# What does air vanishing on contact sound like?

I'm about to break rule seven of superpowers: don't think too hard about what it does to the air.

A superhero has the ability to disappear any atoms that come within 2 cm of their skin (atoms that are already in that region do not disappear unless they exit and re-enter, so their clothing is fine if they stick to the standard tight-fitting superhero attire). More specifically, they pick any area(s) of their skin, and any atoms that enter the area 2 cm above it simply vanish. It also doesn't affect living things (dunno if anyone here reads Worm, but it's similar to the Manton effect from that).

This power has a lot of interesting implications. Effortlessly walking through walls, leaving a person-shaped hole! Vaporizing bullets as they hit them! Draining a room completely of air in a second! Leaving a vacuum in front of their fist so that they punch twice as hard! Heck, atmospheric pressure on the surface area of a person is hundreds of g's - they could fly if they could control it well enough!

...wait a second, those last few open up a Pandora's box of sorts. Realistically any time this power is used, it should create a region of total vacuum, implying that it produces a deafening blast.

So my question is: what does using this power sound like?

If they activate a small area of skin very briefly, how loud would it be? What does it sound like if they use the power continuously? (Assume for that last one that it's being used outside, so the answer isn't just "it doesn't sound like anything because they sucked out all the air.")

• I am a confused. Why does creating a vacuum imply a blast of sound? Especially since the result is a total vacuum. The only sound you would get is the sound of wind, AKA air passing by as it goes to the location where it dissapears. Sound also needs air to propagate, so you would quickly end up with a situation where there can be no sound. – Tyler S. Loeper Apr 29 at 18:06
• If activating the bottom of your hero's feet, they may want to avoid walking. Likewise, sitting in the case that they activate their posterior. Actually it's a great weakness, you just stand behind them and yell BOO so they loose their balance and fall into the center of the Earth... – user3067860 Apr 29 at 20:30
• So general comment applicable to all answers below: max speed of air blowing into a vacuum through a hole is the speed of sound, due to compressability of effects in the inrushing air causing choked flow. No supersonic flows or weird sound propagation issues. The speed of sound will rise as pressure decreases, but that will only happen in a sealed or confined space, cos the earth's atmosphere has Quite A Lot Of Air to get rid of before ambient pressure drops. – Starfish Prime Apr 29 at 20:52
• @user3067860 that might be a good way to hide from the effects of shutting off their power once a good strong wind has built up and debris is flying around... – Starfish Prime Apr 29 at 21:00
• To clarify, 1) the vanished atoms are literally, permanently gone, and they vanish without changing the momentum of any other atoms they might have been bonded to. Yes, this violates energy conservation. Like most superpowers, you could probably spin a turbine with it. 2) They don't pull air toward them or anything like that. But it just so happens that air molecules are moving at an average rate of 500 m/s or so, which means they will quickly enter the regions of empty space left behind by the molecules that vanished. – Gilad M Apr 29 at 23:35

While the various answers to the effect that "sound can't escape a vacuum" are technically correct, anyone nearby is certainly going to hear something, and it will likely be loud.

If the power is used for a just a moment, then the low-pressure shock-wave will propagate outward and make a pop like any other shock-wave. You can listen to it here, although more perfect demonstrations are possible.

If the effect is kept active, then there will be a strong wind toward the person. This probably wouldn't sound like a vacuum cleaner. There'd be no noise from moving parts inside or the exhaust; on the other hand it would be more intense. I'd expect the exact sound to depend a lot on the shape of the obliteration region, and on any nearby stationary objects. In general just a "woosh" noise, but melodic sounds are probably possible.

In general, the forces, momentum, and energy involved in the use of this power will be finite, and as long as the character is careful they'll be safe. 14psi isn't a big deal when applied over a single square inch, but, but applied across a square foot it's over a tonne.

You mention that the power could be used to remove the air from a room. This is unlikely to work, and very dangerous to attempt. The air in the room will be replenished through leaks, if there are any. If the room is large enough and leaky enough this won't be much different than using the power outside. If it's small and leaky then the rushing wind through those small gaps will rapidly enlarge those gaps, like an earthen damn collapsing from a small overflow. If the room is tightly sealed then it will probably implode. A space that wasn't designed to house a vacuum is unlikely to survive.

I think JBH is wrong that it doesn't matter how big the obliteration area is; the energy involved is going to scale more-or-less with the area. I do think that if the character were to use this on a large fraction of their body (excepting, at minimum, the bottoms of their feet), then they'd start a tornado-like phenomenon just like when you pull the plug at the bottom of a pool. It could take a few minutes for the rotational aspect to build up though.

