| ▲ | entropicdrifter 3 days ago |
| It does act as a thermometer, if and only if the altitude remains constant. The speed of sound fluctuates with both temperature and altitude |
|
| ▲ | amluto 3 days ago | parent | next [-] |
| 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 |
| |
| ▲ | KennyBlanken 3 days ago | parent | next [-] | | 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. | | |
| ▲ | amluto 3 days ago | parent [-] | | > 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.) |
| |
| ▲ | pants2 3 days ago | parent | prev | next [-] | | 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! | |
| ▲ | adolph 3 days ago | parent | prev [-] | | Can an ideal gas of same volume, mass and temperature be brought to different pressures? https://courses.lumenlearning.com/suny-physics/chapter/13-3-... | | |
| ▲ | amluto 3 days ago | parent [-] | | 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. |
|
|
|
| ▲ | _0ffh 3 days ago | parent | prev | next [-] |
| 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. |
| |
| ▲ | KeplerBoy 3 days ago | parent [-] | | one might say air pressure changes constantly as we speak. | | |
| ▲ | adammarples 3 days ago | parent [-] | | Isn't air pressure the only thing that microphones actually measure? | | |
| ▲ | KeplerBoy 3 days ago | parent | next [-] | | 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. | | |
| ▲ | jpc0 3 days ago | parent [-] | | 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 | | |
| ▲ | KeplerBoy 3 days ago | parent | next [-] | | 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 | | |
| ▲ | jpc0 3 days ago | parent [-] | | 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. |
| |
| ▲ | sanderjd 3 days ago | parent | prev [-] | | 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. |
|
| |
| ▲ | sojsurf 3 days ago | parent | prev | next [-] | | Air pressure differentials, to be precise! | |
| ▲ | immibis 3 days ago | parent | prev [-] | | Many types measure the derivative of air pressure. One that measures absolute air pressure can be used for calibration. |
|
|
|
|
| ▲ | t0mas88 3 days ago | parent | prev [-] |
| The speed of sound fluctuates with density. Altitude and temperature both change density. |
