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

Posted by Eridanus2 2 days ago

Comments

Comment by VladVladikoff 1 day ago

When I was a teenager I was friends with an extremely poor kid who literally lived on the wrong side of the tracks. He couldn’t afford a microphone and used an old pair of busted headphones to rap into as a microphone. He had recorded and produced a whole album like this with Fruity Loops on an old computer he found discarded at the side of the road.

Comment by dustractor 1 day ago

Busted headphones taped to a broomstick was my friend's microphone of choice when he did punk shows

Comment by QuercusMax 1 day ago

My little brother and I did this with an old Panasonic tape recorder. We were in elementary school so it wasn't very good, but it got my brother into music production.

Comment by gfiorav 1 day ago

what happened to him?

Comment by IncreasePosts 1 day ago

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

Comment by metrix 1 day ago

So authoritative!

You had me for a second :)

Comment by nullsanity 1 day ago

What do you think? He was poor in America, so he stayed poor in America. 99.999% of stories about people in hopeless poverty end with them in hopeless poverty. To expect otherwise is ludicrous.

Comment by Evidlo 1 day ago

Wow, what are the odds you were friends with him too!

Comment by analog31 1 day ago

A fun fact is that the ability of a single transducer to function as both a speaker and a microphone is the basis for establishing an absolute measurement of sound pressure.

https://nvlpubs.nist.gov/nistpubs/jres/25/jresv25n5p489_A1b....

Comment by alexjplant 1 day ago

Using a speaker as a kick drum mic ("subkick") has been a thing in recording for years [1]. I've never used one but it makes complete sense.

[1] https://bobbyowsinskiblog.com/build-subkick/

Comment by dickfickling 2 days 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

Comment by jpc0 2 days 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...

Comment by userbinator 2 days 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...

Comment by analog31 1 day ago

I think you have this backwards. Condensers and electrets (a form of condenser with a permanent charge on one terminal) almost always have a built-in preamp. The reason is that they cannot drive a capacitive load of any magnitude, and their outputs must be buffered before being fed to any wiring.

Like another post mentioned, dynamic mics like the Shure SM58 mentioned here, can drive a cable directly or through a small built-in transformer. They're still used in live sound, though condensers have become quite common there too. Condensers still tend to have somewhat better behavior, such as signal-to-noise, than electrets.

Of course everything has to be amplified or fed to a digitizer at some point. The issue is where the preamp needs to be physically located.

Comment by Anechoic 1 day ago

* The vast majority of mics for PCs are condensers and electrets.*

These can be run in reverse as well, it requires CB custom electronics so it’s not something a lay person can do out of the box.

Comment by atoav 2 days 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).

Comment by userbinator 1 day ago

Yes. I don't think many PCs would have a stage mic plugged into them.

Comment by yen223 2 days 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.

Comment by bigbugbag 2 days ago

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

Comment by docjay 1 day ago

What’s wild is that most things having to do with light, magnetism, and/or electricity are interchangeable and reversible. Put electricity through a wire and it’ll create a magnetic field, or wave a magnetic field near a wire and it’ll create electricity. That means that putting electricity into an LED creates light and a magnetic field, or putting light into the LED creates electricity and a magnetic field, or waving a magnetic field near it will create electricity in the wires and light from the LED. Granted for that last one you’ll need a spinning magnetar nearby, or just add some more wire to the LED and it becomes a kitchen counter experiment.

Same interchangeability with solar panels, transformers, thermoelectric devices, etc. The effect might be big or small, depending on the setup, but the physics is happening either way.

I’ve spent time lost in space thinking about how much stuff is really just a copper wire in various configurations.

Have a copper wire - it’s an antenna, magnet, inductor, fuse, thermometer, heater, and strain gauge.

Put another copper wire near it - it’s a capacitor.

Curl one more than the other - it’s a transformer.

Put iron on it - it’s a thermocouple.

Put electricity through it - it’s a peltier cooler.

Add salt water - it’s a battery.

Put electricity through it - the iron is now a permanent magnet.

Wave the permanent magnet near it - it’s a generator and a microphone.

Put electricity through it again - it’s a motor and a speaker.

Heat it up and it’ll make Cuprous Oxide - it’s a solar panel and a diode.

Put electricity into it - it’s an LED.

Comment by d3Xt3r 2 days ago

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

Comment by amelius 2 days ago

[flagged]

Comment by kqr 2 days ago

What's their spectrum?

Comment by DoctorOetker 2 days ago

near infrared

Comment by m463 1 day ago

You can also get fluorescent tubes to light up under transmission lines.

Comment by akoboldfrying 2 days ago

> perfectly sound.

I hear what you did there

Comment by anonymousiam 1 day ago

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.

Comment by antirez 1 day ago

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.

Comment by jimmydddd 1 day ago

Yes. Mostly the same basic components--Just optimized for which direction you are going in. I recall using an single in ear speaker as a microphone as one of the experiments in my Radio Shack 101 eperiments electronics kit.

Comment by userbinator 2 days 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.

Comment by Anechoic 1 day ago

This is how drive-thru kiosks work (principal, not the specific implementation).

Source: I used to measure the “microphone” frequency response for a kiosk OEM.

Comment by angg 1 day ago

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.

Comment by userbinator 1 day ago

Many intercom systems and walkie-talkies do the same. The downside is they're half-duplex.

Comment by maqp 2 days 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.

Comment by lelandfe 1 day ago

E.g. this music video (evoking old Just Jam sets): https://www.youtube.com/watch?v=_xQKWnvtg6c

To really take it to that next level, snap the headphones in half when you get up on stage for a lollipop. Even seen one bring a corded phone and cradle to a set.