• Any music generated by the superhero's superpowers would probably suck. Which is, incidentally, the very origin of the term "suck" in the sense of being of low quality. – John Dvorak Apr 30 at 7:49
• Good analysis. I'd suspect no melodic sounds though, as cool an application of the power as that may be. For discernible pitches, you need to have a continuous oscillation of the air at high frequencies (compared to human reaction times). So one shock wave will make more of a percussion sound, and people can't really make the shock waves fast enough to generate tones from repeated waves either. – Gilad M Apr 30 at 16:11

The actual diameter of a lightning channel is one-to two inches.1 (Source)

I had the privilege of being outside, walking for exercise in Texas, when a lightening bolt struck within a quarter-mile of me and my house. I was literally blown off my feet by the resulting thunder, which hit so fast my eyes barely had time to register the light of the lightning strike. Yup, it took me a bit to stand back up.2

So, 1-2 inches of channel diameter is enough to blow me off my feet. But before we start analyzing this useful piece of information, let's look at your situation.

The "channel diameter" caused by your superhero is zero. An infinitely thin surface that, whenever any atom or molecule touches it, said atom or molecule vanishes. How big is that channel?

No bigger than the largest molecule that touches it an any given moment.

How large a molecule is depends on the molecule, but they tend to be measured in angstroms or 10-10. I know that, worst-case, 5.08 cm worth of lightening 400 meters away will blow me off my feet. If the channel was only one angstrom wide, I would need to stand about 0.79e-5 millimeters away from the discharge.

Which is a fancy way of saying, it ain't gonna happen. The size of the vacuum (angstroms, at most) is so small that the "crash" of air wouldn't be heard by the superhero, much less anyone else.

But we're forgetting something!

Thunder is the sound made by the air refilling the vacuum from all sides,3 like hands clapping. You don't have that situation! At least, not until the superhero turns his superpower off. Until then you have, for example, air, rushing in to be obliterated by the danger zone! Woo-hoo! When your superhero turns this power on he quickly becomes the center of a tornado with air rushing toward him as fast as air possibly can under any terrestrial ground-level conditions. And it doesn't matter how small the area of his skin he involves in the superpower! It's like punching a hole in the side of a space ship. "Bad Things Happen."

The obvious consequence is the lawsuits filed against him for all the property damage and death (oh yeah! Death!) due to people, places, and things being sucked toward him at beyond hurricane-force wind speeds. And, of course, when they come within 2cm of him, they vanish (in neat little atomic slices!).

But those lawsuits aren't really a problem because, unless he has super durability (like, Superman level super durability!), he's dead the moment he shuts off his superpower. Because that's when all that air/matter/stuff/people hits him at more than the speed of sound. Being dead, he won't hear a thing.4

But the "bang!" it would make would be heard the proverbial hundreds of miles away.

1Totally unrelated to the question is the reality that anyone reading that quote and then thinking about the fact that the channel is pure, unadulterated electron flow should now be peeing their pants. It's a good example of why Mother Nature usually wins.

2I don't actually remember the sound of thunder, I do remember having all the wind knocked out of me. I don't recommend experiencing this.

3"As the superheated air cools it produces a resonating tube of partial vacuum surrounding the lightning's path. The nearby air rapidly expands and contracts. This causes the column to vibrate like a tubular drum head and produces a tremendous crack. As the vibrations gradually die out, the sound echoes and reverberates, generating the rumbling we call thunder." (Source) Yes, the development of thunder is more complex than described in the text, but not much and it's beside the point — you still don't have that situation.

4Let's look at this from opening only a dime-sized area (2.5 cm) rather than his entire body surface area. the world-record tornado wind speed record is 301 mph or 134 m/s - and that wasn't sinking air into a vacuum. I found a calculation online for a 1 ft square hole (929 sqcm) that suggests roughly 1.74 SCFS or 49.3 l/s for a dime-sized area. People standing next to the SH and the SH himself are in danger, but the force is dispersed quickly with distance. However, when he deactivates his power, the force of impact will break bones and potentially punch a hole in his hand (it depends if anything other than air is being carried by the wind). While active, the dime-size hole will try to pull his hand with the same force as the force of wind being sucked toward his hand (Newton's 3rd law), which is going to dislocate his shoulder and drag him around like a wild fire hose. This guy can get very hurt playing with this superpower.

