Scientists Watch as Heat Moves at the Speed of Sound | Hacker News Reader

Scientists Watch as Heat Moves at the Speed of Sound

scientificamerican.com

94 pointspseudolus32 comments

This is so strange. Just 2 days ago I realised that although excessive sound production is usually a symptom of a machine that is not working properly, if a machine is designed to produce sound to dissipate energy, that energy would not be converted into heat within the machine. The energy is carried away at the speed of sound.

I have been thinking about using a similar process to make a "middle of the room" air conditioner that creates a localised cold spot, and sends the heat absorbed out of the room through the walls and into the surrounding atmosphere using low frequency pressure waves.

All of these processes are likely related, and macroscopic systems based on these principles almost seem magical.

I'm curious if the numbers work out to yield you anything within the galaxy of useful.

Yeah, that's the actual question and probably a reason why it's not an established technique

Also, low-frequency waves naturally are lower energy. Why not bury the output and increase the frequency?

It would have the added benefit of supposedly keeping away ground-dwelling pests; supposedly.

>> low-frequency waves naturally are lower energy

Just to be clear here: are they really? Sound waves have an amplitude which also defines the amount of energy they contain.

Yes ... a sinusoidal sound wave has an RMS amplitude based on the peak amplitude regardless of the frequency.

Sound waves also have a maximum amplitude (without distortion) because the minimum of the pressure wave can't be below atmospheric pressure. Although Wikipedia indicates that maximum is quite loud.

For a sine wave, the energy scales like

    E ∝α ω^2

where α is amplitude and ω is frequency. So lower frequencies need propotionally larger amplitudes to carry the same energy.

I recall reading an article about thermoacoustic cooling years ago, but couldn't find anything more current than this old wired article [1]. More recent articles about Ben and Jerry's refrigerators have been about changing refrigerants, so I suspect the research didn't work out.

Also, sort of sound and heat related, piezo fans seem to be a niche way to move air around without a spinning blade.

[1] https://www.wired.com/2003/01/hear-that-the-fridge-is-chilli...

It wouldn't be possible because you need a temperature gradient. Creating sound from plain heat is the wrong direction for entropy.

For the idea to work for cooling, he would need to expand the air in a different location than it was compressed in. (I wonder if standing waves exhibit this effect?) IF it works out that this is possible, I imagine it would be like standing in front of a CO2 fire extinguisher (gets cold as pressure is released).

This is essentially what the pressure waves would be... mild compression waves would travel outward as you pull a cylinder creating a vacuum. You've compressed the entire atmosphere slightly, then you let some air expand back in through a small nozzle to fill the cylinder again.

Then you open a port and push the air back out without changing it's volume, and close the port to complete the cycle...

It seems like there's got to be something wrong with the concept, but I can't put my finger on what exactly.

Maybe relevant to your nutty idea: Heat is just motion on a small scale. In solid state materials, just like there are quantized configurations of electrons, there are also quantized modes of motion. These are called "phonons". (Analagous to electron / hole model of solid state electronic structure). (It's also true for single molecules, and this is what low energy IR spectroscopy probes. But less relevant for bulk behavior in a liquid or gas state). If the material is anisotropic, probably there will be anisotropic phonons too. The existence / density of thermal modes at IR energies can also determine how much heat a material radiates into the environment versus conducts through its bulk.

SO, just like materials can have an electronic bandgap, materials can have a thermal bandgap too!!! But I can no longer envision what a thermal transistor would behave like. And I'm not sure how this relates to the electronically tuned thermal materials which show you when you google "theal transistor"

There is a company in the netherlands that uses excessive heat with a similar idea to create air conditioning (SoundEnergy BV[1]). Unfortunately I can't really find a good English article explaining the principle but it's like a sterling engine without moving parts and without pistons.

[1] https://www.soundenergy.nl

As I understood the article, in one part of the experiment they set up alternate regions of heat and cold within a sample of graphite using interference patterns from two laser sources. When they turned off the lasers, instead of the hot regions dispersing their energy until the troughs were the same temperature, the hot regions "overshot", becoming cooler than the (former) troughs, with wave-like behaviour.

This seems to me like energy is momentarily flowing from a cooler region (the former "peaks") to a hotter area (the former "troughs").

Would a Real Physicist be kind enough to explain to me why this doesn't violate the 2nd law? Probably the answer is "you misunderstood".

Not a "Real Physicist" but the information is not being lost, it's only being kept static-ish. This is the same as that waves don't violate the 2nd law (it's telling that they being called the phenomenon second sound.)

> The second law of thermodynamics states that the total entropy of an isolated system can never decrease over time.

So instead of entropy increasing, it's hanging around for a while. No violation, still unusual though.

Doesn't seem as if total entropy is increasing, simply rebalancing to the other side (which is still kind of interesting, like a wave in a pool).

Yep, and energy is lost in the rebalancing process, and it oscillates until equilibrium, just like a pendulum.

Heat often correlates with entropy, but they are not the same thing. Increasing heat doesn't always mean increasing entropy.

I'd highly recommend Arieh Ben-Naim's books on entropy for anyone looking for more on this point.

TLDR he makes the argument that entropy is a measure of the probability of probability distributions.

Sounds like this would explain the Mpemba effect:

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

The experiment describes an unusual process observed in a system driven out of equilibrium (i.e., non-equilibrium), where, strictly speaking, the 2nd law does not apply.

So the effect was known for a long time, and the new development is that it was now observed at higher temperatures. But 120 K is still pretty cold, so it doesn't really matter much for practical applications.

Depending on the concentration of defects and the length scale of the system, theory suggests that it may be possible to observe second sound at much higher temperatures beyond 120 K.

Here is the arxiv version of the Science paper mentioned in the article: https://arxiv.org/abs/1901.09160.

This is the theory of phonons: pseudo particles which transfer heat in materials.

Asking because it's an interesting phenomenon and I don't know enough to understand the question I'm putting forth:

If conductive heat is radiated through quanta, phonons[0], why would it be limited to the speed of sound? Is there a limitation to waveforms in matter that limits the speed at which it travels or is it purely dependent on the energy "type"? (Thinking of light travelling in waveform, when I posit this, which is the reason the ridiculous ask.)

[0] - https://en.wikipedia.org/wiki/Phonon#Acoustic_and_optical_ph...

Long wavelength phonons are the familiar sound waves. Smaller wavelength phonons have different group velocities due to dispersion.

The speed of second sound is not the same speed of first sound. The speed of second sound is essentially determined by a weighted contribution of the different of speeds of the different phonon modes that collectively participate to produce a temperature wave.

The effect appears at 120 Kelvin, or -240°F, so the article's assertion that this could revolutionize microelectronics seems like a stretch

Right, and the article says you wouldn't need to do it at "cryogenic" temperatures. I thought this was funny: "Nobody ever thought that you would actually be able to do this at such high temperatures."

-150℃ is almost halfway up to ambient temperature though, so it halves the energy requirement.

A quick first impression question: how important is the laser interference? Could you do this with a multi-centimeter graphite rod, by heating one end?

Laser interference is not required. The original second sound measurements in superfluid Helium used a geometry similar to the one you describe, where a heat source (as a pulse or periodic in time) is positioned at one end and the temperature is recorded at some point(s) along the sample.

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