
The Sound So Loud That It Circled the Earth Four Times - aatish
http://nautil.us/blog/the-sound-so-loud-that-it-circled-the-earth-four-times
======
jbert
Nice. I hadn't thought of there being a limit (due to vacuum) to sound
intensity.

Reminds me of this recent comment on reddit:
[http://np.reddit.com/r/space/comments/2h9y9g/the_first_launc...](http://np.reddit.com/r/space/comments/2h9y9g/the_first_launch_from_cape_canaveral_1950/ckr6mev)

"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,"

~~~
dandelany
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](http://forums.prosoundweb.com/index.php?topic=98023.25;wap2)

Here's a good example of the sound I'm talking about:
[https://www.youtube.com/watch?v=OnoNITE-
CLc&t=1m44s](https://www.youtube.com/watch?v=OnoNITE-CLc&t=1m44s)

~~~
disillusioned
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).

~~~
jonifico
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?

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aatish
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.)

~~~
double0jimb0
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.

~~~
kordless
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.

~~~
SerpentJoe
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.

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ivan_ah
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...

~~~
dkhar
Sound intensity actually falls off as 1/r², and here's a source:

[http://hyperphysics.phy-
astr.gsu.edu/hbase/acoustic/invsqs.h...](http://hyperphysics.phy-
astr.gsu.edu/hbase/acoustic/invsqs.html#c1)

~~~
sillysaurus3
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?

~~~
EGreg
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?

~~~
sillysaurus3
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.

~~~
jonsen
_Sound waves can diffract around objects, which is why one can still hear
someone calling even when hiding behind a tree._ :

[http://en.m.wikipedia.org/wiki/Difraction](http://en.m.wikipedia.org/wiki/Difraction)

~~~
sillysaurus3
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.

~~~
jonsen
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.

~~~
sillysaurus3
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?

~~~
jonsen
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.

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beloch
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!

~~~
Balgair
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.

------
SeanDav
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.

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kevinskii
For anyone interested, I'd recommend Simon Winchester's book "Krakatoa":
[http://www.amazon.com/Krakatoa-World-Exploded-
August-1883/dp...](http://www.amazon.com/Krakatoa-World-Exploded-
August-1883/dp/0060838590)

Also interesting are how quickly the volcano has risen out of the sea again
after being totally obliterated, and how much life now thrives on it.

~~~
jgalt212
I second that opinion. Great book.

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danielweber
So the sound got harder to hear as you were farther away, but what if you were
at the exact antipode on Earth? Would it have been audible at all?

~~~
aatish
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.)

~~~
stromgo
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.

[1]
[http://en.wikipedia.org/wiki/Caloris_Basin#Antipodal_chaotic...](http://en.wikipedia.org/wiki/Caloris_Basin#Antipodal_chaotic_terrain_and_global_effects)
("It is thought by some to have been created as seismic waves from the impact
converged on the opposite side of the planet.")

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skosuri
I've been really impressed with Nautilus. Awesome to have a new magazine that
seriously covers scientific topics, and do it quite well and with style.

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th0ma5
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?

~~~
jychang
Sound doesn't travel at the same speed given different air pressures and
temps. I think the waves wouldn't meet up at certain point.

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ChristianGeek
Not only is this article fascinating, even more so is the 600-page report it
links to! Thanks.

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Shivetya
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)

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robjekt
Fantastic article. Thanks for sharing

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hypertexthero
"Holy smokin' toledos"

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MattWard
interesting article