• Comments are not for extended discussion; this conversation has been moved to chat. – L.Dutch Apr 30 at 14:50
• Even if the wind gets up to $500\mathrm{ms}^{-1}$ through a $2\mathrm{cm}^2$ hole in his hand, he'll only be sinking $100\mathrm{ls}^{-1}$ of air. A few meters away that'll seem like nothing, it's only $0.1\mathrm{ms}^{-1}$ through a $1\mathrm{m}^2$ surface. Even if he activates his entire body, the wind is about $1\mathrm{ms}^{-1}$ $10\mathrm{m}$ away. – patstew Apr 30 at 15:52
• "Thunder is the sound made by the air refilling the vacuum from all sides" - That's a persistent 19th Century myth. Wiki cites this book. – TKK Apr 30 at 17:16
• @patstew, the world-record tornado wind speed record is 301 mph or 134 m/s - and that wasn't sinking air into a vacuum. I found a calculation online for a 1 ft square hole (929 sqcm) that suggests roughly 1.74 SCFS or 49.3 l/s for a dime-sized area. So you're correct! The air flow would be dangerous to him and anyone standing next to him, but almost no one else (for a dime-sized hole.) – JBH Apr 30 at 18:31
• @JBH Air pressure is about $10\mathrm{Ncm}^{-2}$, so the pressure difference on a dime sized area will be like holding ~2.5kg. It'll actually be less, since the air pressure in the gap between the magic field and the skin will be reduced but not zero. So it'll probably be like holding a bottle of drink, not enough to dislocate anything. Another way to look at it is like a household vacuum cleaner that sucks 5x harder (they can pull ~$2\mathrm{Ncm}^{-2}$ according to wikipedia). It'd be pretty damn effective on the carpets, but I think it'd be a lot less spectacular than you're saying. – patstew Apr 30 at 19:36

# Initially, there will be no sound

The air will rush towards the person so fast, that no sound can travel away from the person against that rush of air. What people farther away will hear is the rush of the air passing them, and it will sound like a strong wind.

# Then there will be the sound of the world crashing into that person

When you remove all the air from a room in a normal house, the atmospheric pressure will crush that room.

The weight of the atmosphere is ten metric tonnes on one square meter of the ceiling of a room. If your room is, say, five by four meters (as my living room), there will be 200 metric tonnes pressing down on the ceiling of that vacuum. No common house will withstand that.

And then the ceiling will disappear when it hits the 2 cm zone, and the whole world will start to implode on your superhero, and the sound will be like the sound of a tornado picking you up and throwing you around, when you are near that person, and you will hear a huge blast until you hit the 2 cm zone and die.

But the effect will be catastrophic outdoors, as well. As Tyler S. Loeper has noted in their answer, air will be sucked into a vacuum at a speed between 380 m/s and 500 m/s. The fastest wind speed that has ever been recorded was 113 m/s. This means that your superhero will cause air to flow towards him at a speed three to four times as fast as the strongest winds naturally happening on Earth. That is a tremendous force, and will likely destroy most of his surroundings – and everything else on the planetary surface, if he keeps the zone active for long enough.

All that is assuming he doesn't activate the soles of his feet. Because if he does – of if he stumbles –, he'll fall into the ground as if he were falling through thin air and drop into the Earth's core and suck up the Earth.

• Flow into a vacuum will be limited to mach 1 through the orifice. You won't get supersonic flow on the high-pressure side. If you follow the two air speed links you'll see contributors there pointing out that the figures are wrong due to the flow being choked. – Starfish Prime Apr 29 at 20:56
• @StarfishPrime What orifice? There is no bottleneck if the person is standing outside. And if atmospheric pressure has crushed the building and the person has sucked in the rubble, there is no small hole, either -- only the person-sized hole of the superhero himself... – user64555 Apr 29 at 21:16
• The orifice is the space through which the air is moving, as defined in this case by a funny-shaped 2d layer wrapped around a person. A person has a surface area of perhaps 2 square metres. The air is therefore reasonably described as flowing through an orifice of that size. It will choke; that's physics. – Starfish Prime Apr 29 at 21:36

This would be very dangerous.

Let's replace our superhero with a spherical cow with a surface area of about 2 square meters. That's roughly the surface area of a human, but now we can ignore a whole ton of details. Spheres are simple!

