If you’ve ever wondered what noise cancellation means, then you’ve come to the right place.
Active Noise Cancellation is a highly sought-after feature of headphones and earphones, and for good reason. It removes external noise from your listening experience, making everything quieter and easier to listen to at low volumes. However, how noise cancellation works on headphones like the Sennheiser Momentum 4 Wireless and the Sony WH-1000XM5 is a mystery to many. We sat down with Christian Ern, Principal Product Manager of Consumer Headphones at Sennheiser, at this year’s IFA 2023 to help us sort it out.
Noise reduction is the first step in noise control
First, the most important thing to understand about noise-cancellation is that it is all about frequency, sound waves, and most importantly, physics. Sound travels in waves, which are created by vibrations that travel through the air. As these vibrations increase, the frequency of the wave increases (as the name suggests), and the length of the wave determines the amplitude or lift.
When dealing with very high pitches, this has a hard time settling in, and the pitch goes the same for all types of frequencies. Although the sounds used by humans cannot exceed 20kHz (as that is the limit of human hearing), mmWave 5G, for example, goes up to 40GHz. At 40GHz, mmWave has trouble penetrating all kinds of places, which is why I’m showing the spectrum that’s most common outdoors in big cities.
Based on this understanding of the problem of high-frequency noise at the input point, it is why many high-end headphones with active noise canceling use thick, soft cups. While low frequencies can penetrate the sound more easily, higher frequencies are filtered out more by thick surfaces like this. This method is called noise reduction, not noise cancellation.
The physics of destructive interference
When two coherent (simply put, equal) waves combine, this is called interference. If they combine to amplify the current, then this is positive interference, but if they cancel each other, then this is called destructive interference. The concept of destructive interference is a big part of the noise control picture.
Looking at the picture above, notice that the top line is the combination of the two waves below it. On the left, all signals are in phase, so they amplify and create strong waves as a result. Right, out of phase anti-wave causes destructive interference. In audio, one signal may be external noise, and the anti-wave that is designed to destroy it will be played through the ears of the person listening to the music.
If so, then why do you still hear audio from outside your headphones? Some sounds are completely removed, but there are no good noise canceling headphones. Again, for the same reason, it’s all just physics. This is where the microphones outside your headphones come into play when it comes to noise-cancellation and the difference between “feed-forward” and “feed-back” ANC.
Feedforward ANC versus ANC
There are two different types of ANC used in headphones, and Ern tells me that the best headphones will use a combination of both. Feedfoward listens to the audio outside the headphones and tries to tune it out, while ANC solutions have a microphone inside the cups and listen to the difference between the headphones and the sound of the content. they should to be playing.
Feedforward ANC is very interesting, because it shows the special function that goes into creating noise-cancelling headphones. Instead, the sound entering the ear cup will enter one of the microphones, and in the milliseconds it takes for the sound to reach your ear (as the microphone is a few centimeters away), it will try to correct the sound. and anti-wave generation. For the sound facing the microphone, it works well in milliseconds, but the sound above and below is not, because it does not enter the microphone directly and can reach the ear before it can be removed. This is why the ANC solutions can close the gaps.
With ANC feedback, it uses a microphone inside the earpiece to measure the difference between what should be played and what is heard inside the ear. When the sound from the outside reaches the microphone, the listener already hears it, but it can be used to remove continuous noise, such as airplanes or other objects. However, it won’t help with sudden noise, so a combination of feedforward and feedback is better.
As a side note, have you ever rubbed your fingers outside of your eardrums or in your ears and heard a loud noise? Feedforward ANC is why this happens, and wireless headphones with ANC are the same as high-end headphones, on a smaller scale.
Bringing it all together
When we combine feedforward and noise canceling feedback, we get a powerful combination that can cancel very low noise levels consistently. This is why noise canceling headphones are great for filtering the noise of things like public transport and other constant noises because it’s the natural sounds that the ANC response can predict and remove. Combine this with thick ear cushions that work well at high frequencies, and you have all the technology that makes up for today’s most advanced noise cancellation.
Where this goes wrong is with speech filtering, and why noise cancellation is difficult. The 1kHz to 3kHz range is where most human speech is, and this is high enough to make it difficult for ANC feedback to cancel but too narrow to work well with feedforward ANC. The combination creates a lot of complexity (which is why mid-to-low-end headphones tend to use one of these methods rather than both), and can also affect sound quality.
In this case, Ern tells me that either headphones or earphones can be ahead of sound or ANC, but not both. The sound must be tuned to turn ANC on, but if you’re removing frequencies that are being heard from outside, you run the risk of removing frequencies from your actual music. That’s why noise cancellation is such a technical challenge, but the best ANC headphones all do it very well.