Detecting sound with old-style tungsten-filament light bulbs.

jim bell jdb10987 at
Sat Jun 13 12:28:01 PDT 2020

[partial quote follows]

>From Popular Mechanics
At Ben-Gurion University of the Negev, researchers have discovered it's possible to recover full conversations or sounds just by observing a light bulb.

You can see the vibrations created when sound hits the surface of the bulb by looking at slight fluctuations in the bulb's light.

They call the attack "lamphone," and it's outlined in a new paper the team plans to present at this summer's BlackHat USA security conference.

One of the best pieces of magic in the Harry Potter universe is a set of Extendable Ears, which wizards use to listen in on secret conversations behind closed doors. As far as we know, spies can't (yet) slip replicas of fleshy ears under a doorframe to eavesdrop—but a new light bulb device might be the next best thing.

Scientists at the Ben-Gurion University of the Negev in Israel have discovered it's possible to recover full conversations from the vibration patterns in a light bulb. The hanging bulb acts as both a diaphragm (sound waves cascade across its surface) and transducer (it converts air pressure from sound to small changes in light), which means it could be a useful gadget for secret agents and nosy people everywhere.

"We show how fluctuations in the air pressure on the surface of the hanging bulb (in response to sound), which cause the bulb to vibrate very slightly (a millidegree vibration), can be exploited by eavesdroppers to recover speech and singing, passively, externally, and in real time," the researchers write in their new paper, which they plan to present later this year at the Black Hat USA security conference.

Aptly, they call the new attack method "lamphone." It comes from some of the same minds behind a Tesla Autopilot hack from earlier this year, which proved a simple projector could fool the computer vision system into slowing down or speeding up for fake pedestrians and speed limit signs.

You can get your hands on everything required to pull off the lamphone feat, including a laptop, telescope, microphone, and remote electro-optical sensor—which coverts light into an electrical signal—for under $1,000, the researchers say. Right now, it's possible to propagate the attack from over 80 feet away, but with improved audio equipment and a better telescope, it could work from even farther distances.
[end partial quote]

Jim Bell's comment follows:
While I won't say that tungsten-filament light bulbs are 'dead as a doornail', they are getting there.  The last 30+ bulbs I bought were 100-watt-equivalent LED's, for which I paid $1 each. 
It's probably much more practical to bug by building a modified LED light bulb, which has a real microphone, and using the sounds detected from the microphone to AM modulate the light output of the LEDs.  Or, possibly modulate a frequency-modulated (FM) carrier frequency, depending on the bandwidth of the LEDs.  
Modulation bandwidth enhancement of white-LED-based visible light communications using electrical equalizations  (says that the modulation bandwidth of white LEDs is 'several megahertz'.)

           Jim Bell

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