How Noise Cancelling Actually Works

Every pair of active noise cancelling headphones is running a small argument with the world. A microphone on the outside samples the noise arriving at your ear, the electronics invert that waveform, and the driver plays the inverse back at you. Two opposite waves meet and, for the most part, cancel.

That trick works best on sound that is steady and low: engine drone, cabin rumble, the hum of an air conditioner. It works far less well on sudden, high-frequency noise, which is why a keyboard in an open-plan office is harder to remove than a jet engine at cruise.

The part nobody puts on the box is that cancellation quality depends on the seal. A cushion that has flattened after two years of daily use leaks low frequencies straight past the microphone, and no amount of processing recovers them. If your noise cancelling has quietly got worse, replace the ear pads before you replace the headphones.

Ambient or transparency mode does the opposite: it feeds the outside microphones back into the drivers so you can hear a platform announcement without lifting a cup. On our current range it is a single tap on the right ear.