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I am familar with current reactor (experimental) fusion reactor design.

But imagine a fusion reaction happening without a reactor, so there is no reactor hull to shield the sound from an innocent bystander.

It is a small reaction and it is continuus and controlled. (so not all reactive matter is consumed at once)

  • Would it make a sound at all?
  • If yes, what would that be?

In Sci-Fi shows and the like you will very often here humming and zapping sounds and so on, resembling electricity. Would that be the case?

Please do not consider how realistic such an occurence would be or what happens to the energy 'gained'. I have taken care of that part already :)

Additional Information in reaction to current answers:

The device I am talking about is a sort of beam emitter.

Input: Matter in form of small metal spheres.
Output: a "energy beam" of variable width and intensity.

The most plausible reaction to explain that (magical) behaviour is a fusion reaction, at least that's what I am thinking. Am I right?

I know you cannot get that (presumably) if you want to keep things realistic. But thats not what I am aiming for.

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  • $\begingroup$ Should be tagged onomatopoeia? $\endgroup$ – Spencer Mar 16 '17 at 2:48
  • $\begingroup$ I think the "electric sound" is pretty close. Both electric discharges and fusion reactions would make their noise by ionizing air with high energy particles so the sounds should be similar. The differences would come from the shape and distribution of ionization. ie Electric discharges tend to create long and narrow high density channels, while your magic fusion might create a diffuse ball shape. So the "electric sounds" used in movies are probably wrong. A diffuse spherical ionization might be nearly silent, making noise only at start and collapse. $\endgroup$ – Ville Niemi Mar 16 '17 at 3:10
  • $\begingroup$ For that matter the "electric sounds" tend to come from creation and collapse of discharge channels, which your fusion might or might not have depending on how it works. $\endgroup$ – Ville Niemi Mar 16 '17 at 3:13
  • $\begingroup$ well when you get right down to it it is still a steam engine at its core so a good turbine hum would be good. this could also come from the coolant pumps. $\endgroup$ – John Mar 16 '17 at 14:02
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    $\begingroup$ It turns out that you can simply find the sound up on youtube. youtube.com/watch?v=c2Pi4gYIlE4 $\endgroup$ – Ezra Mar 16 '17 at 21:14
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How close have you ever stood to an operational nuclear reactor? I've been within about 20 feet of at least two different ones. It sounds like boring, heavy piles of shielding and maybe you sort of hear the coolant pumps through the hull. If you want to get into the physics, I can do that, but to keep it light hearted, the saying is 'hot rock make boat go'. Fission reactions literally come down to a fancy metal plate that gets hot when you put it close enough to other fancy metal plates at just exactly the right geometry.

Fusion doesn't just happen naturally anywhere except in a star, but that's pretty much just because nature didn't find a better way on its own. We have the opportunity to shove hydrogen into a magnetic bottle and pummel it with particle beams until it gives us its lunch money. As long as the hydrogen is close enough to itself when it goes kerflooey, it could keep going kerflooey. There's some really boring math here, but the jist of it is that the reaction has to add enough energy to the system to make it cause another reaction.

This is easier if the reaction is statistically likely to create a neutron, because making atoms beat each other up is super way easier than actually going down there to beat them up yourself. But you won't usually get that from slamming two hydrogens into each other, because there isn't a neutron there to spare. It would actually need extra neutrons just to fuse into stable helium.

But that's boring. The point is, this gets nigh impossible if there is anything else there for the neutrons and heliums and whatsits to bounce off of. If you did this in free air just because YOLO, you start losing energy to oxygen going through it's 'goth phase' as a highly unstable neon, and those little bastard nitrogens suck up a bunch of thermal energy and go join the circus with water as nitric acid (as just one possible outcome).

Even if you could contain the radiation and the reaction, you would take steps to optimize this process, which means the reaction is going to take place in a relative vacuum. As a result the reaction itself is going to either be soundless, or imperceptible to human hearing.

All that being said, it's probably going to sound like boring piles of shielding and a thrumming, humming magnetic bottle, as well as whatever other technology you have in place around it to make it go kerflooey without boiling people and scorching eyebrows.

Edit after the edit: But alas, we WANT to boil people and scorch their eyebrows, and do it in a very specific direction.

I stand by everything I have said thus far. The reactions themselves are going to be completely soundless, assuming that they are fusion reactions optimized for energy generation. The hot soup in a pulsing tokamak literally can't touch anything in order to pass on sound, but its magnetic containment can complain very, very loudly.

For a weapon with access to significantly advanced technology or magic, you can actually solve all the problems we have with actual fusion. Assuming you have sufficient energy to pay the cost, you could actually ultra-super-mega heat a completely contained sample of air, squish it to pressures totally obscene, and cut it loose in a fashion that guarantees blazing devastation. The little metal spheres you posit in your question could be bits of lithium, which could work basically the same way - make it stupid hot, squeeze it, point it, pop it.

