Technology
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How Active Noise Canceling Actually Works
I’ve been thinking about how headphones work. What’s actually happening in there? Let me try and break down how active noise canceling (ANC) works, and how pass-through mode decides what sounds to let in.
It’s All About Destructive Interference
Sound is a pressure wave moving through air. And here’s the thing about waves: if you produce an identical wave that’s perfectly inverted, 180 degrees out of phase, the wo cancel each other out. That’s the entire principle behind ANC.
Modern headphones have multiple external microphones on each ear cup that are continuously sampling ambient noise in real time. Those audio signals get fed into a digital signal processor (DSP) chip, which analyzes the waveforms and generates an inverted version. The inverted signal is then played through the drivers into your ear at the same time the real-world sound arrives.
So the noise gets canceled before it hits your eardrum. Simple concept, incredibly complex the more you learn about it.
Speed Is Everything
The entire chain, from the mic capture to waveform inversion to playback has to happen in microseconds in order to beat the incoming sound wave. This is why ANC works best on predictable, steady sounds like airplane engines, road rumble, or HVAC hum. Those low-frequency drones are easy to predict and easy to invert.
Higher-frequency or more modulated sounds, like human voices, are harder to predict. The waveforms change too quickly and irregularly for the DSP to perfectly cancel them out. That’s why you can still hear someone talking to you even with ANC cranked up. The algorithm just can’t keep up with the complexity of speech.
Hybrid ANC: The Second Pass
Modern headphones use what’s called hybrid ANC, which adds microphones on the inside of the ear cups in addition to the external ones. This gives the DSP a second correction pass. The external mics handle the initial cancellation, and the internal mics pick up whatever noise leaked through the ear cups and apply another round of inversion.
It’s basically error correction for sound. Pretty clever.
How Pass-Through Mode Works
Pass-through (or transparency mode, or ambient sound, every brand calls it something different) is essentially ANC running in reverse. Instead of canceling out the world, you’re selectively letting it in.
The external microphones are still sampling ambient audio and the DSP is still processing it. But now the headphones are reproducing a version of the outside world through the drivers, as you would have heard it without headphones on.
The DSP can selectively boost certain frequency ranges, like the range where human speech commonly occurs, while reducing others like wind noise or low-frequency rumble. So pass-through mode isn’t just “turning off” noise canceling. It’s actively shaping what you hear.
In transparency mode, ANC is still running. It’s just operating at a reduced level. The system is always actively managing your audio environment.
Now how much of the outside world you are letting in is maybe a different topic for another day.
/ Technology / Audio / Explainer
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/ links / Technology / fitness / wearable
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Transhumanism (as a fictional genre, not as a philosophy) is about the idea that we can use technology to overcome the problems inherent to human nature, while cyberpunk is about the idea that we can’t.
I propose a new form of philosophy called “Cyberpunk Luddism.” The idea comes from reading a blog post on Kagi Small Web about Molly’s Guide to Cyberpunk Gardening. In it, they mention the interesting quote above, that I have tracked down the original source from 2009. Stephen Lea Sheppard on RPG.net
/ Philosophy / Technology