Mechanisms Behind Noise Cancelling
- Juhwan Lee
- 11 hours ago
- 3 min read
Airpods, Sony, Bose, JBL… These are all leading firms for headphones, incorporating the notable noise cancellation technology. This enables us to immerse in whatever audio we are listening to without external interference. However, we have taken such technology for granted and never considered how headphones—that is, nothing more than an earcap that plays audio—effectively “noise-cancel”. In this article, we will dive into the history of this technology that acts as a savior for music lovers, and the science behind it that makes this possible: sound waves.
In 1978, a scientist named Amar Bose took a flight from Zurich to Boston and was disturbed by the airplane noises that disrupted his music experience. Bose, from this flight on, pursued research that could incorporate technology that could cancel the surrounding noise in headphones. His solution was, in fact, quite simple: creating a headphone that “listens” to its surroundings. It still took 10 years and millions of dollars worth of investment for him to invent a working prototype, and the technology was initially used for special sectors such as the military until much later, when the costs started to go down, noise cancelling headphones were then available to the public. His legacy continues, even after his death, with one of the world’s most famous headphone companies, Bose. But what did he mean by headphones that were able to “listen”? What did this have to do with noise cancelling?
The concept of “noise cancelling” has two sub-categories: passive noise cancelling and active noise cancelling. A simple diagram that shows their differences is shown below:
Notice that for passive noise control, an “absorber” absorbs the noise, and some headphones follow this principle by creating thicker layers that physically block the sound from seeping through into the ears of the listener. The issue with this method doesn’t lie in its functionality but rather in its limitations when it comes to much louder noise. On the other hand, active noise control’s approach is very scientific and promising, and it uses a type of behavior that two sound waves can create: destructive interference.
If you have been disturbed by the noise level in a coffee shop because of the influx of customers, you have just experienced constructive interference, which is when two sound waves combine at the same time, amplifying the sound level of the combined sound wave (see the left portion of the picture above). If sound can be combined to be amplified, it can also be counteracted to become eliminated, and this phenomenon is called destructive interference. If two sound waves are completely opposite of each other, then the sound wave becomes completely flat, signifying that the sound has successfully been “canceled”.
Now, let us apply this to the structure of headphones: in each cap of a headphone, a small microphone is present. This microphone listens to your surroundings at a strikingly high speed and generates a sound wave accordingly, which is an inverted version of that noise. By the process of destructive interference, the external noise has been eliminated, and that is fed into the headphone speakers along with your music. But it also comes to our attention that noise cancelling headphones are not able to magically erase all noise. In a bustling crowd or honking cars, noise cancelling headphones may struggle to serve their function. Why can’t our headphones send off inverted sound waves under these circumstances?
Cancelling noise is like playing a whack-a-mole; There are only so many different sounds that you can eliminate at once; be aware that noise cancelling headphones are very weak in coping with variations. Every sound that we encounter, whether it's the sound of a TV, a car honking, a person talking, or a bell ringing, has its own unique sound waves. If these noises are caught by the headphone, it is quite tricky to send inverted sound waves that account for each type of noise. This is why headphones are much more effective in settings such as train cars or airplane cabins, where the sound waves tend to be much more consistent than what we face in our daily lives. Of course, their effectiveness is determined through price ranges as well: headphones that use higher quality (more expensive) microphones tend to cancel noise better than those that don’t.
Noise cancelling headphones are a promising technology that is improving itself both in terms of function and affordability, contributing to the increase in quality of life. However, it is equally important to for once appreciate the science and the scientists who brought up these technologies so that they are available to use for the public as well.
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