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From LWN's security quotes of the week:

These days audio hardware usually consists of a very generic codec containing a bunch of digital→analogue converters, some analogue→digital converters and a bunch of io pins that can basically be wired up in arbitrary ways. Hardcoding the roles of these pins makes board layout more annoying and some people want more inputs than outputs and some people vice versa, so it's not uncommon for it to be possible to reconfigure an input as an output or vice versa. From software.

Anyone who's ever plugged a microphone into a speaker jack probably knows where I'm going with this. An attacker can "turn off" your TV, reconfigure the internal speaker output as an input and listen to you on your "microphoneless" TV. Have a nice day, and stop telling people that putting glue in their laptop microphone is any use unless you're telling them to disconnect the internal speakers as well.

— Matthew Garrett

I can't tell if Garret is being serious here. While an audio chip being a generic converter makes sense, I don't see how an average speaker can convert incoming audio into useful signal. Are speakers that sensitive?

Can software be used to operate an average consumer speaker as a microphone?

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    Could it be a question for Skeptics? – Serge Ballesta Mar 20 '17 at 9:18
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    @SergeBallesta it was a toss-up between Skeptics, here and SU, but I figured people here would be more likely to know about the technology involved. – muru Mar 20 '17 at 9:19
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    From experience, I know headphones and earbuds can be used as microphones, but they are really bad at picking up on sound. If someone used a laptop's internal speakers as a microphone, they wouldn't be able to hear very much. – ztk Mar 20 '17 at 18:01
  • BTW, I went to the link expecting to see a lot of FUD, but this at the bottom pretty much summarizes my viewpoint: "tl;dr: The CIA probably isn't listening to you through your TV, and if they are then you're almost certainly going to have a bad time anyway." His overall summary of the situation provides a very coherent risk analysis, so while I don't know how accurate his technical claims are, I agreed with his general claims more than I expected. – Conor Mancone Oct 9 '18 at 12:30
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One simple experiment you can run is to plug your headphones into a microphone input, and speak into them.

Fundamentally, both emitting sound and capturing sound is about dealing with vibrations in the air. The processes are opposite (one takes electrical signals as input, and outputs air vibrations, one takes vibrations in the air and transforms them into eletrical impulses).

However, consider the following:

  • both mics and speakers(/headphones) have a diaphragm, and as mentioned in the Wikipedia article:

    Microphones can be thought of as speakers in reverse

  • both incorporate circuitry able to translate vibrations into electricity (or vice versa). In other words, both have an electrical signal on one side, and vibrations on the other. The difference is what is labelled as input.

  • if you think about the original phonographs, the same component was used to record and listen to sound.

So yes, a speaker can pick up vibrations (albeit sub-optimally), and a microphone can emit vibrations (albeit sub-optimally).

Many TV speakers are actually speaker arrays, too, so that you (sort of) have an array of tuned inputs (important because it might allow using post-processing to enhance the wanted audio).

This doesn't address the software question, but mechanically, a speaker can function as a microphone.

There is also 'proof' that the functionality can be abused. badBIOS used the built-in PC speakers as a form of high-frequency modem - this despite the fact that one would expect PC speakers to be wired only for output. So yes, your comment was correct - there is a link to information security, and the functionality can be abused, notably to bridge air-gaps.

So this seems possible - how practical and how effective is another question, but there is precedence that it is usable for certain applications.

Edit thanks to @nulldev, how practical seems to have a fairly conclusive answer: per https://arxiv.org/ftp/arxiv/papers/1611/1611.07350.pdf, it is practical, and software solutions exist.

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    There is research about that, see "SPEAKE(a)R", presented in this paper: arxiv.org/ftp/arxiv/papers/1611/1611.07350.pdf from folks at the Ben-Gurion University of the Negev Cyber Security Research Center. There is a secuirty podcast mentioning this paper and work at grc.com/sn/SN-588-Notes.pdf – nulldev Mar 20 '17 at 9:46
  • Great reads and +1. I think it's worth a mention though that while you have clearly demonstrated that using speakers as microphones is feasible, that is a long way from establishing that the claim from Matthew Garrett is at all feasible. – Conor Mancone Oct 9 '18 at 12:25
  • Also, while I think the answer would vary wildly depending on the actual device in question, is there any basis in reality to the claim that some internal speakers can be switched to microphones by software changes only? – Conor Mancone Oct 9 '18 at 12:29
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Speakers can't be used as a mic, but that's because they use amplifiers, which are one way only (someone please correct me if that's the case), but headphones will work as long as they use the headphone jack and aren't amplified. Though, you will need a +30dB boost for decent volume. I can confirm it by personal experience. I hooked up my 10 euro havit headphones in the mic jack and it worked, despite not having a mic wired for PCs.

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When I was young, when a DJ was playing at a party and someone turned up who considered themselves an "MC", there was often no mic and on several occaisions, people plugged a headphone into the mixer mic input. the quality wasn't great, but it 100% worked.

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