Comment by rf15 1 day ago

As a kid I accidently plugged a mic into the speaker port and was surprised that, when I put my ear close to the mic, I could hear the computer sounds! It made sense in hindsight, and since then I knew they are kind of functionally equivalent.

Comment by fipar 1 day ago

My first “electric guitar” as a kid was my acoustic with an earphone taped to the bridge and plugged to the mic in of my boom box.

It was also my first “fuzz pedal” because the sound never came out clean :)

Comment by f055 1 day ago

As a kid behind the Iron Curtain I had a radio & cassette tape player with a single large speaker built-in. The cassette player had a record button that when pressed together with play used the speaker and I could actually record my voice, and play it back, from the tape. After enough re-recording on a single tape the fade of the old recordings broke through the new ones creating a truly amazing sound experience. Too bad I lost the tapes and the device.

Comment by jnellis 1 day ago

When I did commercial fishing in Alaska, often the boats just had two speakers, one in the wheel house and one on the deck (long liner). You just talked into/toward the speaker.

Comment by olelele 1 day ago

A big speaker element is useful for micing kick drums.

Comment by ssttoo 1 day ago

It was so popular in studios to use an old Yamaha ns10 speaker as a kick (bass) drum mic that now Yamaha started making and selling an “official” version.

I personally used a toy guitar amp for this purpose https://music.stoyanstefanov.com/2017/03/30/diy-sub-kick-mic...

Comment by DoctorOetker 1 day ago

not only can electromagnet voice coils (speakers or microphones), be used as either a speaker or a microphone (transducer), they can even be used as both at the same time using a DC-coupled circulator / isolator:

https://techlib.com/files/rfdesign3.pdf

Comment by m4lvin 2 days ago

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

Comment by GlumWoodpecker 1 day ago

You can use the `hdajackretask` program in the `alsa-tools` package to retask your jacks.

https://github.com/alsa-project/alsa-tools/tree/master/hdaja...

Comment by ChicagoDave 1 day ago

I used my moms Koss headphones as a microphone on her old stereo and used it to broadcast cast my teenage stupidity to the whole house.

Comment by 2 days ago

Comment by me_jumper 2 days ago

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

Comment by saagarjha 2 days ago

Fixed

Comment by Se_ba 2 days ago

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

Comment by jedbrooke 1 day ago

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

Comment by undebuggable 1 day ago

That's all true. As a kid I was watching with sister a children's show in TV. The actor on the screen asked a question followed by silence. My sister yelled the answer at the TV, then the actor said something like "you're right kid". I was flabbergasted.

Comment by p1mrx 1 day ago

Dora the Informer.

Comment by kQq9oHeAz6wLLS 1 day ago

This also works during televized sporting events. The refs can indeed hear you, but you have to yell pretty loud.

Comment by riobard 1 day ago

why is jack retasking a thing…

Comment by adrianmonk 1 day ago

Chips only have a certain number of pins. It probably works out better economically if those pins can be used for either input or output. Chip manufacturers can thus make one product that will fit the needs of more customers instead of (say) 9 different chip variants with 8 inputs and 0 outputs, 7 inputs and 1 output, 6 inputs and 2 outputs, etc.

It could also be useful to the end user. Motherboards have a limited number of ports since the connectors cost money and take up space on the back panel. One user might want a line input (for digitizing old cassettes, for example)[1] and another user might want an extra surround sound output (for 7.1 surround sound instead of just 5.1 surround). With retasking, the motherboard can support both these niche use cases with a single shared port.

---

[1] You can't use a microphone input for this because (a) it's mono and (b) it's a different voltage level.

Comment by villgax 2 days ago

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

Comment by murderfs 2 days 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.

Comment by slow_typist 2 days 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

Comment by AmmarSaleh50 2 days ago

don't let the CIA see this one

Comment by 2 days ago

Comment by nailer 1 day ago

Vault 7 from wikileaks confirmed that yes, the CIA is using this.

Comment by 1 day ago

Comment by BFV 2 days ago

[flagged]

Comment by rigonkulous 1 day ago

Its one of those extremely valuable life-changing hacks that kids who grew up in the 70's with electronics magazines as their primary source of tomfoolery knew all about .. and got into trouble for .. once or twice lets say .. during particularly heated typing classes on selectric machines of the 80's necessitating a headphone distribution mixer for sane memo-taking lessons, with a few crossed wires worth of feedback generation device introduced surreptitiously one particularly memorable summer Friday afternoon - originally intending to impress the girls of the class - not deafen them (albeit temporarily) .. but, in any case, getting the party started early, nevertheless..

(If you are going to attempt this with stereo headphones, keep the streams separated at all times!)

Comment by vidarh 2 days 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.

Comment by ijk 1 day ago

I feel like this is the kind of hack that made early radio tech exciting to play around with. The basic parts are incredibly simple to assemble from scratch, so it feels like magic. Speakers and microphones are the same thing in reverse. And so on.

Comment by hecanjog 2 days ago

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

Comment by saagarjha 2 days ago

It's probably downvoted because it sounds somewhat nonorganic.

Comment by 3form 1 day ago

Even with the em-dash. New account and other comments seem to be here-and-there. Maybe LLM with some editing after.

Comment by dnnddidiej 2 days ago

Motors can be dynamos too

Comment by maqp 2 days ago

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

Comment by mkl 1 day ago

It's pretty obvious if you did high school physics. I experimented with earphones as microphones as a teenager but couldn't get any meaningful audio data.

I think they're being downvoted because their comments all seem to have AI features.