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Comment by Marthinwurer | Hacker News Reader

I love these kind of inadvertent measurements. One of my favorite examples is that a sufficiently accurate IMU can get you relatively accurate longitude measurements from the Coriolis effect.

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Slight correction, latitude, not longitude.

The earth’s surface closer to the poles has less distance to travel for any rotation than the surface closer to the equator. As a result the inertial navigation systems of long distance systems must be adjusted. Iirc, this is also the case for artillery firing computations.

https://www.oxts.com/blog/going-round-circles-earth-rotation...

https://www.britannica.com/science/latitude

Coriolis corrections are thrown into sniper ballistic calculations, too. Not a huge effect in most conditions, but not zero, and there have been a lot of long shots in the past two decades.

"Oi! Suzy!"

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

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.

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.

https://github.com/AsahiLinux/speakersafetyd

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.

This is true of all speakers

It's true of all dynamic speakers -- the sort with a voice coil and a magnet.

(But not all speakers are dynamic speakers.)

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

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.

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.

Of course.

Everything is a microphone if you're brave enough.

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).

But not true of all codecs…

Do you think Apple put a hidden microphone in their devices by pure accident?

Exactly.

Also used in electric motor controllers to monitor winding temperature.

Is that the same thing where a flat-earther tried to measure something with an expensive laser gyro and kept finding that Earth was rotating?

I think the most you can tell from an IMU or gyro is that there is a change in velocity in a direction aligning with East-West when there is a change in location and that the change in velocity is greater when the location changes in line with North-South. The change in velocity would be greater as one approaches the poles and lesser at the equator.

Thought experiment: if I zeroed my IMU at the North pole and traveled in a straight line away from the pole along longitude zero, following the guidance of the IMU. By the time I got to 45° latitude I’d be traveling Westward at 1,180 kph (.95 Mach) to keep the IMU at zero.

The flat earther used a fibre optic gyro. You don't "zero" it, it continuously outputs a measurement of its own angular rate around it's sensitive axis. For a 3-axis gyro placed still on earth, it will read about 15 degree/hour around wherever the axis of earth is oriented.

I believe this is one of the initial steps an aircraft INS uses to find north while it is aligning, but it's been too long since I had aircraft systems theory in the front of my brain.

Yes, from earth rotation the INS could figure out true north if the latitude is known. Or figure out the latitude if current heading is known. But normally it's aligned with a starting position from pilot input or GPS.

As I recall, it's a combination of things all happening at once, to include using last known location at shutdown, GPS inputs (if available) and inertial inputs.

There were various workarounds you had to use in the Navy, where the ship has not only usually moved from the last place the jet thought it was, but it's generally moving as the jet tries to re-align itself. GPS made things easier, and there wasn't a whole lot of thinking involved, but it always took the jet a bit more (or a lot more depending on model) thinking to get a stable platform underway.

If you are at an airport you will sometimes notice large signs giving Longitude and Latitude of the individual stands at an airport. These are used to give the initial position to the INS via the FMS. Of course these are now all built into the database these days so are only used (if at all) for gross error checking.

https://en.m.wikipedia.org/wiki/Inertial_measurement_unit

Latitude or Longitude?

Do you mean latitude?

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