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

https://www.usenix.org/system/files/conference/woot17/woot17-paper-guri.pdf

69•Eridanus2•2h ago

Comments

BFV•2h 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

hecanjog•2h ago

This shouldn't be downvoted. Transducers being reversible is a neat and non-obvious thing.

dnnddidiej•2h ago

Motors can be dynamos too

maqp•1h ago

and many LEDs are weak photo-diodes, i.e. you get weak current when you shine a light to them.

saagarjha•2h ago

It's probably downvoted because it sounds somewhat nonorganic.

vidarh•2h 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.

atoav•47m ago

It is basically the same as turning a motor into a generator.

me_jumper•2h ago

This needs a (2017), I was so confused why this was published again, seemed so familiar.

saagarjha•2h ago

Fixed

jpc0•2h 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...

bigbugbag•1h ago

same with solar panels, they can be reversed to emit light.

kqr•1h ago

What's their spectrum?

DoctorOetker•1h ago

near infrared

d3Xt3r•1h ago

Same with LEDs, they can be reversed to generate electricity.

userbinator•1h 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...

atoav•47m 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).

akoboldfrying•1h ago

> perfectly sound.

I hear what you did there

yen223•9m 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.

dickfickling•2h 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

m4lvin•1h ago

Okay, but how do I use this as a replacement when the mic is not working on Linux?

userbinator•1h 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.

villgax•1h ago

If this or an accelerometer based recording is what Meta uses to eavesdrop on in-person talk then color me pink

murderfs•1h 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_typist•37m 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

maqp•1h 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.

AmmarSaleh50•1h ago

don't let the CIA see this one

Se_ba•1h ago

Tbh it's crazy that you can do it in some of the microwaves

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

Comments

Archive of Byte magazine, starting with issue #1 in 1975

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

Game devs explain the tricks involved with letting you pause a game

What are skiplists good for?

NIST scientists create 'any wavelength' lasers

Anonymous request-token comparisons from Opus 4.6 and Opus 4.7

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Ask HN: How did you land your first projects as a solo engineer/consultant?

The electromechanical angle computer inside the B-52 bomber's star tracker

Binary Dependencies: Identifying the Hidden Packages We All Depend On

Updating Gun Rocket through 10 years of Unity Engine

Why Japan has such good railways

Changes in the system prompt between Claude Opus 4.6 and 4.7

The seven programming ur-languages

The world in which IPv6 was a good design

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It's cool to care (2025)

Why Zip drives dominated the 90s, then vanished almost overnight

Frank Dudley Beane's Experience with Ergot and Cannabis Indica (1884)

State of Kdenlive

Modern Common Lisp with FSet

Migrating from DigitalOcean to Hetzner

Metatextual Literacy

Optimizing Ruby Path Methods

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