(Writers note: if you can do this, energy in your world either is, or should be, free, and the difference between 'is' and 'should be' can make for an interesting problem to explore.)

If your question is NOT about the reaction, but about the beam emanating from a process like this, there isn't a lot of macro scale difference between this beam and a bolt of lightning except the method of delivery. They are both essentially going to superheat the air they pass through, leaving a relative vacuum in their wake which will lead to a clap. The beam itself could in theory be sustained beyond a pulse-bolt, which I would expect to not sound like much right out of the gate, but I expect when/if it became diffuse and incoherent, it would sound a lot like Satan playing with his blowtorch.

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    $\begingroup$ This. I've been on top on working nuclear reactor too. Inside reactor's chamber. All that was really heard was people talking physics and security ;) $\endgroup$ – Mołot Mar 16 '17 at 13:58
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    $\begingroup$ Humans. shoving hydrogen atoms into reactors just to take their lunch money. Goth phase O2. N2 joining the circus. This is the best SE reply I've ever read. $\endgroup$ – CaM Mar 16 '17 at 18:00
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    $\begingroup$ The point that the reaction is taking place in a vacuum seems very important. This means the sound you hear is not going to actually be the sound of fusion, but the sound of all of the hardware keeping the fusion alive. That's convenient from a story point of view because we're really good at making wizzbangy mechanical noises! $\endgroup$ – Cort Ammon Mar 16 '17 at 18:36
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    $\begingroup$ A magnetic containment is probably the nearest thing to the original question's unrevealed method of containment. And you wouldn't hear anything from within it, correct? I take it you mean the thrumming of the electrical equipment generating the magnetic flux. $\endgroup$ – can-ned_food Mar 16 '17 at 20:35
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    $\begingroup$ Downvoted since this completely failed to answer the question, and totally ignored the OPs statement that there is no shielding. $\endgroup$ – Innovine Mar 16 '17 at 21:22
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So you're firing a fusion type reaction in a beam?

Let's ignore the science and use some creative use of the 'known knowns'.

A fusion reaction, such as the ITER Tokamak project, involves isolating a plasma in a magnetic field. Now you've specifically requested you remove the 'housing', which would then remove the magnetic field... which leaves us with plasma.

Plasma itself is just matter in the '4th' state... beyond turning into a 'gas', matter turns into a plasma when it reaches a high enough energy state to shed it's electrons and therefore it ionizes. The device or weapon you need would likely absorb, utilize or otherwise discharge the electrons as 'waste' and would be firing the ionized material as the projectile.

So perhaps you don't really require 'fusion', as such, but a device that perhaps uses fusion to heat other pieces of matter (such as the metal spheres (although it doesn't have to be metal)) to incredibly high temperatures and then shoots them in a particular direction.

Now the biggest problem with plasma is that it will cool down over distances. Which makes me think you'll likely be heating it to incredibly high temperatures, perhaps temperatures that are inconceivable in nature (and why not? there is no known 'maximum' temperature.) This would increase the longevity of your ionized plasma beam, allowing it to travel greater distances.

So, now to the crux of your question. What would this sound like?

Well, now we just have to imagine what a burning stream of super-heated matter sounds like as it rips it's way through the atmosphere. I think a great analogy for this in nature is the noise lightning makes just before it 'cracks'... that very first initial opening part of the sound... but drawn out for a long time. When the beam passes by or is switched off, then you would hear the final 'boom' part of the lightning sound, as it cracks and echoes as the air rushes in the fill the hole left behind.

Even more interesting is if you propel the beam at speeds faster than sound, creating sonic booms along it's path :D

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It is hard to answer the question while ignoring the energy gained issue, as some of that gained energy will likely express itself as vibrations which human ears would interpret as sound. Beyond this small portion of the yielded energy which directly becomes sound, most of the rest would interact with surrounding matter in ways that could indirectly create large volumes of additional noise.

If by "I have taken care of that part already", you mean that all of the energy being released by the fusion is being funneled off for some other use, then the noise generated by the reaction would either be a side effect of that funneling or the result of inefficiencies in that funneling process.

Postulations on what those funneling sounds might sound like, would require more information on how the funneling occurs and in what form the energy is funneled away.

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I imagine it'd sound a lot like the sun or a star would, but it wouldn't necessarily be extremely loud.

Most sound comes not from the fusion reaction, the fission reactions, burning coal, or whatever the source of energy is, but the equipment that is being used to gather the power being produced. And the sounds those make vary widely depending on application. A coal-fired power plant is going to make a lot of noise because there's no need to spend the extra effort in making it quiet, while a ballistic missile submarine is designed to be almost perfectly silent when it has to, while still generating plenty of power to do its job.

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I would challenge your continuous operation specification. Many things that we humans think of as continuous are in fact just oscillating really really fast. You think that light is "on" when it is actually fluctuating 50-60 times per second.

