Active noise reduction technology functions primarily by exploiting a principle of physics called phase cancellation. As you probably know, sound travels in waves, moving the air molecules. These waves travel through the air and into your ear canal, where they vibrate your eardrum. However, if a sound wave meets another sound wave that is the exact same in frequency and opposite in amplitude, the two largely negate each other.
Picture the air molecules as a string stretched between two points. If someone were to press down on the string in the direct center, that would disturb the string, causing a ripple. If, as you pressed on the string on one side, someone else were to press on the string from the opposite side at the exact same place with the exact same force, the string would barely move. Although this visual doesn’t precisely communicate how sound waves work, it helps you to picture how a wave, when matched with its opposite in phase, is effectively cancelled out.
Active-noise-cancelling headphones use tiny microphones on the inside (and sometimes the outside) of the earcups to process the sound headed toward your ears and immediately play the opposite phase of that sound through the headphone drivers. The opposing forces effectively reduce the air-molecule movement, and you get a reduction in perceptible sound. Again, this description is a simplification, but it’s the basic concept that all ANC headphone designs currently go by.
Generally speaking, this type of active noise cancellation is most effective on lower frequencies of sound, between 50 Hz and 1 kHz. (If you’re curious about what 1 kHz sounds like, watch this video.) This is partly because lower frequencies produce longer waveforms that are easier to line up properly. Also, at higher frequencies, if the waveforms don’t line up just right, you’re more likely to encounter feedback. So most active-noise-cancelling headphones have a noticeable dip in usefulness right at the 1 kHz point.
This is why ANC is better suited for reducing low, sustained sounds like those of motors and airplane engines, and it’s why such headphones can’t filter out screaming kids.