Your superhero is going to create a discontinuous boundary condition. Usually air obeys wave-mechanics rules because information propagates upstream at the speed of sound. When this ceases to happen, air does "different" things. Sonic booms occur because an object moves fast enough to disrupt this simple assumption. Air ends up moving subsonic with respect to the object near the object, and subsonic with respect to the outside airflow far from the object. Between them, we get a "discontinuity," a region a few nanometers thick where we cannot treat air with simple wave-mechanics equations. This is the shockwave that leads to a sonic boom. In these regions, you have funny things like "there is no pressure." Pressure occurs in all directions, thanks to the rules of wave-mechanics. This means the thermal velocity of the particles is isotropic. Inside a shockwave, it's anisotropic -- it's directional. Very directional.

In this case, we are creating one of those anisotropies. There will be no air molecules moving away from our sphere. They all got absorbed. So this means all of the thermal velocity of the air, which is usually in all directions, is now going to be highly directional.

There's going to be some stratifiction of the air in this strange situation, but we can consider the mean thermal velocity of air: 464m/s. Without any molecules of air pushing back, the thermal motion of the air molecules will cause air to dissapear into our sphere at 464m/s. Other answers have mentioned numbers between 380 and 500 are considered reasonable, depending on how choked the airflow is. My expectation is that it won't choke much in this unusual configuration.

So now you have the scary number. 464m/s multiplied by 2 square meters is nearly 1000 cubic meters per second. 1000 cubic meters of air is going to disappear into your superhero/sphere every second.

For perspective, one of the largest buildings in the world is the Boeing Everett Factory. It's 14 million cubic meters. If you ran this ability full bore for 4 hours, you'd suck all of the air out of that building.

Other perspective: a 3000 square foot house with 10 foot ceilings is on the order of 1000 cubic meters. So you'd pull all the air out of a house in a second.

So what's the sound of this vacuum superhero ability? It's the sound of wood snapping and shattered glass as entire buildings collapse under strains buildings were never built to withstands.

## There will be almost no sound, except for air passing by very fast.

To understand why, we have to understand the following things about sound.

• Sound requires air to propagate through. If there is no air, most
likely there will be no sound reaching our ears.

Though propagation through the ground is possible. Our hero might also be falling to the center of the planet as they destroy all the ground 2cm underneath them, constantly.

• Sound travels at … well the speed of sound: 343 m/s.
• Sound is the propagation of energy.

The Speed of Sound vs. the Speed of Air Being Sucked into a Vacuum:

I googled a bit, and found a guestimation for the speed at which room temperature air is sucked into a vacuum, which is somewhere between 380 m/s and 500 m/s. You may notice that both of these numbers are larger than the speed of sound, 343 m/s. This means that ambient air will fall into your vacuum faster than any sound from it will be getting back out. So for sure there will be no sound travelling outwards.

Your hero is a blackhole for sound. People will be able to see what your hero is doing, but not hear anything he is doing. This is because the direction of sound travel is only towards the hero, not away from him/her.

An Energy Source for the Propagation of Energy:

Sound is the propagation of energy. So where is the energy for the sound coming from? This is not an implosion, so there is no rebound of energy (air sucked in then expelled). The air is sucked in, and then it is gone. All anyone would be able to hear, even if the speed of sound was fast enough, is the whistling of air as it rushes by towards the hero.

What might be observable is the visible implosion, as things get displaced and sucked towards the hero, rather than the sound itself.

No Air:

Obviously no air means no sound travelling through the air, so for sure by the time a room is cleared it will be silent.

Conclusion:

Most likely there will be no sound, at least as it relates to a sonic boom or any other kind of sound blast. There will however be the sound of air passing by at 380-500 m/s, which is quite fast. The sound of air forcing its way through cracks (whistling), the sound of things falling over further upstream, these are the things that an observer might hear.

• "it will just be the sound of air passing by at 380-500 m/s". A powerful hurricane will sustain winds of under 100m/s. An F5 tornado won't reach 150m/s. Those things are loud. Wind of 380m/s will be terrifying to hear. – Starfish Prime Apr 29 at 19:42
• @Rob to be clear; sound won't travel up the direction of flow of a supersonic air mass, but that's not to say that it can't transmit sound laterally into adjacent air masses or anything solid it hits. It'll be difficult to hear it for long, of course. – Starfish Prime Apr 29 at 20:04
• You'll also end up with some quite limited volume of supersonic flow, I think, because air simply can't move into the region fast enough. The area of the vanishment region is limited (~2$m^2$ if it were at the skin) and so you'll end up with the flow of air being choked in a similar way to the air flow through a rocket nozzle being limited by compressability effects. I'm not able to work out how big that region is, alas. – Starfish Prime Apr 29 at 20:14
• (Tyler, FYI: both the links you provided suggesting supersonic flows have contributors pointing out to the other answerers that the flow will be choked at mach 1 and not reach the quoted figures. Might wanna update your answer) – Starfish Prime Apr 29 at 20:54

