"the Saturn V rocket produced a SWL (Sound Power Level) of about 220 decibels, which is sufficient to melt concrete nearby and set grass aflame a mile away,"
Yes! My favorite example of this is the sound made by the space shuttle's solid rocket boosters at liftoff. Techies who have seen recordings of shuttle launches generally assume the guttural crackling, popping roar sound they hear is due to a badly adjusted microphone clipping the loud sounds. In fact, this was a physical phenomenon that could be heard in real life, due to the pressure waves physically "clipping" against a vacuum upon reaching the atmospheric pressure limit! Sorry I can't seem to find any better sources, but this is an anecdote I've heard from several witnesses: http://forums.prosoundweb.com/index.php?topic=98023.25;wap2
I had the privilege of watching STS-134 launch from the press site and everyone tells you: "it's loud. louder than loud. you will be surprised by how loud it was." and that still wasn't enough to prepare me. It is loud. It is literally like the sound is clipping, the brapping noise moves through your teeth, it vibrates your clothes, you feel it inside of you. It is incredible, and I was so lucky to have watched it from the closest any humans are allowed to be (except for the brave rescue team in an APC 1 mile away).
Wow! That sounds like an amazing experience. Kind of a dumb question, but were you also impressed by how fast it goes up, or does it 'float' into the sky?
Yesss! I got to see STS-3 (!!!) from the press site. I was just a kid at the time on a school field trip. Our bus was misdirected to the VIP/press site, and they let us stay.. ;-) IIRC, several of the huge rental-only Canon telephoto lenses were in attendance on the photog's hill. The photographers were incredibly cool, talking with us, letting us look through their spotting telescopes and such.
The launch was far and away the loudest phenomenon I've ever heard, or ever hope to hear. Impossible to adequately describe. You don't hear the sound, you are the sound. The universe transforms, and is made of nothing but sound and vibration.
I recall being thrilled to be able to see STS-124 in May 2008, only because the previous launch had been delayed, and the new date coincided with our holiday. Among other things it was taking up parts to fix a toilet on the ISS!
I remember also being struck by not only how loud it was, but also how bright. It's something that photos or video just can't capture.
Speaking of the Saturn V... it's one of the most powerful machines that humans have ever created. It was equipped with five F-1 rocket engines, with each engine producing an absolutely staggering 1,500,000 pounds of thrust; that's a total of 7,500,000 pounds of thrust!
Can you imagine being tucked into the small, cramped Command Module, sitting on top of this magnitude of power at lift-off?
The whole thing, the technology, the sound, the people coming together to make it happen... it's soul-stirring.
The Falcon Heavy total thrust is supposed to be 3.9 million lbf, according to Wikipedia. However it will have a larger specific impulse than a Saturn V (282 sec vs 263 sec).
Standing next to the thing makes you feel like an ant. I visit it once and a while and I just think to myself "we did this, this thing blasted off of the face of the earth with humans at the top"
"This limit happens to be about 194 decibels for a sound in Earth’s atmosphere."
Therefore, wouldn't 220db be like -280°C?
EDIT: Somewhat of an answer:
194.09(P) = 1 (ONE) AIR ATMOSPHERE = 14.6962 POUNDS PER SQUARE INCH = 14.6962 P.S.I.= 1 ATM
SOUND WAVES DISTORT AND ARE NOW DEFINED AS SHOCK WAVES AND THEY BEGIN TO FOLLOW SHOCK WAVE BEHAVIOR. PARTICLE VELOCITY (BLAST WIND) = 590 FEET / SECOND = 180 METERS PER SECOND = 402 MILES PER HOUR. -REF.2.
That link has many decibel levels, and examples, and factoids about the things producing the sound, unrelated to the sound itself.
I think the chart intends to say that the rocket melts concrete, sets grass on fire 1 mile away, and produces sound that is 220 decibels. Not that the sound itself is melting concrete or setting grass on fire.
The reddit post mentions "At 2 kilometers (140+ db), ..."
2km is a little more than 1 mile. So if the sound alone could set grass aflame a mile away, then that would mean that 140db at distance zero is enough set grass on fire.
If it can set grass on fire, then it can set a lot of other things on fire. Paper, clothing, hair, etc...
Some extremely loud rock concerts hit 150db.
So, while I guess I'm not 100% sure that this is an error, I just doubt that rock concert speakers can/would output enough volume to set things right next to them on fire. There would be huge safety issues, and risk of the speakers destroying themselves and other equipment.
I know nobody is right next to speakers this loud, but still... I doubt the accuracy of this 'melting concrete and setting grass aflame via sound' factoid.
