How Do Noise Cancelling Headphones Block Out Noise?

Sourced from [1]

There are two types of noise cancellation used in headphones: passive and active.

Passive noise cancellation (PNC) is an intuitive concept. It imposes a physical barrier between the user’s ear and the environment, using specialised materials that absorb sound waves.[3]

Active noise cancellation (ANC) uses a more complicated technology, making use of destructive interference to cancel out unwanted sounds. Destructive interference occurs when two waves with the same amplitude and frequency meet 180° out of phase and they cancel each other out. This phase difference is effectively the opposite mirror image.[4]

Sourced from [4]

Noise cancelling headphones use a microphone to pick up noise, followed by delivering the frequency, amplitude and phase of the wave to a processor which produces an identical wave, phase shifted by 180°. This new sound wave is played into the speakers of the headphones to cancel out the original noise.[3]

Active noise cancellation effectiveness varies with the type of sound it has to combat. The noise cancellation system has a delay between the reception of the sound wave and the delivery of the inverse sound wave.[5] The system effectively has to guess how the sound will continue in the future. Noise cancellation is optimal at low frequencies – consistent, low pitched sounds that are not likely to fluctuate massively, such as aeroplane hum and air conditioning systems.[3] 

High frequency sounds, such as people talking and babies crying are not as easy to predict, and thus the noise cancellation is not as effective in combating these types of noise.[5]

Noise cancelling headphones vary widely in cost, starting around €50 for in-ear buds, to upwards of €700 for the newest, state-of-the-art wireless over-ear headphones. Of course, the price will reflect the degree of noise cancellation, as well as other aspects such as sound quality and battery life for wireless models.

Not only do noise cancelling headphones improve audio listening experience, but they can also be an aid for individuals with sensory issues, making high-noise environments a less stressful experience. Noise cancelling headphones are an interesting example of an everyday product whose operation may seem complicated, but can be explained by a simple physical concept.

[1] “Bose QuietComfort SC Headphones,” Bose.ie, 2024, https://www.bose.ie/en_ie/products/headphones/noise_cancelling_headphones/QCSC-HEADPHONEARN.html#v=QCSC-HEADPHONEARN-BLK-WW

[2] OpenStax, “College Physics,” LibreTexts, Apr. 15, 2025. Available at: https://phys.libretexts.org/Bookshelves/College_Physics/College_Physics_1e_(OpenStax)

[3] J. Caesar, “Active vs. Passive Noise Cancellation: Which is Better?,” techreviewadvisor.com. https://techreviewadvisor.com/active-vs-passive-noise-cancellation/

[4] OpenStax, “University Physics I – Mechanics, Sound, Oscillations, and Waves,” LibreTexts, Available at:

https://phys.libretexts.org/Bookshelves/University_Physics/University_Physics_(OpenStax)/Book%3A_University_Physics_I_-_Mechanics_Sound_Oscillations_and_Waves_(OpenStax)

[5] Ashish, “Why Noise Cancelling Headphones Cannot Block High-Pitched Sound?,” ScienceABC, May 10, 2018. https://www.scienceabc.com/innovation/why-are-noise-canceling-headphones-less-effective-at-blocking-high-pitched-sounds.html