SPEAKE(a)R: Turn Speakers to Microphones for Fun and Profit [pdf] (2017)

He ended up producing a documentary you can watch, called 7 Mile.

Similarly most leds are photo diodes, electric motors can be used to generate current, Peltier cells can be used to generate current, and so so forth. Many of such physical processes are invertible.

While staying in a high-end D.C. area hotel, I once discovered a hidden hard-wired speaker under the bathroom sink. Somebody had written "F.B.I." on it with nail polish.

I already knew that speakers could be used as microphones, and it occurred to me that putting a speaker in a hotel room in the name of "safety" would be a great cover story for a surveillance operation.

Fun fact, an electric guitar can also be used as a microphone if you shout in to it loud enough.

We discovered this while fooling around with some guitars and such as teenagers. We had a 4 track input device that was separating vocals and instruments, but even after turning down the vocal track, we could still hear it in the instrument track. We then of course followed it up with some experiments deliberately shouting into the guitar and enjoying the distorted recordings that came out of it

This comment caught me off guard and I couldn't believe this to possibly be true but as it turns out, yes, drive-thru speakers used the *literal same physical membrane* to act as both the speaker and microphone, and this was apparently widely commonpractice as recently as the 90s.

And they literally just used off the shelf, bog standard stereo speakers to use as a mic. Insane.

Given that such a mic would be several feet from the driver and poor audio quality could directly result in daily lost revenue for this business that sees revenue 100x to 1000x more than even high end audio equipment during this time period every month, I would've assumed they would've at least used a special membrane or more optimal type of speaker, but apparently not.

Sidenote: Obviously I used an LLM to research this (not to write, this is all certified organic human-generated text), and I just gotta say, isn't it absolutely delightful to be able to satisfy such random, obtuse curiosities like this one on a whim? This kind of question would've normally required a fair bit of googling to confidently validate, to the point I most likely wouldn't have even attempted to do so.