Electronic – Speakers as microphone

amplifiermicrophonesound

I've seen this following post regarding using the speakers as a microphone:
https://skeptics.stackexchange.com/questions/5664/can-a-computer-be-hacked-to-use-a-connected-speaker-as-a-microphone

There are some things I don't understand about it. He says that an amplifier doesn't pass any signal from it's output to it's input. Is this correct? if I have an amplifier such that output=100*input and I force 100*x on the output, won't I get x in the input?

To my understanding, speakers are always plugged through an amplifier to the sound card. The question is whether the amplifier resides in the sound card (where it is also controllable and we can lower it) or in the speaker. In any way, say that when I produce a voltage of x from the sound card I get some sound signal level in the air of gx right on the speakers (g in the appropriate units). Now if I produce a sound in the air that is equal to gx right on the speakers I will get the voltage level x to the sound card. So reducing the amplifier is just a matter of reducing this g. Is this the accurate explanation? and not the explanation inside the link?

Best Answer

An amplifier... amplifies. Takes a tiny signal, and beefs it up with more voltage / current / power.
A speaker needs that power, to make a coil move in a magnetic field. Attached to the coil is the cone, this displaces air when the coil moves - and we get sound.

A speaker WILL act as a microphone. It's just not very efficient.

If you get real close, and shout really loud, you will get a mediocre signal. In theory, this get passed back to the input where, since we are reducing signal strength along the way, we would have a microscopic signal.

Then comes the final hurdle: recording this signal.

Computer data, in the form of ones and zeroes, gets chewed through a chip called Digital-Analog-Converter [DAC] before being sent to that amplifier. To record our eensy signal, we need an Analog-Digital-Converter [ADC].

ADC's are weird and wonderful beasts. Your computer has one - it's what the "mic" input goes to. No matter how you quibble about passing signals backwards through an amplifier, I fail to see how you get a digital version without adding your own hardware [ie. planting a bug].