What is Active Noise Cancelation?

Active Noise Cancelation is a majorly sought-after feature on earphones and headphones, and for good reason. It removes a lot of outside noise from your listening experience, making it both significantly more peaceful and easier to listen at lower volumes. However, how noise-canceling works on headphones like the Sennheiser Momentum 4 Wireless and the Sony WH-1000XM5 is a bit of a mystery to many. We sat down with Christian Ern, a Principal Product Manager of Consumer Headphones at Sennheiser, at this year’s IFA 2023 to help break it down.

Sound dampening is the first step to noise-canceling

First and foremost, the most important thing to understand about noise-canceling is that all of it has to do with frequencies, sound waves, and most of all, physics. Sounds travel in waves, which are made up of vibrations that pass through the air. The more frequent these vibrations are, the higher the frequency is (as the name suggests), and the height of the wave essentially determines the amplitude or loudness.

When dealing with higher-frequency sounds, these have a harder time penetrating surfaces, and this property goes the same for all types of frequencies. While sounds in content aimed at consumption by humans will typically never go past 20kHz (as that is the limit of human hearing range), mmWave 5G, for example, typically goes as high as 40GHz. At 40GHz, mmWave has trouble penetrating all kinds of surfaces, which is why it’s a signal spectrum predominantly found outdoors in major cities.

Taking that understanding of the difficulty of high-frequency sounds in penetrating surfaces, this is why many high-end headphones with active noise-cancelation use thick, soft cups. While lower-frequency sounds can penetrate these with ease, higher-frequency sounds will be greatly filtered by a thick surface such as these. This technique is called sound dampening, and not active noise-cancelation.

Physics of destructive interference

When two coherent (to put it simply, similar) waves combine, this is called interference. If they combine to amplify the wave, then this is constructive interference, but if they cancel each other out, then this is called destructive interference. The concept of destructive interference is a huge piece of the active noise-cancelation puzzle.

Taking a look at the above image, note that the top line is a combination of the two waves below it. On the left, both signals are in phase, so they amplify and create a stronger wave as a result. On the right, an out-of-phase anti-wave results in destructive interference. In an audio context, one signal would be an outside sound wave, and the anti-wave generated to destroy it would be played through the headphones of the wearer listening to music.

If that’s the case, then why do you still hear audio from outside of your headphones? Some sounds are removed completely, but there are never a perfect pair of noise-canceling headphones. Again, for similar reasons, it’s all just about physics. This is where the microphones on the outside of your headphones come into play when it comes to noise-canceling and the difference between “feed-forward” and “feed-back” ANC.

Feedforward ANC versus feedback ANC

There are two different kinds of ANC in use in headphones, and Ern tells me that pretty much all flagship headphones will use a combination of both of these. Feedfoward will listen to audio outside of the headphones and try to cancel it out, whereas feedback ANC will have microphones inside the cups and listen for the difference between the inside of the headphones and the sound of what should be playing.

Feedforward ANC is particularly interesting, as it shows the specialized work that goes into creating noise-canceling headphones. In essence, a sound wave entering the ear cup will enter one of the microphones, and in the milliseconds it takes for that sound wave to reach your ear (as the microphone is a few centimeters away), it will try to process that sound wave and generate an anti-wave. For sounds facing the microphone, it works well in just milliseconds, but for sounds above and below it doesn’t, as they don’t directly enter the microphone and may reach the ear before they can be canceled out. That’s why feedback ANC can close the gaps.

With feedback ANC, it uses microphones on the inside of the earcup to measure the difference between what should be playing versus what actually can be heard inside the earcup. When the sound from outside reaches this microphone, it’s already being heard by the listener, but it can be used to remove constant noises, such as the hum of an airplane or other things. However, it can’t help with sudden noises, which is why a combination of feedforward versus feedback is best.

As a side note, have you ever rubbed your fingers on the outside of your earphone or headphones and heard a high-pitched noise? Feedforward ANC is exactly why that happens, and high-end true wireless earphones with ANC are essentially the same setup as high-end headphones, just on a smaller scale.

Bringing it all together

When we combine feedforward and feedback active noise canceling, we get a powerful combination capable of canceling out lower-frequency sounds with great consistency. That’s also why active noise-canceling headphones are great at filtering out the sound of things like public transport and other constant noises because they’re predictable sounds that feedback ANC can successfully predict and remove. Combining that with the sound-dampening properties of thicker earcups that work well on higher frequencies, and you have all the technology that makes up the vast majority of active noise-canceling today.

Where this still falls apart is filtering out speech, and why active noise-canceling can struggle. The range of 1kHz to 3kHz is where most human speech resides, and this is just high enough to be harder for feedback ANC to cancel out but too directional to work well with feedforward ANC. Combining them introduces a lot of complexity (which is why typically mid-to-low-end headphones may only use one of these techniques and not both), and can also influence sound quality.

In this case, Ern tells me that headphones or earphones can be prioritized for sound quality or ANC, but not always both. The sound needs to be tuned for when ANC is on, but if you’re removing frequencies that can be heard from outside, you run the risk of removing frequencies in your actual music, too. That’s why noise-canceling is such a complicated feat of engineering, but the best ANC headphones all do very well at it.