4 ms·

Asahi Linux (and likely MacOS too) uses the resistance of the speakers coils to detect overheating of same speakers and reduces volume.

derhuerst 2y ago
https://github.com/AsahiLinux/speakersafetyd https://github.com/AsahiLinux/speakersafetyd
- CamperBob2 2y ago
  Interesting. If the voltage across the speaker voice coil can be sampled with enough sensitivity at a fast-enough rate, you have an undocumented microphone.
  - xyzzy_plugh 2y ago
    This is true of all speakers
    - CamperBob2 2y ago
      Exactly.
    - ssl-3 2y ago
      It's true of all dynamic speakers -- the sort with a voice coil and a magnet. (But not all speakers are dynamic speakers.)
      - planewave 2y ago
        Would this also be true for electrostatic speakers as well? Though would probably would require greater gain/amplification or, potentially the application of some kind of bias voltage for the capacitive diaphragm of the speaker. Just speculation based on the shared operating principal with condenser microphones
        ssl-3 2y ago
        With bias power, I think an electrostatic loudspeaker turns into a condenser microphone (a thing that provides varying capacitance in response to changes in pressure). I don't think that electrostatic loudspeakers all require bias power, so it's not quite as simple as using a dynamic loudspeaker backwards is. It is a neat idea, though. A big, flat-panel microphone would be interesting to play with.
        robotresearcher 2y ago
        You can use a window or any large panel as a microphone without even touching it by observing its vibrations. You can bounce a laser off it, or even go fully passive using a camera with some sensitivity tricks: I recall a paper that reconstructed a remote conversation by watching a houseplant through a window.
        ssl-3 2y ago
        Of course. Everything is a microphone if you're brave enough.
    - BobbyTables2 2y ago
      But not true of all codecs… Do you think Apple put a hidden microphone in their devices by pure accident?
    - TacticalCoder 2y ago
      Others who know that better than me and commented but... First time I read that, as a kid, here was I plugging my headphones into the input jack of my parents' soundsystem and, sure enough, it worked as mic (although at as super ultra low volume but I clearly remember it worked).
squarefoot 2y ago
That's the same principle used by cheap solder stations to regulate the tip temperature without employing a thermal sensor: they measure the heater resistance, presumably during the off state of the PWM signal that drives the heater. In that case the measurement is less accurate than using a real sensor, still good enough for cheap solder stations where a few degrees don't make a big difference.
- floating-io 2y ago
  Why would they do it during the off state? If they know the voltage and can measure the current that they're driving it with -- or vice versa -- they can use Ohm's law to calculate the resistance. They'd probably have to do that anyway.
nick3443 2y ago
Also used in electric motor controllers to monitor winding temperature.

derhuerst 2y ago

https://github.com/AsahiLinux/speakersafetyd https://github.com/AsahiLinux/speakersafetyd

CamperBob2 2y ago
Interesting. If the voltage across the speaker voice coil can be sampled with enough sensitivity at a fast-enough rate, you have an undocumented microphone.
- xyzzy_plugh 2y ago
  This is true of all speakers
  - CamperBob2 2y ago
    Exactly.
  - ssl-3 2y ago
    It's true of all dynamic speakers -- the sort with a voice coil and a magnet. (But not all speakers are dynamic speakers.)
    - planewave 2y ago
      Would this also be true for electrostatic speakers as well? Though would probably would require greater gain/amplification or, potentially the application of some kind of bias voltage for the capacitive diaphragm of the speaker. Just speculation based on the shared operating principal with condenser microphones
      - ssl-3 2y ago
        With bias power, I think an electrostatic loudspeaker turns into a condenser microphone (a thing that provides varying capacitance in response to changes in pressure). I don't think that electrostatic loudspeakers all require bias power, so it's not quite as simple as using a dynamic loudspeaker backwards is. It is a neat idea, though. A big, flat-panel microphone would be interesting to play with.
        robotresearcher 2y ago
        You can use a window or any large panel as a microphone without even touching it by observing its vibrations. You can bounce a laser off it, or even go fully passive using a camera with some sensitivity tricks: I recall a paper that reconstructed a remote conversation by watching a houseplant through a window.
        ssl-3 2y ago
        Of course. Everything is a microphone if you're brave enough.
  - BobbyTables2 2y ago
    But not true of all codecs… Do you think Apple put a hidden microphone in their devices by pure accident?
  - TacticalCoder 2y ago
    Others who know that better than me and commented but... First time I read that, as a kid, here was I plugging my headphones into the input jack of my parents' soundsystem and, sure enough, it worked as mic (although at as super ultra low volume but I clearly remember it worked).

squarefoot 2y ago

That's the same principle used by cheap solder stations to regulate the tip temperature without employing a thermal sensor: they measure the heater resistance, presumably during the off state of the PWM signal that drives the heater. In that case the measurement is less accurate than using a real sensor, still good enough for cheap solder stations where a few degrees don't make a big difference.

floating-io 2y ago
Why would they do it during the off state? If they know the voltage and can measure the current that they're driving it with -- or vice versa -- they can use Ohm's law to calculate the resistance. They'd probably have to do that anyway.

nick3443 2y ago

Also used in electric motor controllers to monitor winding temperature.