How do headphones cancel noise by making more sound?

At a glance

Listen

Microphones measure unwanted sound.

Oppose

Electronics create a matching opposite signal.

Reduce

The waves combine near your ear.

Sound is a changing pressure

A noise makes air pressure rise and fall. The wave diagrams are a simple way to draw those changes over time. They are illustrations, not recordings from a real headset.

Source: Bose: What is active noise cancellation?

A microphone notices the noise

Tiny microphones measure sound around the headphones. Electronics analyse this signal so the speaker can add a carefully timed opposing sound. Headphone designs vary; microphones may monitor outside the ear, inside it, or both.

Source: Sony: What is noise cancellation and what can I expect? · Bose: What is active noise cancellation?

How does an opposing sound wave reduce noise?

Where the unwanted sound pushes pressure up, the cancelling sound aims to pull it down, and vice versa. When the timing and size line up, the combined pressure variation is smaller. This is why more sound can mean less noise at your ear.

Source: Bose: What is active noise cancellation?

It does not silence the whole room

The system targets what reaches the listener inside the earcup or earbud. People beside you still hear the room. Real noise is more complicated than two perfect waves, so the result is reduced noise rather than guaranteed silence.

Source: Sony: What is noise cancellation and what can I expect? · Bose: What is active noise cancellation?

Why is a steady hum easier to reduce than a sudden shout?

Active noise cancellation generally works best on steady low-frequency noise, such as an engine or air-conditioning hum. A changing voice or a sharp sudden sound is harder to counter as effectively. Performance depends on the headphones and the sound.

Source: Sony: What is noise cancellation and what can I expect? · Bose: What is active noise cancellation?

Fit and power still matter

Earcups and ear tips also form a physical barrier against sound. That passive isolation works alongside electronic cancellation. Many ANC headphones can work while music is paused, but their microphones, processor and speaker still need power. Check the mode and fit if the effect seems weak.

Source: Bose: What is active noise cancellation? · Bose: Do noise cancelling headphones work without music? · Sony: The Noise Canceling effect is weak or not working