I would love to be shown wrong, because it is really interesting if it's true.
As an example, its intensity (in W/solid angle) is 14 orders of magnitude (AKA 100 billion times) highter than the 80 dB people usualy use as a safety limit.
Those things are already too subtle and hard enough to learn in a second language. People defining the same expression with widely conflicting meanings in different places do literally turn it into a maze :)
Hey, I'm the author of the post (on twitter @aatishb). Thanks for voting it up. Would love to hear your thoughts and constructive feedback. Cheers. (PS the references linked to at the bottom of the post are packed with fascinating information about Krakatoa, for anyone who wants to dig deeper.)
It's a great article - makes me wonder how long the shockwave of the impact that killed the dinosaurs must have been echoing across the globe, or the one that created the moon (well, that one was before earth had an atmosphere, but it probably was strong enough to make the entire planet vibrate for quite a while).
I once saw a talk about this at a scientific meeting, so the papers are out there. All I can remember from it is that they estimated the ejecta at "about the mass of the Sierra Nevada". This was quite a while ago so I'm sure they've got a better estimate by now.
How does something like the Tzar bomb compare in terms of sound pressure? Would the immense volume of ejected material from the volcanic explosion move more air than a nuclear explosion? I'd guess lots of mass (air molecules) is just vaporized in a nuclear explosion, leaving much less mass to create pressure deltas.
The Tsar bomb was in the ballpark of a 50 megaton explosion, whereas estimates that I've seen for Krakatoa put it at up to 200 megatons. That said, there would be differences due to the fact that h-bombs are exploded at a height to maximize the damage from the blast wave, and that a significant fraction of Krakatoa's energy would have been spent on throwing earth around. It's always hard to compare different explosions because it's difficult to know what fraction of the energy output goes into acoustic energy, but I think it's reasonable to assume that Krakatoa was louder (at least in that the sound was heard further).
Wikipedia says it was four times as powerful as Tzar Bomba, which would put it in the neighborhood of 200Mt. Technically Tzar Bomba was suppose to yield about 100Mt, but the scientists dialed it down a bit because they were worried about it.
My understanding is that the yield was halved by the use of a lead tamper, rather than a uranium one. The tamper is a shield around the core that slows down the explosion in order to consume the reactants more fully. In production they commonly make the tamper out of waste uranium, which undergoes fission during the blast and contributes a significant amount of energy, but if you don't need the boost then lead works fine, or anything else heavy. The other thing about uranium is it greatly multiplies the radioactive fallout (fusion bombs are actually pretty clean when lead is used), which may have contributed to the decision not to use it.
"The Tsar Bomba's fireball, about 8 kilometres (5.0 mi) in diameter, was prevented from touching the ground by the shock wave, but nearly reached the 10.5 kilometres (6.5 mi) altitude of the deploying Tu-95 bomber."
I'd love to hear the pilot's thoughts on this. From what I can guess, he couldn't have traveled very far away by the time the explosion occurred. At the very least, he must have felt tremendous turbulence.
I wonder if the scientists knew how close the explosion would be to the aircraft.
One thing I'm surprised you didn't mention is infrasound[1], which is sound so low that it can potentially damage the human body. Some of the newer non-lethal weapons the US military use for crowd control include a directional infrasound "gun" that basically vibrates your inner organs fast enough to make everyone super nauseous.
Infrasound is about a specific frequency range of sound which isn't audible but still carries energy (which is why it can be dangerous).
This article was about a wideband sound that was so loud that it defies comparison. Loudness is related to the amplitude, or "height" of the wave, not frequency.
Some of the newer non-lethal weapons the US military use for crowd control include a directional infrasound "gun" that basically vibrates your inner organs fast enough to make everyone super nauseous.
Do you have a cite for this? A "directional" outdoor infrasound weapon would be huge (10s of feet) and quick web search seems to indicate these types of weapon are at best "not mature" and at worst fiction.
That article describes a theoretical infrasound weapon, not something that is being used today:
But this [infrasound weapon] has seemed to be not a very practical weapon, since large banks of speakers were required to provide directionality, and power demands were deemed excessive.
The article goes on to describe LRAD which does exist and has been deployed, but is a high-frequency weapon, not an infrasound weapon (and can be defended against with a good pair of earplugs).
Interestingly, the sound intensity will decay as a 1/r law, where r is the distance from the source. (I'm assuming conservation of sound energy as it travels horizontally, i.e., no loss to interactions with atmosphere and terrain).
Compare with the 1/r² laws like the strength of an electric field at a distance r from charge Q: E(r)=kQ/r².