Your fusion reaction is likely to be operating like the light, fluctuating cyclically very quickly and only appears to be continuous to a human.

When things cycle quickly like this they tend to make a sound, anywhere from a low hum to a very high pitched whine depending on the frequency. These sounds are very commonly associated with electrical equipment which commonly cycle in similar ways. This is what your fusion reaction would sound like.

It is of course entirely possible that this sound would be swamped by the sounds of associated equipment in the area. You couldn't hear the sound of the reaction over the sound of the giant coolant pumps, the sparking of power supplies, or the blast of the ventilation system.

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Probably the same sound as a bare fission reaction would make.

You only hear that which is registered by the temporal lobes in the brain. Usually those are receiving input from the auditory nerves, which are usually only excited when the tympanic membrane is vibrated by sound waves carried across molecules.

So, in the roughest manner, sound is perceived when longitudinal waves of energy travel through media — such as the air, the ground, your body, and the like — and vibrate your eardrums.

Nuclear fusion occurs when the distinct nuclei of two or more atoms approach near enough that it becomes easier for some of their neutrons and protons to fall into mutual nuclear “orbitals”. ‘Orbitals’ are, of course, a slightly antiquated name for the set of energy states which are considered to be stable for a certain set of conditions: why electrons stay near an atom, and why the nucleus of that same atom stays intact.

There are a few, and not too many, ways we know this could happen. In particle colliders at comparatively low energies, it is possible to bombard heavy nuclei with $\alpha$ particles, a.k.a. Helium nuclei, so as to synthesize heavier elements. This is how elements like Darmstadtium (a.k.a Ununnilium et al), for example, have been produced.
Lighter, smaller nuclei are less likely to collide at lower energies, so to make them stick you need an adequately dense and energetic — hot — plasma, where many of them are bouncing around.

If the energies of a stellar plasma get too dense, of course, then the nucleus ceases to exist, per se — but that's not relevant. (Neutron star.)
Then, of course, to perform a nuclear fusion at colder energies would require some other means of relaxing the repulsions between the electron shells of atoms — but you didn't ask about that.

When such fusion occurs, the energy is released in several forms:

  • One of them is simply an imparting of additional kinetic energy to the resultant nuclei.
  • A few neutrinos are emitted. These are largely hypothetical, as detecting them is difficult and indirect, but thus far the predicted characteristics haven't been counterindicated. They don't interact with much.
  • $\gamma$ radiation — electromagnetic photons with a very short wavelengths.
  • free neutrons, protons, or $\alpha$ particles a.k.a. Helium nuclei.

What manner of sound waves would these produce? Well, I suppose you could always see if the gamma rays, incident on the neurons in the brain or auditory nerve, would excite in a way that would either make them more likely (agonism) or less likely (antagonism) to fire off their neurotransmitters. Gamma radiation has a very small wavelength — well, what we call a wavelength — and isn't likely to interact with matter except on the nuclear scale. Mutogenic damage, sure. Perception of sound? The Hum, perhaps?
Not so likely, so far as I would expect.

Most of the sound would come from the resultant heat and thermal activity of the air. In rawest form, it would probably sound like an explosion: a rush of expanding gases.
However, depending on the nature of your containment, you would probably either hear nothing or you'd hear whatever passed through the containment. You don't explain enough about that for me to make accurate assessments.

Now, because you say that the reactions are contained and fed at a controlled rate, the only thing which you haven't adequately explained is the nature of the output beam.
Is it a beam of $\gamma$ radiation? A stream of $\alpha$ particles (Helium-4 nuclei)? Most of the sound from your reactor would probably come from this.

In summary,

The size of the atomic nucleus with respect to the atom itself is quite remarkable. It has been compared to the distance between the sun and Pluto — i.e. the orbit of Pluto is the atom, and the sun is the nucleus. Tiny.
When nuclear fusion occurs, most of the emitted products are far too small to be sensed directly, more or less, by us. The only thing you could hear would be that which agitated the motion on the atomic or molecular scale.
Ergo: the heat of the fusion.

Note that I am ignoring the concommitant operational sounds coming from any equipment which are used nearby, or from the containment of the plasma itself, as your question doesn't ask for such things.

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  • $\begingroup$ @SeanBoddy Admittedly, yes I did swipe that off the Wikipedia page. It's all hypothetical, isn't it? I mean, it's not like astrophysicists have actually probed the internals of a star — sheesh. Anyways, I was trying to say that any sound wouldn't come from the fusion itself: But, now that you mention it, I can see that writing out the nuclear reactions isn't necessary to the answer. $\endgroup$ – can-ned_food Mar 17 '17 at 18:07
  • $\begingroup$ Don't forget that he clarified that the device in question is a beam emitter that appears to be a weapon. If you're still available for this, I know I'd like to hear your take on that. $\endgroup$ – Sean Boddy Mar 18 '17 at 3:54

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