Hacker News Reader

Comment by pixxel | Hacker News Reader

> hooking up an electro-optical sensor to a telescope and pointing it at a light bulb, but that's the meat and potatoes of the sub $1,000 (£800) hardware hack at least. Fluctuations in air pressure on the surface of the hanging bulb are created by the sound of conversation, or music, and make a hanging bulb vibrate.

Isn't this the same technique used to listen in on a conversation using a glass window?

View on HN

The technique used to listen in on a conversation using the window is an active technique (an IR laser is emitted towards the window to get the bounce), so it can be detected, and requires LOS to the window + to the reflected path if you're not hitting the window completely perpendicularly. For example you can't hit a window a floor above from ground level, and architectural treatments that result in angled windows can reflect light away in directions that are more difficult to observe.

You can point your telescope to any reflection or light (even fluorescent ones) on a room, and the induce vibration will be enough to create a signal to listen.. I accidentally figured that out when I was 13 or so, and tried to build a led walkie-talkie.. I kept hearing other voices, and that is what it was..

One thing that's not obvious to me from TFA is why an incandescent bulb is called out specifically. It seems to me that a hanging LED bulb would be just as vulnerable, as might a glass chandelier of sorts.

I also wonder whether Christmas trees with hanging ornaments are vulnerable. These are often placed in a prominent window, so it might be quite easy to start listening in on conversations in people's living rooms or business lobbies.

Maybe the filament is more susceptible to vibration and "flickering". But it may just be my interpretation, the text doesn't really spell it out as such.

A couple of other comments mentioned the filaments, but the article says nothing about clear bulbs. They are very much in fashion these days, but most incandescent bulbs are frosted, so it seems to me that if you want to use a laser on the filament, there would need to be some considerations around photons passing through the frosting and reflecting off the filament.

The bulb doesn't have to be clear, the filament vibration would make the light flicker visibly (for sensitive equipment that is) even if the glass isn't clear. So it may be that the filament is far more susceptible to vibration than the glass on any type of light bulb. As I said, this is my interpretation, the text doesn't make it clear to me.

I don't think it's about observing the filament directly, just that the filament basically being a spring is more susceptible to vibrations which would change the amount of light emitted.

I am totally guessing at that, but it seems reasonable.

The filament is in a vacuum...

Almost no incandescent light bulb has a vacuum inside. They are filled with low pressure inert gas (mostly argon sometimes mixed with nitrogen, some bulbs have krypton or xenon for lower temps) not only because it makes them safer to handle and harder to break but mainly because it slows down the decay of the filament by slowing down its evaporation. It bounces tungsten atoms back on the filament.

LED lights and fluorescent lights are vulnerable too per the Wired article at https://www.wired.com/story/lamphone-light-bulb-vibration-sp... , just with different degrees of sensitivity

That article seems to say that this technique works better with LED lamps vs. incandescent bulbs.

I wonder if they're talking about direct line of sight to the LED chip. Most of the LED "bulbs" in my house use a frosted plastic housing around the LED chip, which I would guess to interfere with the sound recovery.

>> One thing that's not obvious to me from TFA is why an incandescent bulb is called out specifically.

Because they produce light continuously. An led, CFL, or other will actually flicker at 120Hz.

An incandescent bulb still has 120Hz power going in, but because it depends on heating to produce light and the filament has some thermal capacity, this is largely smoothed out.

> An led, CFL, or other will actually flicker at 120Hz.

In 60Hz countries. Also, if it has a constant current driver there will be PWM noise from the regulator.

I have noticed LED bulbs keep glowing for a short period of time after turning them off:

https://www.quora.com/Why-do-LED-bulbs-keep-glowing-even-aft...

I imagine there is still some flicker, but it seems to be smoothed out pretty well on LEDs also.

I've seen that too. I think they're using a phosphor of some kind to convert UV wavelengths to visible light. That would have some persistence for those wavelengths.

Maybe it’s because an LED bulb is much heavier?

Reply on news.ycombinator.com