In both cases there is a total of something (sound energy or electric field lines) and that total must be split over all possible directions. The total electric field must be split over the surface area of increasingly larger and larger spheres hence the 1/r² behaviour (1/4πr² to be precise). The sound energy is spread uniformly over a circle with circumference C=2πr, hence giving a 1/r decay, over short distances.
For longer distances, the curvature of the earth will play a role. Come to think of it, it must have been really loud somewhere diametrically opposite to Krakatoa, 17 hours after the eruption...
If sound traveled as a sphere, then it wouldn't be able to circle the Earth. If you imagine a sphere growing from a point on the Earth's surface, most of it will wind up in space, and none of it will reach the opposite side of the Earth. The 1/r^2 falloff is probably just an approximation that holds true for short distances or smaller intensities.
In this case, the soundwave was able to follow the curvature of the Earth, which implies that it wouldn't decay as a sphere but rather a plane, which would be a 1/r falloff. I wonder why sound of massive intensity will follow Earth's curvature?
Why don't you think that the sound waves would go in all directions, and be absorbed / redirected at the edges? What you see following the curvature of the Earth is the result of that, and once you factor in the second-order wave reflection, it's still on the order of 1/r^2 isn't it?
Nah, imagine a point on the earth. Now trace rays from that point in every possible direction. All of those rays will eventually lead to space or to the ground, and none will reach the other side of the Earth. Since molecules are physically further apart the higher you go in the atmosphere, it seems unlikely that the energy would be redirected at the edges, only dispersed. That must mean the wave is literally following wherever the atmosphere is thickest rather than simply being reflected.
When I hear "energy dispersed at the edges", I think "lost" (or effectively absorbed), not "reflected back the way it came". I can't think of any obvious mechanism that would lead to significant coherent reflection of sound waves off the upper atmosphere. Similarly, I think a lot of the sound intensity would be absorbed by material on the ground rather than reflected back up. I vote for predominantly 1/r^2 behavior.
[As a physics prof, I probably ought to be doing some sort of calculation to justify that, but I don't have the time. I'm sure it's been analyzed formally somewhere already. Just off the cuff, though, the speed of sound is lower at low temperature, so refraction of a sound wave in the upper atmosphere will tend to bend it outward, away from the Earth. I'm thus imagining that loud sounds may tend to locally push the atmosphere out from the planet a little bit, and I expect that coming back to equilibrium will be a lossy process that doesn't preserve the shape of the wave.]
Is diffraction the reason why soundwaves could travel around the whole Earth? My previous understanding of diffraction was that obstacles cause waves to propagate in different ways. But the thinning of the atmosphere isn't really an "obstacle." The molecules that soundwaves use to propagate are simply further apart from each other, meaning waves are more likely to disperse and lose energy than to keep traveling or bounce. That would imply the boom from the volcano should disperse into space and go silent rather than travel around the Earth. But since that doesn't happen, it seems like the waves follow wherever the atmosphere is thick.
I'm having trouble understanding how diffraction would cause that end result of "waves go where the atmosphere is." If waves could bounce off of the thin atmosphere near space, that would make total sense. But they can't bounce due to thin atmosphere, only disperse, so it seems like there's some other phenomenon in play.
Sound waves are points of high pressure and points of low pressure. At each point of high pressure the pressure tends to spread uniformly in all directions. Likewise at points of low pressure there is sucking from all directions. Opposite directions cancel, i.e. orthogonal to the wave direction. All in all the sum of all points of pressure creates a moving wavefront. Which naturally bends around obstacles. The earth is just a very big obstacle.
Hey, thank you for taking the time to explain this. I really appreciate it.
I'm having trouble seeing how that explanation would explain the case at hand. Your explanation is likely correct, and I'm probably just thinking about it incorrectly. Would you mind pointing out the flaw in my logic?
In this scenario, a volcano's boom was so loud that it traveled through the atmosphere, all the way around the Earth. Your explanation is perfectly reasonable for thick atmosphere. In thick atmosphere, a soundwave is a pressure differential, and since molecules are densely packed together (since the atmosphere is thick), there's no choice but for the molecules to "slosh around." The high pressure areas will spread to the low pressure areas within the thick atmosphere and create a moving wavefront, exactly like you said.
But as the soundwave travels closer to space, the atmosphere becomes thinner. There are fewer molecules for the soundwave to travel through. That means a pressure differential will have less medium through which to traverse. Since there are fewer molecules, there's more room between them to absorb a pressure differential, right? For example, the reason sound travels so well underwater is because water is extremely dense in comparison to the atmosphere, so less energy is needed to travel an equal distance underwater. Correspondingly, near space where the atmosphere is thin, more energy would be needed to traverse an equal distance. That must mean that as the wavefront approaches space, the wavefront should dissipate. Since more energy is required to travel through less atmosphere, then as the atmosphere approaches zero, the energy required for a wavefront to travel one meter should approach infinity, and that's why it seems like the wavefront should dissipate near the edge of the exosphere.