# Forget the effects of air, gravity is a bigger problem.

atoms that are already in that region do not disappear unless they exit and re-enter, so their clothing is fine if they stick to the standard tight-fitting superhero attire

As you take a step, the material that makes up your socks and shoes deforms and compacts under your weight. It then elastically reforms and returns back into a form similar to its shape. This causes the atoms making up your socks and shoes to exit and re-enter the region of disappearance.

This means that the mere act of walking cause the hero to make holes in their shoes, socks and whatever else is under their feet, including the ground. This will cause them to sink into the earth (or floors in a building) as the particles beneath them disappear. Activating the power too long will cause them to disappear into the depths of the earth. Even a short period of activation can cause them to be trapped in the hole caused by them sinking down (moving around before releasing the power can cause a large enough gap that they are able to exit).

Let's say your shoes are exactly 2cm thick and do not expand beyond that, when they touch the ground the material will still contract, resulting in the matter making up the ground being consumed in the small gap caused by that contraction, causing you to create a hole that you sink into.

The only way for this to work is to have the outfit be made of some magical material that cannot expand or contract (or requires high forces to do so), and is at least 2 cm thick. Due to the nature of this material (since it cannot expand or contract), it would have to be cast onto the feet of the superhero to never be removed (think of a metal cast).

This is because the material would be unable to form itself to fit the hero as a sock or chain mail would because it cannot move or else it risks expanding and then contracting, thereby causing a gap for atoms to disappear and the hero to fall inside the earth. Additionally, due to the nature of this shoe, it would be incredibly uncomfortable to walk in, let alone run-in.

Then we have the fact that this material would need to be removed and recast often in order to adjust for physical changes to the hero's feet. Otherwise we risk a gap in the cast that causes the feet to shift around inside, thus beginning to cause the ground to disappear and them to fall again.

• Shouldn't this issue be negated if you just have a really tight outfit? If your shoes never extend 2 cm past your body, they'll never vaporize. – Gilad M Apr 29 at 23:40
• @GiladM See my updated answer. – william porter Apr 30 at 5:30
• Feet shouldn't be a issue since the OP defined the ability as being able to select which skin areas have the effect so excluding the feet should be possible (second paragraph second sentence) – masterX244 Apr 30 at 6:56
• ^This. I think that's where your confusion lies. You don't need the shoes to be exactly 2 cm either. They can be less, and it won't interfere with the ground unless you specifically have the soles of your feet be the area projecting the annihilation field. – Gilad M Apr 30 at 9:40
• Ah, that's the hangup I was having, I didn't notice that they could only do select areas of skin (though it's completely obvious now that I reread the question). I'll leave my answer up as a caution against using it on the foot. – william porter Apr 30 at 15:13

I think JBH etc. have offered great answers for what it would likely sound like, and how when the power is switched off the great wind generated may impact pretty hard.

But what would the power look like?

But, if I can, I would like to maybe go slightly beyond the scope of the question to ask what the power would LOOK like.

As each atom approaches the "line of death" the electron orbitals will be the first part to hit this line. As they do it will constitute a position measurement which will essentially find that either the electron fell in before the nucleus, or after it (at random).

This means that the approaching atom (charge neutral) will first loose some electrons (you can think of this as some of the electrons orbiting like planets happened to be in front of the nuclues) becoming net positive in charge. It will then loose its nucleus and the remaining electrons (those who happened to be behind the nucleus on their orbits) are released as free-floating electrons.

So while the power is on many charged particles will briefly appear near the surface, attract/repel one another and be accelerated, releasing electromagnetic radiation.

I expect the spectrum of all this light will be fairly broad - your superhero may appear to be glowing or white-hot over the parts of their body using the power. If their power allows photons to enter the region around them (so that they can see while using it) then they will see all this light, and feel all this heat.

That's what the power does as it cuts atoms in half. But a perfect particle-cut guillotine is worse than that. Zoom in further. The nucleus was at one point half missing! Depending on the order the neutrons and protons are lost you may have had a nucleus that was very, very, unstable for a split second before falling in entirely, so expect some radioactivity.

