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"Smart" devices are all around us nowadays and many of them have built-in mics which can be be misused to spy on you in cases when the device is hacked (which is rather probable to happen as there are countless and growing vulnerabilities in all the software and hardware and botnets grow and flourish).

The question is how to find all the mics to take care of them a way or another?

Laptops and phones are the obvious hits. Smart TVs, set-top-boxes and game consoles, different kinds of video cameras etc may contain tiny built-in mics with (if that is a visibly advertised feature) or without you knowing.

Is there a way to find them all? E.g. by detecting magnets or something...

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    If not mistaken, any speaker or audio output device can be use as recorder or input device.
    – Baptiste
    Aug 21, 2017 at 9:19
  • Maybe this question might be better suited for dsp.stackexchange.com
    – Tom K.
    Aug 21, 2017 at 13:50
  • Which are you asking: (1) how to be sure there is no microphone in a particular electronic or smart device or (2) how to be sure there is no microphone in the room or (3) how to be sure there is no electronic or smart device in the room? Sep 10, 2017 at 19:43

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This is very broad without a context. If you simply define a microphone as a device able to record sounds, it will be very hard, because microphones can use various technologies, and you will only be able to protect you from technologies.

  • microphones sending in real-time what they hear: the common attack against them will be a wave scanner to detect their emission - that will be the easier part
  • common microphones with local storage. Depending on the technology used, you could try to detect the magnetic fields generated by the electronic machinery, but they can be low and hard to isolate from the ambient noises generated for example by modern lamps, not speaking of other electronic devices: do not hope to detect them at a large distance
  • laser microphones: glass windows can be used as a passive microphone with a device analyzing its vibrations through a laser. It is easy to detect a glass window, much harder to detect whether it is actively spyed
  • you could even build a purely machanical microphone by copying the disks ancestors: a horn to capt the sound, a needle attached to its top writing on a wax disk moved by a spring. Ok, it will be much bigger than an electronic device could be, but you would not detect it by the same tools...
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  • your last 2 points do not seem to be relevant to locating mics in smart devices (your points are about external threats, not localised threats)
    – schroeder
    Aug 21, 2017 at 10:15
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    @schroeder: you are right, but I was unsure whether smart devices was just an example of possible microphones or whether the whole question was limited to smart devices. Aug 21, 2017 at 11:10
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    And also listening devices with no electronics: cryptomuseum.com/covert/bugs/thing/index.htm (mainly pointing that one out because of how neat it is...)
    – derobert
    Aug 23, 2017 at 1:34
  • On the subject of listening devices with no electronics, be aware of radar remotely bugging the vocal chords. For example "A 94-GHz millimeter-wave sensor for speech signal acquisition." (2013) says "This result suggests that the MMW radar sensor has better performance than a traditional microphone in terms of speech detection for detection distances longer than 1 m." ncbi.nlm.nih.gov/pubmed/24284764 Sep 10, 2017 at 19:29
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First, Assume that every "smart" device has a mic.

After that the only thing stopping you from finding all mics in a room is to which degree you are willing to be satisfied that you have no mics.

If you are talking about company secrets some sort of electric sweep will do.

If you are talking about state secrets the process might become more involved.

If they really want to know what you are talking about and they have the means($) to do it, they will. Alone some sort of electronic device or process wont work you also need to have controlled access and maybe even randomized rooms etc.

If you want a more specific answer you will need to be more specific about your situation.

You might also want to look up the Russian spying device "The Thing", it used no electricity until it was activated.

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