The Science Behind Active Noise Cancellation in Headphones

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You must have come across ‘Active Noise Cancellation’ written on the packaging of headphones you bought or saw somewhere, or maybe you read one of the many positive blog reviews of noise cancelling headphones from sites like Lightningcans, Verge, or Macworld. Most of us have never wondered what it really could be or how could a device such as headphones have the ability to not block but cancel out unwanted noise. Active noise cancellation is a technique to reduce unwanted ambient sounds which are used not just in headphones but in cars and other audio devices as well for this very purpose.

The sound is an audible wave of pressure that travels through a medium and alternates between compression and decompression. For active noise cancellation, a noise cancellation speaker emits a sound wave equal in amplitude to the sound waves of the noise to be cancelled out. Although it is equal in amplitude to the noise, the phase of this wave is opposite or inverted to that of the noise sound waves. Due to this, the two waves with equal amplitude but inverted phases to each other cancel out and in an ideal case leave us with zero resultant sound.

This is a scientific principle known as interference where two waves join to form a resultant wave that might have a magnitude higher, lower or even zero to that of the original two waves. When the resultant wave generated as a result of interference between two waves has zero amplitude it is known as destructive interference. Active noise cancellation allows for better audio quality and for a louder audio sound at lower volume. Noise cancellation in aircraft is used to cancel out engine sounds. In cars, active noise cancellation is used to reduce the sound of the car engine and the sound of the car moving on the road as well as outside sounds.

In headphones, a dedicated mic is used for this purpose. A built-in microphone in the earpiece of the headphones takes in the unwanted ambient sounds like traffic, etc. and generates opposing sound waves of the same amplitude. However, a wave with the exact inverted phase is hard to achieve and that is why not all the sound gets cancelled out but rather most of it. Active noise cancellation headphones can usually reduce up to 70% of unwanted background noises.

The term ‘active’ means that energy is required for the noise cancellation to work. A rechargeable battery inside the headphones helps with this for active noise cancellation to be successful. This means you can use ANC headphones even if you don’t want to listen to audio but just rather have some peace from the environmental sounds around you. The opposite of these is passive noise cancelling headphones that focus on blocking out the sound rather than cancelling it with opposing sound waves. Passive noise cancellation uses soundproofing. Soundproofing is also employed into ANC headphones to block out higher frequency sounds which are harder to counter with active cancellation.

Although active noise cancellation is achieved at the cost of slightly reduced audio quality, regular users claim the trade-off is more than worth it as the lost audio quality is made up for by the cancelled-out noise which greatly improves the listening experience.

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