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Sound can't travel through outer space. But if it could, how loud would the Sun be? Would the sound be dangerous to life on Earth, or would we barely hear it from this distance?

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    $\begingroup$ Nice question. Something I would never have wondered $\endgroup$ – Rimian Dec 15 '15 at 7:52
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    $\begingroup$ Sound can travel through outer space. $\endgroup$ – Rob Jeffries Dec 15 '15 at 8:22
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    $\begingroup$ @RobJeffries But not at frequencies that make us deaf. $\endgroup$ – gerrit Dec 15 '15 at 14:11
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    $\begingroup$ And indeed nothing is producing sounds at frequencies we can hear. $\endgroup$ – Rob Jeffries Dec 15 '15 at 15:18
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    $\begingroup$ Wow, nice question! I remember having dreams where I could hear the Sun. $\endgroup$ – noncom Dec 16 '15 at 12:22
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The Sun is immensely loud. The surface generates thousands to tens of thousands of watts of sound power for every square meter. That's something like 10x to 100x the power flux through the speakers at a rock concert, or out the front of a police siren. Except the "speaker surface" in this case is the entire surface of the Sun, some 10,000 times larger than the surface area of Earth.

Despite what "user10094" said, we do in fact know what the Sun "sounds" like -- instruments like SDO's HMI or SOHO's MDI or the ground-based GONG observatory measure the Doppler shift everywhere on the visible surface of the Sun, and we can actually see sound waves (well, infrasound waves) resonating in the Sun as a whole! Pretty cool, eh? Since the Sun is large, the sound waves resonate at very deep frequencies -- typical resonant modes have 5 minute periods, and there are about a million of them going all at once.

The resonant modes in the Sun are excited by something. That something is the tremendous broadband rushing of convective turbulence. Heat gets brought to the surface of the Sun by convection -- hot material rises through the outer layers, reaches the surface, cools off (by radiating sunlight), and sinks. The "typical" convection cell is about the size of Texas, and is called a "granule" because they look like little grains when viewed through a telescope. Each one (the size of Texas, remember) rises, disperses its light, and sinks in five minutes. That produces a heck of a racket. There are something like 10 million of those all over the surface of the Sun at any one time. Most of that sound energy just gets reflected right back down into the Sun, but some of it gets out into the solar chromosphere and corona. No one can be sure, yet, just how much of that sound energy gets out, but it's most likely between about 30 and about 300 watts per square meter of surface, on average. The uncertainty comes because the surface dynamics of the Sun are tricky. In the deep interior, we can pretend the solar magnetic field doesn't affect the physics much and use hydrodynamics, and in the exterior (corona) we can pretend the gas itself doesn't affect the physics much. At the boundary layers above the visible surface, neither approximation applies and the physics gets too tricky to be tractable (yet).

In terms of dBA, if all that leaked sound could somehow propagate to Earth, well let's see... Sunlight at Earth is attenuated about 10,000 times by distance (i.e. it's 10,000 times brighter at the surface of the Sun), so if 200 W/m2 of sound at the Sun could somehow propagate out to Earth it would yield a sound intensity of about 20 mW/m2. 0dB is about 1pW/m2 , so that's about 100dB. At Earth, some 150,000,000 kilometers from the sound source. Good thing sound doesn't travel through space, eh?

The good folks at the SOHO/MDI project created some sound files of resonant solar oscillations by speeding up the data from their instrument by 43,000 times. You can hear those here, at the Solar Center website. Someone else did the same thing with the SDO/HMI instrument, and superposed the sounds on first-light videos from SDO. Both of those sounds, which sound sort of like rubber bands twanging, are heavily filtered from the data -- a particular resonant spatial mode (shape of a resonant sound) is being extracted from the data, and so you hear mainly that particular resonant mode. The actual unfiltered sound is far more cacophonous, and to the ear would sound less like a resonant sound and more like noise.

