How Do Active Noise Cancelling Headphones “Erase” Noise Out of Thin Air?

In our noisy world, noise cancelling headphones have become essential for many seeking quiet and focus. But how do these devices seemingly make unwanted sounds disappear? The secret lies in two main principles: passive noise cancellation and active noise cancellation (ANC). This article explains how both work, with a focus on how ANC uses sound wave physics to “cancel” noise.

Passive Noise Cancellation: Physical Barriers at Work

Passive noise cancellation is the simplest form of noise reduction. It relies on the physical design and materials of the headphones to block sound waves from reaching your ears.

Thick ear cushions, snug-fitting ear cups, and dense foams act as barriers that absorb and reflect external sounds.
This method is effective mainly for mid- and high-frequency noises, such as human voices or keyboard clicks.
However, low-frequency sounds like airplane engines or subway rumbles can still penetrate these physical barriers more easily1 2.

In short, passive noise cancellation is about physically isolating your ears from ambient noise using the headphone’s structure.

Active Noise Cancellation: Cancelling Noise with Anti-Noise

Active noise cancellation takes a more sophisticated approach by using sound wave interference.

The Science of Sound Wave Interference

Sound travels in waves, characterized by peaks (compressions) and troughs (rarefactions). ANC works by generating a sound wave that is the exact inverse (opposite phase) of the ambient noise.

When two waves of the same frequency and amplitude but opposite phase meet, they cancel each other out through destructive interference.
This means the positive pressure of one wave fills the negative pressure of the other, resulting in a net sound pressure close to zero2 3.

How ANC Works in Headphones

Microphones on the headphones constantly “listen” to ambient noise around you.
The headphone’s processor analyzes this noise and creates an “anti-noise” wave with the opposite phase.
This anti-noise is played through the headphone speakers simultaneously with the noise.
The two waves combine and cancel each other out, reducing the noise that reaches your ears.

This process happens in real time and adapts continuously to changes in the environment.

Practical Considerations

ANC is most effective at cancelling low-frequency, constant noises such as airplane engine hum or air conditioning.
It is less effective against sudden, irregular noises like people talking or clattering dishes.
Some users may notice a slight hissing sound or a feeling of pressure when ANC is active, which is a side effect of the technology but not harmful to hearing.

Summary: Passive vs. Active Noise Cancellation

Feature	Passive Noise Cancellation	Active Noise Cancellation
How it works	Physical barriers block sound waves	Generates inverse sound waves to cancel noise
Best at blocking	Mid-to-high frequency sounds	Low-frequency, constant noises
Power needed	None	Requires battery and processing chip
Examples	Ear cushions, earplugs	Sony WH-1000XM series, Bose QuietComfort series
Limitations	Less effective on low-frequency noise	Less effective on sudden, irregular noises

Conclusion

Active noise cancelling headphones use built-in microphones and processors to generate anti-noise waves that destructively interfere with ambient sounds, effectively “erasing” noise out of thin air. Passive noise cancellation complements this by physically blocking sound through headphone design. Together, these technologies offer a much quieter listening experience in noisy environments4 5.