But in this case, the wavefront didn't dissipate. The volcano's boom kept on going all the way around the Earth, and it was somehow able to maintain its energy. If the soundwave travels as a sphere from its point of origin, then that sphere should have a hard time traveling all the way around Earth, shouldn't it? So it must not be travelling as a sphere, but something else.
You're saying that "something else" is diffraction. I'd like to understand that. How is it that a wavefront of such intensity can approach the exosphere where it should dissipate, yet not dissipate and instead keep traveling all the way around the Earth due to diffraction?
It is enough to think about the part of the sound wave, a ring, that travels horizontally to see that it bends with the curvature of the earth. The front of the wave will at every point "shoot" some of the energy horizontally forward, and horizontal is at every point tangential to the earth surface.
What happens to the sound energy going upwards, well, energy can't disappear. And certainly not into the nothing between the air molecules in thin air. The wavefront going vertically up will eventually push some air molecules away from earth without them hitting any other molecules further out. And then gravity will pull them back. That is in fact a kind of reflection.
The speed of sound changes with density, so the wavefront will not move perfectly spherically. That and the gravity induced ripples on the top of the atmosphere will distort and dissipate the energy around the earth in a somewhat complicated pattern. I think. I'm not a physicist.
The /r^2 factor is due to the fact that the surface of a sphere grows proportional to r^2. That will hold for the wavefront half way around the earth, but from there it will be reverse. The energy will converge to full strength at the point opposite from the origin. That is if there were no loss. A significant portion of the energy is converted to heat and otherwise dispersed from being a compact wavefront.
The energy is dispersed in all directions through air molecules. At the top it dissipates and at the bottom it is absorbed by the earth. The only extra boost you'd get along vectors parallel to the surface would be reflections of the energy, which other commenters have said is small ... so this term doesn't change enough to get the function out of O(r^-2) does it?
You do get a boost from the wave contracting and converging towards the opposite point from halfway around the earth. I think it is not at all obvious what the order of damping will be.
I don't think this is correct. A sound wave propagates in three dimensions the same as an electrical field (spherically) and so should also obey the inverse square law.
In this case, the atmosphere is essentially a thin (compared to the radius of the Earth) two dimensional sheet wrapping the Earth, so it's probably valid to think of the sound as propagating on a surface. There wouldn't be much loss of energy to space. Would the atmospheric interface with the ground reflect or absorb?
The divergence is probably even less that 1/r because the Earth is roughly sperical, and the sound will focus at the antipode of its source. There might even be a region, approaching the focus, where as the sound propagates its sound level is actually increasing, not decreasing!
It would be interesting to know what history has to say about Medellin, in Colombia, in 1883. Being at coordinates 6N 75E, it is roughly at the antipode of Krakatoa and could well have had an awful lot of energy dumped in it general direction a few hours after the eruption.
Would gravity affect soundwaves at all? If sound is traveling parallel to the Earth's surface, will it follow along the curvature of the Earth (implying gravity has an effect on it) or was it able to propagate around the Earth four times simply because the intensity was so great that it was able to still be heard despite the loss of energy in the "up-down" directions relative to the origin of the sound?
If I had to completely guess, I would think that gravity might have something to do with its behavior in this case, because if sound decayed at 1/r^2, it seems highly unlikely that the sound could survive four trips around the Earth. Yet we also have to explain how the sound was able to preserve itself even though sound travels "sort of spherically" (if you're up in the sky and someone shoots at you, you can hear it, so sound must be going up-down as well as left-right and every other direction) but if it was simply spherical in this case then it would decay into the upper atmosphere. If you imagine a sphere that grows from any point on the surface of the Earth, most of it will wind up in space, but since it circled the Earth, it must not have traveled exactly as sphere, which implies it didn't fall off as 1/r^2.
The most likely explanation seems to be that sound of huge intensity somehow follows the curvature of the Earth, which is very interesting. I wonder if gravity is influencing the soundwaves directly (which might actually be nonsense, since waves aren't matter, but something that travels through matter) or if gravity somehow sets up the propagation medium in a way that lets the sound curve around the earth without losing intensity as 1/r^2.
You're partially correct about Gravity, but there's more to it.
