Hey Reader, Happy Thursday!

Last week, I watched American football for the first time at a stadium in Berkeley.

Let’s look at how noise-canceling headphones work this week. I aim to write my newsletter issues so that one can follow them while traveling on a bus, having a coffee, waiting for food, etc. Let’s jump in!

Shoutout!

Before diving into today’s topic, I’d like to give a special shoutout to Gowtham Sivaraman for suggesting this week’s topic: how noise-canceling headphones work! Thanks, Gowtham, for the suggestion. You can do so as well here and I'll write about it in the upcoming editions. Thank you!

I had written about this gadget more than a year ago but revamped it with added details to make it more exciting to read. Hope you like it!

How it works: 15-second answer

Noise-canceling headphones, like the Bose QC 35, use Active Noise Cancellation (ANC) to block out unwanted background noise. They have built-in microphones that capture external noise. A processor inside the headphones analyzes these sounds and generates a sound wave through the speakers that is the exact opposite—180 degrees out of phase with the noise. This inverted sound, when combined with the original noise, cancels it out through destructive interference, allowing you to hear just the music or audio.

How do noise-cancelling headphones work?

I use Bose QC 35 noise-canceling headphones and genuinely love them (not a promotion, haha!). These headphones use Active Noise Cancellation (ANC).

Headphones, as you know, have speakers that play audio directly into your ears. But your ears don’t just pick up the music; they also catch the unwanted external noise around you.

What is sound?

Sound is a vibration that travels as a wave through a medium, such as a solid, liquid, or gas. It is a type of energy created when an object vibrates, causing the air molecules around it to move. These molecules then bump into other molecules, causing them to vibrate as well, and so on. This chain reaction of molecular collisions is what creates sound waves.

How do we visualize sound?

Now, sound is made up of waves, with different frequencies blended together. Essentially, sounds can be visualized as a wave, as shown in Figure 1 (Left), where the amplitude of the external noise (as an example) is plotted.

Amplitude is a measure of how much energy the sound wave carries, and it corresponds to the volume or loudness of the sound. The louder the noise, the higher the amplitude in general. So, how do we cancel out this external noise?

How is the noise canceled?

Headphones have one or more microphones built into them, even if you're not using them to talk. If you check your headphones right now, you'll probably notice a small matrix of holes on the exterior—those are the microphones (see Figure 1, Right). These microphones pick up sounds and noise from the external environment.
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A computer inside the headphones digitally analyzes the waveform of these external sounds. To cancel out the noise, the speakers produce a sound that has an amplitude 180 degrees out of phase with the detected noise (Figure 2). This inverted sound is then played along with the original music or audio.

Thus, when the noise and the inverted sound are combined, they cancel each other out through a process called destructive interference, allowing you to hear just the music without any of the background noise. This is the principle of a noise-cancelling headphone!

Question of the week

If noise-cancelling headphones can cancel all noises, why do we still hear high-frequency sounds like a car horn, etc.? Reply to this email with your thoughts, and we’ll discuss. I'll answer this in the next week's edition.

Gadget of the week

AI notetaker: PLAUD's NotePin is a wearable AI notetaker that transcribes and summarizes conversations in real time. It can be worn as a necklace, wristband, or clip, offering hands-free audio capture. With high-definition recording, AI speaker labeling, and rephrasing for clarity, it supports 59 languages and has a 20-hour battery life.

That's all folks. Thank you for reading!

Have an amazing rest of the week, and take care!
Until next to next Wednesday,
Chendur

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