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Comment by frankus | Hacker News Reader

"As part of the calibration, the speed of sound is also a parameter which is optimized to obtain the best model of the system, which allows this whole procedure to act as a ridiculously overengineered thermometer."

Reminds me of the electronics adage: "all sensors are temperature sensors, some measure other things as well."

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Back in high school, I built (with some parental assistance) an apparatus to measure how quickly the pressure would drop (in a pressurized cylinder) when a very small hole allowed air to leak out.

Turns out, not only can you measure temperature that way, but can extrapolate the graph out to find absolute zero (IIRC my result was out by about 20 kelvin, which I think is pretty damn good for a high-school-garage project).

No, there you are measuring R, assuming the air inside the cylinder was an ideal gas.

>>>> Reminds me of the electronics adage: "all sensors are temperature sensors, some measure other things as well."

A corollary that's one of my rules to live by: Never measure anything over time without also measuring the ambient temperature.

Not really related, but the way you chose to indicate a comment here immediately made me think you had merge conflicts.

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.

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

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!"

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.

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

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

Exactly.

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.

But not true of all codecs…

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

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).

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.

Also used in electric motor controllers to monitor winding temperature.

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.

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.

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.

Latitude or Longitude?

Do you mean latitude?

Is there one saying “All electronic devices are smoke machines, some can compute too”?

"All diodes are light-emitting if you try hard enough"

"All diodes are light-emitting at least once"

Hahaha yea

I've seen that in electronics lab a few times. The "temporarily light emitting diode"

I have a Temporarily light-emitting harddrive cable. Really old 40 MB hdd connected to an old computer with a cheap power supply that most likely couldn't handle the slightly lower than standard power in a friends house.

All diodes are also light SENSING is you try hard enough.

You don't have to try hard. Just use it as a photodiode and it magically works. However, if it's inside a plastic case that blocks light, it doesn't.

Due to some law about entropy, efficient processes are necessarily reversible. That's why electric motors - some of the most efficient machines ever invented - are also generators.

> However, if it's inside a plastic case that blocks light, it doesn't.

You want an ordinary diode to allow current to flow easily when it senses light? Simple: shine a powerful laser at the plastic-encased diode and it will melt the plastic and liquify the metal, fusing it together and allowing current to flow again. See? You just needed to try harder.

Or if the hammer don't work, the sledgehammer is over there.

All diodes are photodiodes, one has to be esp careful of glass encapsulated diodes. I have had that bite me before.

Especially true for LEDs, tried that in the lab once with a flood light, got a few μA out of the LED shortened with the multimeter. Did that with 8th graders, we did other experiments mainly with pv, LEDs and bipolar transistors as well.

The logical question came up more than once: “can we use photovoltaic cells as a light?“. Pretty sure that‘ll work, too, but didn’t try because stuff was expensive then and we didn’t have any broken parts of cells at the time. They probably learned a few things on that day.

Steve Mould of Youtube fame did this:

Why all solar panels are secretly LEDs (and all LEDs are secretly solar panels) - https://www.youtube.com/watch?v=6WGKz2sUa0w

I did try to do that in 8th grade, it worked for a bit but it was quite dim and uneven.

Ah, the light emitting resistor. The moment when you realize why it's called Ohm's Law.

Similarly, diesel engines come with a reserve fuel supply that you can accidentally use once. (diesel engines will happily run on engine oil when warm)

This happened to me once in a Peugeot 306 2L turbo diesel.

Over filled it and kinda had to do one 1600m trip.

Fortunately it was manual so I was able to stall it fairly swiftly in third gear with my foot on the break.

Didn't seem to have any impact on the engine as far as normal operating and how it sounded. I didn't do any internal inspection.

"All electronics are hand-warmers if miscalibrated correctly enough."

"Inside every amplifier is an oscillator trying to get out."

The one I've heard is "Every machine is a smoke machine, if you operate it wrongly enough."

I just learned how the Duracell Powercheck© worked, which was done with temperature.

https://youtu.be/zsA3X40nz9w?si=oGg2wdUlLXSDxpsN

a colleague of mine spent months analysing fluctuations in narrow band signal from a geophone only for a more senior colleague to get fed up with it and demonstrated that actually the fluctuations simply correlate with the air temperature and do so within the spec sheets reported temperature tolerance.

> Reminds me of the electronics adage: "all sensors are temperature sensors, some measure other things as well."

I wanna say that’s a Bob Pease quote but I can’t find an attribution to it.

I first encountered it in Elecia White's book Making Embedded Systems, but the attribution is anonymous and whom it's attributed to may have heard it elsewhere.

The highest grade gauge blocks use laser interferometry from Mitutoyo have a measured coefficient of thermal expansion AND a uncertainty of that coefficient. And they have a size variance of plus or minus 30nm. That is only about 410 oxygen atoms.

