Active Noise Cancelling (ANC): What is it and how does it work?
14.10.2021

Active Noise Cancelling (ANC): What is it and how does it work?

active noise cancelling diagram next to apple airpods pro
Words by Sam McNiece

A look at the technology that allows modern headphones to block external noise.

Originally developed by Bose, Active Noise Cancelling (ANC) has become one of the most used features in headphones and earbuds on the market currently. From small to large manufacturers, it has been a staple in recent years, ensuring that when you’re listening to music, you’re not hearing the outside world.

With the current climate of new consumer and professional products offering ANC, we’re taking a deep dive into how it works and how it’s being used across the board.

Summary:

  • Active Noise Cancelling (ANC) uses microphones and circuitry to generate a phase inverted audio signal that nullifies external noise.
  • The technology was thought of in the 1930s and put into practise by Bose for the US military in the early 1980s.
  • ANC is a primary selling point for headphones in the present day and will continue to be a major factor in consumer decision making on earphones.

Read all the latest features, columns and more here.

How Active Noise Cancelling (ANC) works

ANC technology was implemented for the US military as a way to combat their US$200 million a year payments to veterans with hearing loss developed during their service. The first product Bose introduced, was used by pilots on the Voyager aircraft which was particularly noisy. The technology is similar to how modern Active Noise Cancelling products work today, by using a microphone to record external sounds and phase invert the signal within the headphones to nullify the external sound.

This works, because when a wave, and an inverse of a wave arrive at a source at the same time, the output produced is net zero. In the case of ANC, the external noise must be processed extremely quick to be able to replicate the external sound. The speed of sound is 343 meters per second at 20°C and generally the distance between a microphone and the headphone driver is only a few centimetres, if not less. At a 2cm distance, the DSP or circuitry that processes the audio must function in less than 0.000059 parts of a second (59µs) to successfully counter external noise.

Due to the way our ears function, Active Noise Cancelling targets lower frequencies more than higher frequencies. Within all headphones, there is some passive noise cancellation, done through the choice of materials that cover our ears, with higher frequencies being the easier to target due to their shorter wavelengths. The lower the frequency, the larger the wave length with 20Hz being the lowest frequency able to be heard by humans measuring at 17 metres long (!) which would be impossible to target through passive noise cancelling within headphones.

One way to imagine Active Noise Cancelling outside of the technical jargon is to imagine two people hitting a tennis ball at the same time with the same force from opposite sides. In practise the net force would be zero and instead of the ball flying across the tennis court, the ball would fall to the floor. Now this ball is representative of what we hear with one person being the external sound and the other being Active Noise Cancellation. It applies the equal and opposite force (sound wave) to the ball (our ears), cancelling out the force from having an effect on the ball.

How Active Noise Cancelling is used

In the present day, ANC is offered in basically all modern wireless headphones and earbuds. Products from Apple’s Airpods Pro to Nura’s range of earphones utilise Active Noise Cancelling to provide a clearer picture of the sound, excluding people chattering away on your train commute to work from your favourite songs.

There’s three common types of ANC—feed-forward, feed-back and hybrid. Feed-forward utilises a microphone on the outside of the headphone, feed-back uses a microphone on the inside of the unit and hybrid is a combination of the two. Apple’s wireless earphones use an external microphone (feed-forward ANC) while Bose chose both internal and external microphones to get the job done (hybrid ANC).

Modern products, mainly over ear headphones, offer a function called transparent or social mode which utilises the microphone used for ANC but doesn’t invert the phase, to allow you to have conversations and hear the outside world like you weren’t wearing headphones. Now I’m definitely in the camp of just taking off your headphones but this might be useful when you’re ordering a coffee and don’t want to stop listening to your favourite mix.

There are even some products that allow for a sliding scale of ANC, which allows you to inherently turn up the effect from doing nothing to a fully immersive audio experience.

It’s been over twenty years since Active Noise Cancelling was introduced in consumer products by Bose and if things continue the way they are, the technology will continue to play an active role in headphone design for years to come. For accurate music mixing and critical decision making, a quiet space will beat ANC every time, but for consumer products that are used everywhere from the office, to the train and the gym, they provide a step away from outside distractions, allowing you to hear your music in full.

Check out this video for more on Active Noise Cancelling.