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I understand that it's possible to use the headphone port of a laptop as a microphone, but what are the physical limitations to using any speaker in the same way?

I'm curious about what makes amplification a one way street and if it is necessarily always the case.

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  • \$\begingroup\$ There are non other than the practicality of having a driver and microphone circuit together with some arbitration circuit to decide direction. \$\endgroup\$ – Andy aka Jan 29 '18 at 16:54
  • \$\begingroup\$ one thing to consider is that mic diaphragms are super thin and light, while speaker ones are heavy and glued to wires, severally reducing their sensitivity. a sub woofer won't move from whistling... \$\endgroup\$ – dandavis Jan 30 '18 at 6:14
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If active speakers (i.e. speakers that require power) are connect to a computer then the amplification circuitry is a one-way street and it would be impossible for microphonic signals to be received by the computer.

If passive speakers or headphones (these are essentially the same) are connected to the computer then there is a more complicated answer.

Modern computer audio codecs often support multi-purpose audio jacks. This allows each jack to be software configured to function as: headphone output, microphone input, line out output, etc.

It is therefore possible for headphones connected to what the user presumes to be a headphone jack, to function as a microphone. Such a hack has already been demonstrated:

http://vrzone.com/articles/new-software-lets-hackers-turn-speakers-microphone/117315.html

This hack is only possible if the headphones are not being used to output sound. It is plusable that an audio codec could be hacked to have a jack simultaneously function as a microphone input and audio output. However, any microphonic current induced will be absorbed by the output driver so that the microphonic signal is never manifested as a measurable voltage. Even if the audio output was 'silence', the output driver would still be driving a zero voltage signal and absorbing microphonic current.

In summary: Active speakers cannot be used for listening/surveillance. Passive speakers or headphones can be used for listening/surveillance but only if they are not being used to output audio.

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I take your question to mean "can software use the speakers in computers at the moment as microphones".

And not to mean "can we change computer electronics to use their speakers as microphones".

Currently, computer speakers are generally (almost always) driven by an amplifier, with no electronic circuitry to read that speaker signal. So there is no mechanism there to 'listen' through the speaker using the existing electronics.

As you suspected, amplification itself is a one-way street. It is possible to design more elaborate circuits that aren't one-way. But that's not what your desktop or laptop PC has driving the speaker in it.

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Yes it is possible to hook up a circuit to a speaker to turn it into a crude microphone... for example..

enter image description here

However, your laptop speakers will not have such circuitry built in, and as such, you can not use them that way.

The sound quality from said microphones is also very limited, and the output and frequency range is highly dependent on the type and sixe of the speaker.

The speaker can not be driven as speakers at the same time as they "listen" as the transmitted sound will swamp the receiver.

Generally, this method is not used. It is so much simpler to add a small microphone which gives much better performance.

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  • \$\begingroup\$ I would accept this answer too. I was curious if it was possible without hardware modification. \$\endgroup\$ – user3737396 Jan 29 '18 at 17:40
  • \$\begingroup\$ @user3737396 no not without additional hardware. \$\endgroup\$ – Trevor_G Jan 29 '18 at 17:43

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