29 ms·

I don't understand how laser light, even from a small 5 mW laser pointer, can get the membrane to move. My first thought was that it wasn't moving and that the

I don't understand how laser light, even from a small 5 mW laser pointer, can get the membrane to move. My first thought was that it wasn't moving and that the signal was from the photoelectric effect, but they have a section in the paper testing just that and it turns out it is the membrane moving. I doubt it's expanding from heat since it's such a low power and it reacts fast enough to simulate speech, but how does it work?

deepnotderp 7y ago
Radiation pressure would be my initial guess, but this is a very weak laser for that.
- Animats 7y ago
  That puzzles me, too. The thermal effect that drives a radiometer[1] should be too slow to transmit audio. Radiation pressure transfer from photons is very weak, but wouldn't have bandwidth problems. The frequency response of this effect does fall off with frequency, but not very fast; see fig. 6. in the paper.[2] There's still reasonable output at 3KHz. Light pressure of sunlight is about 1mg/m^2. Or one nanogram per square millimeter, a reasonable size for a chip microphone. Will chip microphones respond to a nanogram of pressure? [1] https://en.wikipedia.org/wiki/Crookes_radiometer https://en.wikipedia.org/wiki/Crookes_radiometer [2] https://lightcommands.com/20191104-Light-Commands.pdf https://lightcommands.com/20191104-Light-Commands.pdf
skykooler 7y ago
Maybe it's an electrostatic effect?
superkuh 7y ago
The heat has to go somewhere. The photoacoustic effect of the changing intensity of laser light causes the material of the case to generate sound which is coupled by direct mechanical contact to the mounted MEMs microphone.
DerekL 7y ago
There's some speculation in this Wired article: https://www.wired.com/story/lasers-hack-amazon-echo-google-home/ https://www.wired.com/story/lasers-hack-amazon-echo-google-h... The thermal effect is one, where the membrane heats up and makes the surrounding air expand. The other is the light hitting photosensitive electronic components. Discussed here: https://news.ycombinator.com/item?id=21445043 https://news.ycombinator.com/item?id=21445043

deepnotderp 7y ago

Radiation pressure would be my initial guess, but this is a very weak laser for that.

Animats 7y ago
That puzzles me, too. The thermal effect that drives a radiometer[1] should be too slow to transmit audio. Radiation pressure transfer from photons is very weak, but wouldn't have bandwidth problems. The frequency response of this effect does fall off with frequency, but not very fast; see fig. 6. in the paper.[2] There's still reasonable output at 3KHz. Light pressure of sunlight is about 1mg/m^2. Or one nanogram per square millimeter, a reasonable size for a chip microphone. Will chip microphones respond to a nanogram of pressure? [1] https://en.wikipedia.org/wiki/Crookes_radiometer https://en.wikipedia.org/wiki/Crookes_radiometer [2] https://lightcommands.com/20191104-Light-Commands.pdf https://lightcommands.com/20191104-Light-Commands.pdf

skykooler 7y ago

Maybe it's an electrostatic effect?

superkuh 7y ago

The heat has to go somewhere. The photoacoustic effect of the changing intensity of laser light causes the material of the case to generate sound which is coupled by direct mechanical contact to the mounted MEMs microphone.

DerekL 7y ago

There's some speculation in this Wired article: https://www.wired.com/story/lasers-hack-amazon-echo-google-home/ https://www.wired.com/story/lasers-hack-amazon-echo-google-h... The thermal effect is one, where the membrane heats up and makes the surrounding air expand. The other is the light hitting photosensitive electronic components. Discussed here: https://news.ycombinator.com/item?id=21445043 https://news.ycombinator.com/item?id=21445043