Noise-cancelling headphones are an incredible (and shockingly simple) technology. In this post, you’ll learn how active noise cancellation headphones work and why they may not be the best option for music production or critical listening.
The video above contains audio tests comparing my favorite active noise cancelling headphones (Sony WH-1000XM4) and my favorite passive isolation headphones (Direct Sound EX-29)
How Noise Cancelling Headphones Work
Noise cancelling headphones are capable of playing your music, while at the same time creating interference that combats the noise around you before that noise reaches your ears.
The key principle at play here is called destructive interference. A simple sound wave is a series of positive and negative pressure changes – called compressions and rarefactions. If I copy this simple sound wave and add it to itself, the two copies will sum to create a larger sound wave.
On the flip side, if I reverse the polarity of one of these waves and combine the two waves, they will completely cancel. That’s because each compression (or high pressure phase) is met with an equal but opposite rarefaction (or low pressure phase) from the other wave.
With active noise canceling headphones, the noise from your environment is captured by a microphone. Then, the signal from that microphone is inverted and played back through the speakers alongside your music.
When done properly, this will result in the environmental noise being canceled, leaving only the music.
Limitations of Noise Cancelling Headphones
Now, just because the principle behind active noise cancellation is simple to explain doesn’t mean it’s easy to implement.
First of all, there is a difference in noise level between the outside of the headphones and the inside, depending on how the headphones are positioned on the listener. It’s critical that the noise level is accurately matched to maximize the cancellation, so engineers often design the headphones with two microphones – one on the outside and one on the inside – to measure the difference in level as accurately as possible. This internal microphone also informs the digital signal processor within the headphones on how effectively the noise is being canceled.
Another challenge that engineers face is getting the correct timing or phase alignment. If the inverted signal is even slightly shifted in time, the cancellation won’t be as effective. A complete cancellation only occurs when the level and phase of the two opposite sound waves are perfectly aligned.
In terms of timing, there is a larger margin of error for canceling low frequencies than there is for canceling high frequencies. For example, while a 1 ms shift in time might cause only a small phase offset at a low frequency, that same shift in time could be much more significant at a higher frequency because the wavelength is much smaller. The low-frequency wave will still be mostly canceled, despite the offset, while the high-frequency wave may be radically misaligned. This is one reason that when you turn on active noise cancellation, the low-frequency noise disappears, while the higher-frequency noise remains.
All of this to say that, while the basic principle behind this technology is simple, actually putting the principle into action requires very sophisticated engineering and design by people much smarter than me.
Are Noise Canceling Headphones Good For Music Production?
Active noise canceling headphones are great for use in a noisy airplane or coffee shop. But in my opinion, they aren’t great for music production and critical listening for the reasons I’ve mentioned.
When recording in an extremely noisy environment, I’ve found it much better to use headphones with passive noise isolation.
Among the best options for passive isolation headphones are the Direct Sound EX29s. There aren’t any microphones or DSP built into these headphones, yet they still attenuate the signal by more than 36 dB! Another benefit of passive noise isolation is that it works both ways – keeping noise out and keeping the sound in, which is very helpful for preventing the click track from bleeding into your microphone and ruining your recording.