How Do Noise Cancelling Headphones Work?

For those of you who love listening to only your music and nothing else, it becomes difficult when you want to listen to your favorite song in a busy, crowded place. You might have to increase the volume, which could disturb others sitting beside you as they can hear your music.

If you have tried listening to music on a plane, you will be familiar with what I am talking about. The roar of the plane's engine makes it difficult to hear what is coming out of your headphones. Or if you have experienced listening to music at a train station either waiting for your train or inside the train, you know how difficult it can be. The noise of the train and people talking all over makes it difficult to focus on what you are listening to. For these purposes, noise cancelling headphones have been designed. They provide a more enjoyable and secluded listening experience to the listener by blocking the background noise.

Who Discovered Active Noise Reduction?

Active noise reduction was invented recently by Bose, the best-known seller of noise-cancelling headphones. However, the theory behind this process dates back to 1878, according to Professor Collin Hansen. Hansen points out that the first experiments were done on a telephone, and the first patents were given to Henri Coanda, a Romanian aircraft engineer, in 1932 and to Paul Lueg, a German physicist, in 1933. Both men applied the theory of noise cancellation by adding other waves to the sound waves but in antiphase. Many other people carried on this work including Harry F. Olson, a synthesizer pioneer. In the 1990s, active noise reduction became more than laboratory work and gradually turned into what we see today.

How Do Noise Cancelling Headphones Cancel Noise?

The sound is a pressure wave, consisting of a compression phase and a rarefaction phase. The noise cancellation speaker generates a wave that has the same amplitude and frequency as the original sound but in antiphase i.e. inverted phase. This process is called destructive interference as both waves cancel out each other when they combine, an effect that is known as phase cancellation.

Modern noise cancellation is achieved by using analog circuits or digital signal processing. Algorithms are designed to analyze the waveform of the background noise and depending on the algorithm, a phase shifted or inverted polarity signal relative to the original signal is generated. The antiphase signal is amplified, and the transducer generates a sound wave that is directly proportional to the amplitude of the original waveform. This results in destructive interference and reduces the volume of the background noise.

A noise cancellation speaker needs to be located near the sound source that needs to be attenuated. Also, the transducer that emits the cancellation signal must be located at the position where sound attenuation is required, which is the user's ears. Noise cancellation is difficult at other places because when the two waves combine, the wavefronts of the cancellation signal and unwanted noise may result in destructive and constructive interference, and the noise may be reduced at some points and doubled at other points.

Types of Noise Cancelling Headphones

There are two types of noise canceling headphones: passive headphones and active headphones.

Passive Headphones

They are the simplest kinds of earphones that are designed to block background noise. Either the material with which they are constructed prevents sound waves from entering the listener's ears, or the shape of the earphone, which fit snugly into the ears, blocks noise. The best passive noise cancelling headphones are the circumaural ones, designed specifically to maximize noise filtering. They contain many layers of high-density foam or other sound absorbing material, making them heavier than normal earphones. They reduce around 15 to 20 dB of the background noise.

Active Headphones

They are an advanced version of active noise reduction and contain a microphone. The microphone continuously samples the background noise and sends it to an electronic circuit embedded into the headphone case. The purpose of the circuit is to invert the sound and play it into the loudspeaker that is present in the ear. The logic behind this is that noise heard by the listener and the inverted noise cancel out each other's effect, leaving behind only the desired music that the listener wants to hear. These headphones are used by pilots where noise cancellation is extremely crucial.

Which Ones Are the Best?

This depends on your preference. Passive headphones have the advantage of being cheaper, but their noise reduction quality is low. On the other hand, active headphones are a little expensive but have a very high-quality audio performance. The best solution is to try different headphones and choose what suits you the best.

One thing to keep in mind is that active noise reduction headphones are used to reduce steady and predictable noises like the sound of the plane's engine. It will not suppress people's chatter. If you want to resolve the latter problem, you need some other solution.

How to Get Rid of Noise That Can’t Be Cancelled Out

To cancel out other distractions while working, an effective solution is to record a sample of white noise or pink noise (there are plenty available on the internet) and put into your music player. Play the noise at an average volume and it will cancel out most other noise.

Another solution is to place foam earplugs and place your headphones on top. Then, play pink or white noise. The combination of headphones, earplugs, and white/pink noise cancels out the background noise which seeps into your ears. It is an easy solution that works.