SPEAKE(a)R: Turn Speakers to Microphones for Fun and Profit [pdf] (2017)(www.usenix.org)
96 pointsby Eridanus25 hours ago18 comments
  • analog318 minutes ago
    A fun fact is that the ability of a single transducer to function as both a speaker and a microphone is the basis for establishing an absolute measurement of sound pressure.

    https://nvlpubs.nist.gov/nistpubs/jres/25/jresv25n5p489_A1b....

  • VladVladikoffan hour ago
    When I was a teenager I was friends with an extremely poor kid who literally lived on the wrong side of the tracks. He couldn’t afford a microphone and used an old pair of busted headphones to rap into as a microphone. He had recorded and produced a whole album like this with Fruity Loops on an old computer he found discarded at the side of the road.
    • gfiorav2 minutes ago
      what happened to him?
  • dickfickling5 hours ago
    I have vague memories of iPod Linux (or Rockbox, I can’t remember) having a feature where you could record voice notes using your regular headphones using the same technique
  • jpc05 hours ago
    A magnet in a coil operates both ways, this is non intuitive but perfectly sound.

    Not sure if it's mentioned in the article but microphones can be speakers too...

    • userbinator4 hours ago
      Not sure if it's mentioned in the article but microphones can be speakers too...

      Only dynamic mics, which are relatively rare and seldom encountered without an attached preamp. The vast majority of mics for PCs are condensers and electrets.

      Anything can be a speaker, briefly and only once, if you apply enough voltage to it...

      • analog3110 minutes ago
        I think you have this backwards. Condensers and electrets (a form of condenser with a permanent charge on one terminal) almost always have a built-in preamp. The reason is that they cannot drive a capacitive load of any magnitude, and their outputs must be buffered before being fed to any wiring.

        Like another post mentioned, dynamic mics like the Shure SM58 mentioned here, can drive a cable directly or through a small built-in transformer.

        Of course everything has to be amplified or fed to a digitizer at some point. The issue is where the preamp needs to be physically located.

      • Anechoic38 minutes ago
        * The vast majority of mics for PCs are condensers and electrets.*

        These can be run in reverse as well, it requires CB custom electronics so it’s not something a lay person can do out of the box.

      • atoav3 hours ago
        Huh? The standard stage mic, the Shure SM58, certainly is dynamic and has no preamp.

        But you probsbly think about smaller form mics like found on headsets (Electrets).

    • yen2233 hours ago
      I recall when I was a kid decades ago, being able to plug a speaker directly into the microphone jack and use it as a microphone, without any modifications whatsoever.

      We could do the reverse too, plug a microphone into the speaker jack and hear sounds coming out from it.

    • bigbugbag4 hours ago
      same with solar panels, they can be reversed to emit light.
      • d3Xt3r4 hours ago
        Same with LEDs, they can be reversed to generate electricity.
      • kqr4 hours ago
        What's their spectrum?
    • akoboldfrying4 hours ago
      > perfectly sound.

      I hear what you did there

  • maqp4 hours ago
    Some DJs use this principle when they need a hacky stage mic. They plug their headphones to the mixer's mic input, and shout to the speaker element.
    • lelandfean hour ago
      E.g. this music video (evoking old Just Jam sets): https://www.youtube.com/watch?v=_xQKWnvtg6c

      To really take it to that next level, snap the headphones in half when you get up on stage for a lollipop. Even seen one bring a corded phone and cradle to a set.

  • userbinator4 hours ago
    Not all speakers work well as dynamic mics; and in fact turning on mic mode may enable the bias voltage, which could either burn out the voice coil or hold the diaphragm against the stop, making it even less likely to pick up any sound.

    Jack retasking, although documented in applicable technical specifications, is not well-known, as was mentioned by the Linux audio developer

    This could be a "bubble effect"; the Realtek codecs mentioned have a Windows utility to configure the jacks, which countless otherwise non-technical users would've seen and interacted with, so awareness of this feature is probably higher than they think. Fun fact: the "ALC" prefix in their codec names stands for Avance Logic, which was acquired by Realtek and they just kept that prefix well into the HD Audio era.

  • Anechoic41 minutes ago
    This is how drive-thru kiosks work (principal, not the specific implementation).

    Source: I used to measure the “microphone” frequency response for a kiosk OEM.