Oh yeah. I realised this the day I discovered my fancy digital SLR was a thermometer: https://entropicthoughts.com/does-my-dslr-have-dead-pixels

A lot of people like myself consider heat a form of light but I guess a photographer would be just thinking visible light. They say that about 50% of the sun's light emissions comes in the infrared frequencies.

That seems like a mistake since heat can transfer e.g. via contact without any electromagnetic emission. In fact, that is what I think happens with the sensor also, given that there is an IR filter in front of it.

But I may misunderstand your comment.

Visible light from the sun, or incandescent lights, is also emitted due to heat. Planck’s law is the mathematical relation if you want to know more.

Yup, it's called dark noise. Random generation of electrpns which sometimes find their way into the depletion region.

It does act as a thermometer, if and only if the altitude remains constant. The speed of sound fluctuates with both temperature and altitude

Right, it gets even worse: Air pressure in not only altitude-dependent but fluctuates even at constant altitude. The pressure (altitude) dependence is comparatively weak, though.

one might say air pressure changes constantly as we speak.

Isn't air pressure the only thing that microphones actually measure?

Air pressure differentials, to be precise!

By definition, sure. But one always needs some effect which changes some electrical property. We can't just hook up an ADC (analog digital converter) to thin air and hope for the best.

In practice most microphones measure the displacement of microscopic membranes, which are deformed by the air pressure. The next question then becomes how to measure microscopic movements of a tiny membrane. Turns out the membrane forms part of a capacitor and the electrical characteristics of capacitors depend on their geometry.

That is not necessary true.

There are at least 4 different types of microphones. Condenser which does in fact form part of a capacitor, dynamic which is effectively a linear generator (coil attached to membrane), ribbon which is a change in resistance as a small ribbon flexes and piezoelectric which is some black magic witg crystals

Sure, that's why I wrote most microphones.

There are also some exotic principles like laser or radar microphones using interferometry.

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

https://ieeexplore.ieee.org/document/7808865

I think popular is very situational though.

For me I see a lot more dynamic than condensers but I guess if you are talking about what is in like every single IOT thingamabob then you might be right there.

No, you're right. Most microphones and speakers ever made are dynamic. The speakers do function as microphones, by the way. The difference is simply whether you amplify the electricity generated by the vibrations moving the membrane (microphone) or if you send a pre-amplified signal to the copper coil around the magnet (speaker). They work on the same principle, so any dynamic driver can work as a microphone and any dynamic microphone could work as a driver, all just depending on the rest of the electronics they're hooked up to.

Fascinating. Is there a book about the history of microphones?

I find this to all be in the realm of "I don't believe you that any of this works at all" if I didn't have a lifetime of experience with the fruits of successfully-functioning microphones.

Many types measure the derivative of air pressure. One that measures absolute air pressure can be used for calibration.

I’m not sure how the speed of sound could depend on altitude, even in principle. The air doesn’t know where it is!

Putting that aside, in an ideal gas, the speed of sound depends on the composition of the gas and the temperature and, interestingly, does not depend on pressure, and pressure is the main way that the altitude would affect the speed of sound. So measuring the speed of sound in air actually makes for a pretty good thermometer.

https://en.wikipedia.org/wiki/Speed_of_sound

In liquids the speed of sound is related to the density, I would have thought similar for air but I see your point. Very insightful!

Can an ideal gas of same volume, mass and temperature be brought to different pressures?

https://courses.lumenlearning.com/suny-physics/chapter/13-3-...

Not unless you change the average mass of the molecules.

An ideal gas’ pressure is a function of number of particles per unit volume, its temperature, and nothing else. If you do anything involving adding or removing heat or changing the volume or pressure, you probably also need to know the specific heat at constant volume and the specific heat at constant pressure or, frequency, their ratio. That ratio is called the adiabatic index or the heat capacity ratio, it’s written as gamma, and it’s the last parameter in the speed of sound of an ideal gas. Interestingly, it doesn’t vary all that much between different gasses.

From your own link:

"The speed has a weak dependence on frequency and pressure in ordinary air, deviating slightly from ideal behavior."

"The speed of sound is raised by humidity. The difference between 0% and 100% humidity is about 1.5 m/s at standard pressure and temperature, but the size of the humidity effect increases dramatically with temperature."

"Slight" can matter significantly in an application like this.

> the size of the humidity effect increases dramatically with temperature.

This has little do with the behavior of sound. The fraction of the air that consists of water vapor at 100% relative is very small at cool temperatures and increases to 100% at 100 degrees C.

(Yes, water boils at the temperature at which air that is saturated with water vapor is all water vapor.)

The speed of sound fluctuates with density. Altitude and temperature both change density.

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