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Michael: You’re on a tedious flight, and the drone of the engines is getting on your nerves. So you pop on a pair of noise-canceling headphones, and sweet, blessed silence descends. But those headphones aren’t just muffling the sound -- they’re actually making it go away.

Active noise-canceling headphones get rid of noise using the physics of sound waves, and their ability to cancel each other out using what’s known as destructive interference. Sound is made up of waves. And as a sound wave moves through the air, it compresses and expands the air in repeating patterns. Destructive interference is what happens when a second sound wave combines with the first -- but this second wave has a pattern that expands the air where the first wave was compressing it, and compresses the air where the first wave was expanding it. When you stack these expansions and compressions on top of each other, you get air that isn’t really compressed or stretched. The compressions un-stretch the expansions, and the expansions un-bunch the compressions. So the sound wave fizzles out, because those spaces in the air that were the sound don’t exist any more.

Noise-canceling headphones incorporate a microphone that listens to ambient sound and calculates a sound wave that will cancel it out. Then it plays that second sound wave, and because of destructive interference, you get silence where there used to be noise. But not total silence. That second wave has a pattern that’s designed to cancel out noise, but only the noise outside your headphones. So you get your own personal sound bubble, and it leaves whatever you were listening to alone.

And noise-canceling technology won’t get rid of all outside noise, but it’s good for the equivalent of about 70% of the noise on a loud flight. It works best on noise that’s fairly constant, like an airplane engine, because the sound waves are more consistent. The headphones can cancel out that constant droning with a constant, opposite-sound of their own. Noises that are sudden or change frequently in pitch -- like a baby crying -- are much harder for the device to react to in time, so it won’t cancel them out as effectively. So the headphones might help get rid of the plane’s droning noise, but if you’re sitting right behind a six-month-old, you’re on your own.

Thanks for asking, and thanks especially to our President of Space, SR Foxley! If you’d like to be President of Space, or just want to help us keep making episodes like this, you can go to patreon.com/scishow. And don’t forget to go to youtube.com/scishow and subscribe!