What is the difference between active and passive noise cancelling?

Passive blocks sound physically. Active generates sound to cancel it. They work on different frequencies, which is why good headphones use both.

Passive noise isolation is just physics and materials — a seal that stops sound reaching your ear. Over-ear cups with dense padding, or in-ear tips that seal the canal. It works best on high frequencies, which have short wavelengths and are easily blocked by physical barriers. It requires no power and adds no processing.

Active noise cancellation (ANC) uses microphones to sample incoming sound, then generates an inverted waveform — equal amplitude, opposite phase — through the driver. The two waves interfere destructively and largely cancel.

ANC works best on low frequencies. Long wavelengths give the electronics time to sample, compute and output the inverse before the sound reaches your ear. This is why ANC is transformative on the constant low drone of aircraft cabins, trains, traffic and air conditioning, and far less effective on speech, a crying child, or a keyboard — sudden, high-frequency, unpredictable sounds.

People who expect ANC to silence an open-plan office are usually disappointed for exactly this reason.

Feedforward ANC uses external microphones; feedback uses one inside the cup measuring what reached your ear; hybrid uses both and performs best.

Practical points:

Fit dominates. A poor seal ruins both passive isolation and ANC, because the system cannot correct for leakage it cannot measure. Trying different ear tip sizes typically improves in-ear ANC more than buying a more expensive model.

ANC uses battery and can slightly affect sound quality, though this has largely been engineered out.

Some people experience pressure sensation or mild discomfort from ANC — a real effect, thought to relate to the absence of expected low-frequency pressure variation.

Transparency mode deliberately reverses the process, piping outside sound in.