How do Noise-Cancelling Headphones work?

You are back home after a long day of work or have a long commute ahead. You want to relax so you put on your headphones and put on your favorite song. Although this is easy enough to do in the comfort of your own home, out in the constant noisy environment of modern urban cities, hearing your music over the background can border on impossible. 

Today we have noise canceling headphones which hugely help deal with this issue. Although the technology isn’t terribly new (aviator headphones have used it since 1986) it’s only been easily available to the public market for a couple decades. Bose, the company that first invented the aviator headphones, introduced a consumer grade pair in the early 2000s, and a wider array of tech companies have supported the feature since then. 

One would think, based on the price tag often associated with noise-cancellation, that the actual process used to make them work would be complex and archaic. But the technology is based on a few simple principles of how sound works, making them both very practical to use, and an interesting example of sound fundamentals. 

So how does sound work?

Sound is what we in physics call a wave, which has a very precise definition in academic circles. Most of that definition isn’t important for what we’re discussing here, but there are two properties of a wave that are important in the case of sound:

• All waves move through a medium
• All waves go through cyclical motion, called oscillation. 

You can see both of these properties in action by looking at what we usually think of when waves are discussed; ocean waves. These waves move through the ocean, as they are essentially just the movement of water molecules, so the ocean is the medium through which ocean waves move.

They also go through a cyclical motion, first one wave rises, then there’s a drop, then another wave rises, and so on. In physics, we call the rise of a wave the crest of the wave, and we call the drop the trough of the wave. The distance from one crest to the next crest is known as the wavelength. 

So how does this apply to sound? Sound also has a medium, but unlike ocean waves sound can move through a wide variety of mediums. Although it is typically thought of as moving through air, sound is essentially just a medium vibrating in a particular way, so as long as a material is able to vibrate enough it can transmit sound. Wood, plaster, glass, and even certain metals can all vibrate enough to allow sound to be transmitted through them. These vibrations are rhythmic, with each individual molecule moving back and forth with a consistent wavelength, and it is this wavelength that determines the pitch that we hear when the sound wave reaches our ear. 

Passive and Active Noise Cancellation

Headphones isolate the sound they produce through a few different methods, which fall under two main types. The first, passive noise cancellation, uses the specific materials that the headphone is made of, and specific layouts of those materials, to reduce the amount of noise that reaches the user’s ears. The second, called active noise cancellation, uses sound waves produced by the headphones themselves to cancel out any incoming sound waves from the environment. These two types can be used on their own or in combination, and are described in more detail below. 

How does Passive Noise Cancellation Work?

In general, passive noise cancellation is achieved by using a material that doesn’t vibrate well to stop sound from being transmitted through. Concrete, cloth, and certain types of foam are examples of materials that don’t vibrate in the right way for sound to be transmitted. These materials are what’s used to sound-proof a room or space.

Things like insulation are used to fill a wall, and prevent sound from being transmitted into or out of a room. A similar concept is also used in sound studios, where foam is placed on walls at certain places to deaden any sound waves, and prevent an echo from showing up in whatever’s being recorded. 

A pair of headphones can also be made with these materials and specifically engineered to reduce as much noise as possible between the inside, where the user’s ear is, and the outside. This serves the dual purpose of preventing outside noises from disturbing the user’s listening experience, and it prevents whatever the user is listening to from bleeding out, and becoming audible to the outside world.

Although this is very effective in preventing outside noise, it’s only half of the process used by noise-canceling headphones, and is usually referred to as either sound-isolation or passive noise-cancellation.

Active noise-cancellation takes one step further to prevent outside noise from reaching the user’s ear. 

How does active noise-cancellation work?

Sound is a physical interaction, caused by the physical movement of a particle within a material, and so can be interrupted by other physical effects, including other sounds. When two sounds pass through the same point in a material they interact, causing the affected particles to move differently than either would alone.

If two crests or two troughs overlap, they build on each other, resulting in a louder sound. On the other hand, if a crest and a trough overlap, then they detract from each other, and can even cancel out if the crest and trough are the same size. 

This is the principle that active noise-cancellation uses.

A tiny microphone in the headphone detects any incoming sound and figures out that sound’s frequency. The headphones then output a sound of the same frequency, but at a different phase, so the two waves interact with each other and cancel out. Since they cancel out, nothing is reaching the user’s ear, which in turn means that the user won’t hear either sound.

Between the active noise-cancellation and the passive sound-isolation discussed earlier, headphone manufacturers can ensure that outside noises don’t reach the user’s ear, leaving only whatever the user actually wants to listen to. 

Conclusion

At first inspection, noise-canceling headphones may seem like a marketing strategy to make a pair of headphones more expensive. The truth though is that noise-canceling headphones use very real physics techniques to cut down on the bleed-through of environmental noise. They’re well worth the investment if you’re looking for a new pair of headphones, and are worth examining for their interesting properties even if you’re in the market.