The radius of the Earth is roughly 6000 km, while the thickness of the atmosphere is about 120km. Thus, the ratio of the two heights is about 1/60. Because the ratio of atmosphere thickness to terrestrial thickness is so small, for the purposes of modeling wave propagation through the atmosphere we can approximate the atmosphere as a thin spherical shell around the Earth.
Furthermore, the Earth's surface forms a hard boundary below the shell, further confining the sound to the atmosphere, while the force of gravity also prevents the sound wave from escaping into space via the atmosphere.
As other posters have stated, this forms a 2-dimensional propagation layer which results in a linear falloff rather than an inverse square falloff.
"which might actually be nonsense, since waves aren't matter, but something that travels through matter"
I don't think that's much of a nonsense. These are waves propagating through a medium - in this case mostly air. And particles of air are affected by gravity, therefore I would very much expect them to follow Earth's curvature.
> And particles of air are affected by gravity, therefore I would very much expect them to follow Earth's curvature.
Gravity isn't the reason sound waves curve around the earth. One reason is that the curved surface is the only avenue open to them. There are a number of other, less influential reasons like different temperatures at different altitudes, but gravity isn't on the list.
If a loud noise was heard there, or nearby, it would likely have been reported...? I didn't get very far searching the web for something like that. One would probably have to go to the Bibliotecas de Bogotá and check newspapers from the day.
To put this in context, the Krakatoa eruption released energy roughly equivalent to a 200 megaton bomb, which is roughly four times larger than the "Tsar Bomba", an H-Bomb which is the largest nuclear weapon detonated to date. The globe's average temperature fell 1.2 C after the eruption due to SO2 increasing the Earth's albedo. The Volcanic Explosivity Index (VEI) of the Krakatoa eruption was 6, and it's estimated to be a roughly once in a century eruption.
When you think about it, we're living on a extremely thin crust that's formed on the surface of a molten, roiling ball of rock and metal. People are strangely obsessed with the threat of asteroid impacts when what's beneath our feet presents a perhaps even greater danger, one which we know next to nothing about!
My wife is a geo-chemist. She says that plastic is a better term to get an idea on the mantel. Yeah, it is a liquid, but the pressures and temperatures are just so crazy to us that calling it 'roiling' is a little bit of a misnomer. Also, because of these pressures and temperatures, it's hard to ever know anything about the interior of the planet. Turns out, dense things that are very deep and hot are very hard to study.
> People are strangely obsessed with the threat of asteroid impacts when what's beneath our feet presents a perhaps even greater danger, one which we know next to nothing about!
I'd wager we're much closer, technologically, to do something about asteroids, though. Not quite so much about eruptions.
Another amazing/frightening statistic is that pyroclastic flows killed people living 30 miles away over open ocean. If I was living that far away across an ocean, I would have felt perfectly safe. It just boggles my mind that even at this distance Krakatoa was deadly.
I suspect that due to variations in the earth's surface, and fluctuations in pressure due to things like air currents (and to a lesser extent perhaps even natural and man-made formations) the shock wave didn't travel uniformly in all directions, and wouldn't have been audible from the antipode.
Something that makes this extra intriguing: the exact antipode of Krakatoa seems to be in an inhabited area on the banks of the Rio Magdalena in Colombia.
I wonder if anyone was there at the time to notice a pressure wave coming from all directions.
The sound was only audible up to 3000 miles in radius, or about 1/13th the globe. It wasn't audible in the other side of the Earth (although it was detectable.)
He mentioned the "exact antipode on Earth", so he's probably thinking of an effect similar to antipodal chaotic terrain [1]. The fact that sound was only audible up to 3000 miles in radius is not enough to rule it out, you need other arguments.
Assuming it was powerful enough to get there, you would hear it.
At the antipode you would feel an identical pressure wave from both side. This would manifest as compression which you would feel since you are compressible.
A flag would not move in either direction (i.e. the wave is canceled out), but there would still be a compression wave in it.
Like imagine hitting a piece of wood with two hammers simultaneously.
One thing I only see a little bit about is the negative pressure pole created on the opposite side of the earth? I guess I need to spend some time at the library, but I've only seen this stated as a trivia item, but no further details, like was the effect as pronounced at that point given this is when the reflections outward all met up again?
It would be so amazing to capture such an event from satellite. I wonder how much deformation would be evident, sure some material was pushed into space (not speaking earth - but gases)
Reminds me of this recent comment on reddit: http://np.reddit.com/r/space/comments/2h9y9g/the_first_launc...
"the Saturn V rocket produced a SWL (Sound Power Level) of about 220 decibels, which is sufficient to melt concrete nearby and set grass aflame a mile away,"