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    $\begingroup$ What if we consider space filled with Earth-like air instead of attenuating sound as if it were light? I think that would be more in-spirit with OP's question :-) $\endgroup$ – Andrew Cheong Dec 15 '15 at 6:07
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    $\begingroup$ +1 for a quantitative answer. A fair fraction of the acoustic waves are probably used to heat the chromosphere. Do you have a reference for the 30-300 W per square metre? $\endgroup$ – Rob Jeffries Dec 15 '15 at 9:19
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    $\begingroup$ I would like to know the loudness in audible frequencies. Are there any measurements about it or can we only estimate the loudness of the sounds caused by turbulences (like sound of wind on Earth - but is it created without land?). $\endgroup$ – BartekChom Dec 15 '15 at 16:01
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    $\begingroup$ @AndrewCheong It's difficult to answer, because you have to choose how much physics to throw away when you answer a counterfactual. However, 3 minute or 5 minute or 20 minute waves would form shocks and/or dissipate as heat long before they reached Earth, if they had to travel through 1 AU of air. Also, if the Solar System was filled with that much air, it wouldn't last long. It would fall into the Sun pretty fast, and the Sun itself would get a lot brighter and a lot heavier. It might (given the composition of air) even immediately burst into its red giant phase and engulf the Earth. $\endgroup$ – Sir Cumference Dec 15 '15 at 21:28
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    $\begingroup$ @user2813274 Well, the Sun as a whole doesn't resonate at higher frequencies than about 5 minute period (3mHz). The chromospheric layer (just above the visible surface or photosphere) resonates at about 3 minute period (5mHz). That doesn't mean there isn't sound at higher frequencies, just that it isn't resonant with a well-defined frequency. The photosphere could in principle support audible frequency sounds, but we have no way to detect them at this time. The layers above the photosphere can't, simply because the gas there is too tenuous. $\endgroup$ – Sir Cumference Dec 15 '15 at 21:42
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While Sir Cumference's post is a very intriguing answer, but I'm afraid it's wrong. The sun's surface is clearly in motion, but that does not necessarily result in the radiation of audible sound, even if the sun and earth where in a fluid medium (such as a air) that would allow sound transfer.

To explain why, we can actually apply the same line of analysis to the earth's ocean. The surface moves a lot, so sound should be radiated. However, we hear nothing unless you are really close by and have breaking waves.

Let's run the math with rough numbers: The ocean has a surface area of about 510 million square kilometers. $150 \cdot 10^{12} m^2$. Let's say the average wave height is 1m and the average wave frequency is 0.1 Hz (1 wave every 10 s). If the ocean were a spherical source this would create a sound power of $5 \cdot 10^{24} W$ and the sound pressure at 1000 km away would be 240 dB SPL. That's obviously not the case, otherwise we'd all be dead.

So why not? In order for sound to actually radiate, the surface must move uniformly. For every ocean wave that moves air up there is a wave nearby that moves air down and so the contributions simply cancel. Technically speaking, we need to calculate the power by integrating the normal intensity over the entire surface, the intensity has equal amounts of positive and negative components and the sum over those is zero.

That's the same reason why you put a loudspeaker in a box: in open air, the air motion from the front of the cone and from the rear of the cone will simply cancel out, so you put it in a box to get rid of the sound from the rear.

So I think the real answer here is: you would hear absolutely nothing since the sound contributions from different parts of the sun's surface would cancel each other out. Sound radiation over that distance would only occur if the sun's surface moves uniformly, i.e. the whole sun expands or contracts. That does happen to some degree but only at very, very low frequencies which are inaudible and where sound radiation is a lot less efficient.

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  • $\begingroup$ Sir Cumference's answer says that "we can actually see sound waves (well, infrasound waves) resonating in the Sun as a whole". But you can't see such infrasound waves resonating in the ocean, so something is different in the Sun. $\endgroup$ – JiK Dec 16 '15 at 13:05
  • $\begingroup$ Off course you can see infra sound waves from the ocean. The tides is a good example. You still can't hear them. Same reasoning applies though: Very, very low frequency changes the energy calculation drastically and also makes it inaudible, $\endgroup$ – Hilmar Dec 16 '15 at 14:50
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    $\begingroup$ so what's the bottom line here - will DJ's of the future be able to include samples of the Sun in their music or not $\endgroup$ – coburne Dec 16 '15 at 16:21

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