  • rf152 hours ago
    As a kid I accidently plugged a mic into the speaker port and was surprised that, when I put my ear close to the mic, I could hear the computer sounds! It made sense in hindsight, and since then I knew they are kind of functionally equivalent.
    • fipar2 hours ago
      My first “electric guitar” as a kid was my acoustic with an earphone taped to the bridge and plugged to the mic in of my boom box.

      It was also my first “fuzz pedal” because the sound never came out clean :)

  • riobard28 minutes ago
    why is jack retasking a thing…
  • m4lvin4 hours ago
    Okay, but how do I use this as a replacement when the mic is not working on Linux?
  • Se_ba4 hours ago
    Tbh it's crazy that you can do it in some of the microwaves
  • 5 hours ago
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  • me_jumper5 hours ago
    This needs a (2017), I was so confused why this was published again, seemed so familiar.
  • BFV5 hours ago
    That’s actually a pretty wild concept—turning speakers into microphones sounds like one of those “this shouldn’t work, but it does” kind of hacks
    • rigonkulous6 minutes ago
      Its one of those extremely valuable life-changing hacks that kids who grew up in the 70's with electronics magazines as their primary source of tomfoolery knew all about .. and got into trouble for .. once or twice lets say .. during particularly heated typing classes on selectric machines of the 80's necessitating a headphone distribution mixer for sane memo-taking lessons, with a few crossed wires worth of feedback generation device introduced surreptitiously one particularly memorable summer Friday afternoon - originally intending to impress the girls of the class - not deafen them (albeit temporarily) .. but, in any case, getting the party started early, nevertheless..

      (If you are going to attempt this with stereo headphones, keep the streams separated at all times!)

    • vidarh5 hours ago
      I hadn't thought about whether this would still with modern speakers, but this was the common assumption for several older types of speakers and microphones.

      One of the first "science experiments" my dad showed me was the other direction: Dismantling our telephone and demonstrating that the carbon microphone (yes, I'm old) in the handset would also work as a (really bad) speaker.

    • ijk2 hours ago
      I feel like this is the kind of hack that made early radio tech exciting to play around with. The basic parts are incredibly simple to assemble from scratch, so it feels like magic. Speakers and microphones are the same thing in reverse. And so on.
    • Rekindle809038 minutes ago
      This user is an LLM.
    • hecanjog5 hours ago
      This shouldn't be downvoted. Transducers being reversible is a neat and non-obvious thing.
      • mkl2 hours ago
        It's pretty obvious if you did high school physics. I experimented with earphones as microphones as a teenager but couldn't get any meaningful audio data.

        I think they're being downvoted because their comments all seem to have AI features.

      • saagarjha5 hours ago
        It's probably downvoted because it sounds somewhat nonorganic.
        • 3form2 hours ago
          Even with the em-dash. New account and other comments seem to be here-and-there. Maybe LLM with some editing after.
      • dnnddidiej5 hours ago
        Motors can be dynamos too
        • maqp4 hours ago
          and many LEDs are weak photo-diodes, i.e. you get weak current when you shine a light to them.
    • atoav3 hours ago
      It is basically the same as turning a motor into a generator.
    • 3 hours ago
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  • AmmarSaleh504 hours ago
    don't let the CIA see this one
  • villgax4 hours ago
    If this or an accelerometer based recording is what Meta uses to eavesdrop on in-person talk then color me pink
    • murderfs4 hours ago
      It's pretty unlikely that Meta is actually eavesdropping on your conversations, because it'd be immediately obvious from battery usage. The ability to turn speakers into microphones doesn't help if the speakers aren't actually connected to an ADC, and both of the modern smartphone OSes limit you to on the order of hundreds of samples per second, so it's rather difficult to get anything sensible without either doing a bunch of local analysis or exfiltrating it, both of which would be visible.
      • slow_typist3 hours ago
        It can be done with neural networks [1]. Also, speech doesn’t need much bandwidth to be intelligible. You would need control of the analog filter between the accelerometer and the ADC. With 250/s acceleration samples you can reconstruct a signal of a bandwidth of more than 100 Hz anywhere in the spectrum. That is called undersampling.

        [1] https://dl.acm.org/doi/abs/10.1145/3478102

  • 5 hours ago
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  • naileran hour ago
    Vault 7 from wikileaks confirmed that yes, the CIA is using this.
    • an hour ago
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