We all used them once in our life: Noise canceling headphones. With them suddenly even the busy subway becomes quiet. But how do they work?
For this we have to first think about noise in a physical sense as a wave function. Some sounds like a voice or screaming… have a very high frequency while other sounds like the deep consistent engine roaming have lower ones.
To cancel both of them out there is passive and active noise canceling. Passive noise canceling basically uses just different materials or positioning of the headphone to kind of shield off as much outside noise as possible. This works best for higher frequencies and traditional headphones mostly relied on this type but it filters only around 30% of the noise and especially longer wave functions penetrate through them.
Active noise canceling on the other hand is the real deal, it can filter up to 90% of the surrounding sound and works best for low frequency and repeating sounds.
As you might have already learnd in school, two wave functions with the same speed, frequency and amplitude can go in different phases ( for example one wave peaks while the other is currently cutting the x axis). Also both waves interfere/ add up to create a new and according function. This means that if both waves are in phase the corresponding new wave will have twice the amplitude.
This also works the other way around. If there is a sound wave coming, lets just say from the exterior, and we mirror that wave but with a 180 degrees rotated phase (the peak of the first is the dip of the mirrored one) then what happens is also that they add up and therefore cancel each other out to become 0.
So inside active noise canceling headphones there are microphones placed on the outside of each device detecting any unwanted outside noise. This sound wave is then mirrored, phase rotated and added to the given sound the user wants to hear. The mirrored sound wave is then canceled out by the incoming distracting outside noise and the user can enjoy a more pure sound even in the busy subway.
Newer models also have 360 sound detecting microphones also near the mouth for voice recognition. This means when you speak the results of both the near and far microphone from your mouth detect the same distraction noise but only differ when it comes to your spoken words, which can then be distracted and used for phone calls also without the background noise.
The technology can even be used in rooms or cars to reduce driving noises.
I hope that you understand the world a little better now, thanks for reading.