Zoom in again and it may get even worse, neutrons cut apart, Quakrs cut apart... you may sample some very interesting high-energy physics.

Final Points

Hawking Radiation : The above is somewhat related to the comment of Ruadhan's about Hawking Radiation. Hawking radiation itself is predicted to be a very weak effect for black holes (https://en.wikipedia.org/wiki/Hawking_radiation) IE they emmit very little light. This superpower would presumably also release light by this mechanism, but if it was at all similar in brightness to the black hole effect it would be un-detectable.

Space Suit : When our hero uses his/her power they are not just subjecting the world around them to an open vacuum like a space-craft breach, he/she is also subjecting themselves to one. Even if the power wraps around his/her entire body their is now no pressure to hold the air in the 2cm zone around them in place. (The outside air cannot apply any pressure as it is being spirited away). So they will experience the same effect as being thrown into vacuum themselves.

Its an awesome superpower by the way.

• Your comments about light sent me down a bit of a rabbit hole. I think you're right that separated electrons will be emitted from the surface. Atoms will pass them at 400 m/s, which is as though we were running a 400 m/s current through the air. This will give off BLUE light, similar to an electric spark. However, that's as far as I can figure out, since it's really hard to calculate the number of electrons. It might be a cool, bright flash, a negligible glow, or an instant electrocution. – Gilad M Apr 30 at 16:06
• Okay, I worked it out. If you use the power very briefly on a very small area (1 mm^2 and 1 ms), electrons freed from atoms at the vanishing surface will have a combined charge of about 6 C. To put that in perspective, that's on the order of a million shocks from a taser, or one above-average-sized bolt of lightning. Being anywhere near that is going to almost certainly kill the hero, and it completely rules out any larger-area uses. So... let's just say that nuclei and electrons always vanish together, okay? – Gilad M May 1 at 0:02

I think it would make a very loud roar, but not a blast.

Average speed of an air molecule is about 1.5 times the speed of sound. So air on your side of the barrier is going to deplete very fast. Collisions will slow it down some, but I expect the 2cm close to you will be an effective vacuum in milliseconds.

Around you, you effectively are a vacuum cleaner. Air rushing toward you vanishes. You create a zone of zero pressure that extends outward, limited by the resistance of the air to acceleration, and it's convergence into a smaller volume, and the general cooling effect as it expands into the vacuum. If you stood still, I suspect a large cyclonic storm would form around you. You'd become the centre of a walking hurricane, or perhaps only a tornado.

Note that while to outsiders you are a roar, inside, you hear nothing -- that 2 cm of vacuum around you cuts off all sound.

Note also: What are you standing on? Do we see a Superhero shaped hole in the ground?

• 1.5x the speed of sound? Are you talking about the speed of an air molecule in a collapsing vacuum? – Rob Apr 29 at 17:55
• If the air is sucked in faster than sound propagates, there would be no sound. – Tyler S. Loeper Apr 29 at 18:10
• It would be like a black hole, but for sound instead of light. – Harper Apr 29 at 20:39
• @Harper we can see black holes in part because stuff falling into them crashes into other stuff, heats up and glows. You'll hear stuff rushing out into a vacuum, at least in part because it will crash into other stuff on the way (like your ears) and transmit sound through it. – Starfish Prime Apr 29 at 21:39
• @Rob In normal air, the speed of sound is 2/3 of the average speed of the air molecules. Assumes an ideal gas, but air is pretty close. So speed of sound to first approximation depends on temperature. Lighter gasses such as helium, have molecules that travel faster at a given temp, sound travels faster, so individual peaks arrive faster, and you get raised pitch. – Sherwood Botsford Apr 30 at 13:34

It depends how much and how fast; on the small scale something like a frying oil crackling as tiny bubbles of vacuum only a few molecules across collapse. On the largest possible scale you're talking about 0.03 cubic metres (30 litres), of vacuum collapsing at once, that's probably going to cause a rather load bang, unless it was a continuous effect in which case it would be a roar, either way it will probably be dangerously loud for the hero and those standing too close to them.

• For a full-body activation, hero won't be able to hear any sounds from outside. However, the air from his 2-cm bubble would be also escaping rather quickly. – Alexander Apr 29 at 17:33
• @Alexander In reality full body activation will almost certainly suffocate the hero. – Ash Apr 29 at 17:35