headphones actually play a sound to your ear that is the exact same as the noise incoming from the outside.

This seems counter-intuitive at first. How can noise be reduced by playing more of the same noise? Well, this is where the destructive interference comes in.

As seen below, the waves are either in line with each other and amplify each other or out of line with each other and cancel each other out in destructive interference. This is what happens with sound waves when noise cancelling headphones play replay exterior noise; the sound waves are precisely aligned through calculations involving the frequency of the sound and the speed of sound such that the exterior sound and the replayed sound cancel.

Wave interference taken from UCSD

All of these calculations are done in real time by headphones when noise cancelling is activated, which is pretty incredible. However, noise cancelling has more applications than just headphones. If instead of isolating a single person from all noise someone wanted to muffle a single noise from all people, this same technique can be applied to loudspeakers strategically placed in a room/area with an unwanted noise. For example, if a private conversation were happening in a room, active noise control could be used to hide the conversation from eavesdroppers by creating what the IEEE calls a “quiet zone” within a certain radius. This could also muffle the sound of loud interior HVAC systems or the background noise in plane cabins.

This application of active noise control, called 3-D ANC, has its limitations. With multiple speakers needed to project sound in all directions/areas, these multiple sound sources could create unpredictable or unwanted areas of constructive, rather than destructive interference. Thus, a true “quiet zone” that covers the intended area is very difficult to achieve. However, it still has its useful applications in reducing noise pollution and increasing privacy, as well as possible less useful theoretical applications where I could go around and delete other people’s conversations from existence. That’s not science fiction; it’s science.