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Comment by mSparks | Hacker News Reader

anyone know of an app that can detect these signals?

Maybe decode them.

easiest way to block them is to know they are there in the first place.

i am suspicious that occasionally my laptop makes high pitch noise correlated to mouse movements when its running windows 10.

edit:Thinking about it. seems possible that might actually be facebook. never installed their phone app untill i had android 6 because it wants mic permissions (disabled) and facebook and reading hn is about all i use win 10 on the laptop for.

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Your MacBook microphone can detect ultrasonic frequencies above 22khz, though I'm not sure whether my observed ceiling is an artifact of the microphone, or of the speaker I was using to emit the tones.

In my informal tests, most people are unable to hear any tone above 20khz or so. This is discovered, naturally, by starting with the highest pitch tone I can generate at the loudest volume, and lowering the pitch gradually until somebody covers their ears and starts screaming. This is usually the youngest person in the room.

You can generate and receive audio using web audio APIs. Creating a browser extension to detect ultrasonic comma would be fairly straightforward.

However, most audio output (at least back in the nineties and naughties) is not capable or creating sounds reliably above 16khz. They almost always have a low pass filter on which cuts it out. Apparently most dell laptop speakers cant even manage above 10khz

"You can't hear" doesn't have to be high frequencies though.

It can be more subtle like phase shifts and other "in audible range" fingerprinting.

While I've not used it for getting on 20 years now, I did do a lot of software audio engineering back in the day, everything from RF to DSP.

You most probably have some "coil whine"[1]. It's very unlikely that you are hearing ultrasonic signals (disclaimer: I'm no expert).

[1]: https://en.wikipedia.org/wiki/Coil_noise

sounded like "coil wine".

doesnt stop a phone app decoding it though. it was only when i moved the mouse. and came then went.

And i definately still have reasonable high frequency hearing. for example filing a complaint with Morisons in the UK for those high frequency speakers aimed at stoppping kids loitering. Don't know if they got rid of them or not because i stopped shopping there.

I've noticed this before on another laptop. I believe it is EFI being picked up by the laptop amplifier, not from the touchpad, but actually from the graphics adaptor or display panel controller. You'll likely actually get the noise any time you cause the whole screen to refresh for a long period.

Also sounds like it could be a ground loop (earthing). Somehow isolating the laptop's power from all others you may use (such as powered hubs, speakers, etc) by placing them on separate power feeds tends to help.

The sound is probably coming from the LCD screen. https://news.ycombinator.com/item?id=8862689

I discounted that at the time by playing a muted YouTube video and using keyboard shortcut forward back.

It only altered in any meaningful way to trackpad movements and clicks.

That is just electromagnetic interference.

It may have just been electrical interferance.

But electrical interfance from a trackpad? - made me suspicious at the time.

And one way or another, a proof of concept is only a few lines of javascript.

A very very easy way to link people in a room together.

And it would explain why the facebook app wants mic permissions when afaik it offers no audio features. it doesn't even need to be audio the maker of the app controls - just "fingerprint" sounds and report them back to facebook (or whoever).

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