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Comment by joncrocks | Hacker News Reader

> How can a webpage play a sound if my speakers are muted?

Well it can't obviously, but lots of people (although maybe not the types of people who use tor) browse the internet with their speakers on and active. Most people don't unmute their speakers just before they're about to listen to something.

> How do they bypass the little sound notification on my tabs?

Admittedly they probably can't, but are you sure you're going to notice a flicker as a short sound is played and then stops?

I think the most contrived part is your mobile being always-on/always listening, given that you're likely to notice this due to reduced battery life. But given that certain hardware now has support for always-on keyword detection, you can see a future when this could happen.

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FWIW, there was a BBC article about always-on audio detector apps. Describing an Android proof-of-concept app: "The battery drain during our experiments was minimal and, using wi-fi, there was no data plan spike."

http://www.bbc.com/news/technology-35639549

They don't give much information on what "minimal" battery drain means. I'm skeptical. Keeping an app running in the background and keeping a stream of audio data piped into it to be processed on the CPU is not cheap. Google has a dedicated DSP on phones to do hotword detection (among other things), and IIRC that's not exposed to unprivileged apps. Hell, even iOS needs to be charging to get "hey siri" support (not sure about now; it was like this in previous versions, though).

Either way, it doesn't sound like that's what the article describes: they're talking about collecting and sending all audio wholesale. Sending that much audio data over 3g or LTE would be expensive (transcoding it to decrease payload would be expensive, too), and would surely be noticeable looking at data usage charts.

> using wi-fi, there was no data plan spike

Uh, yeah. Because it's using wifi. Phones are on wifi far less often than you'd imagine.

It's certainly possible, but it's just not plausible.

This example is about people using TOR on a computer, so your phone is more likely to be connected to wifi than the general case.

You're right, the article is talking about seems to be talking about sending the audio wholesale, which would be cheaper from a CPU perspective, but sending the data would probably be noticeable both from a data use and battery perspective.

Having said that, quite a few people have their phones connected to wifi at home, which could mitigate these issues due to both less conspicuous data and power usage.

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