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The Sound So Loud That It Circled the Earth Four Times

nautil.us

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Re: The Sound So Loud That It Circled the Earth Four Times

#61

Post by jschwartzi »

sillysaurus3 wrote: post #57
femto wrote: post #56

Earlier quoted context omitted.

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 w…

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 c…

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.

Re: The Sound So Loud That It Circled the Earth Four Times

#63

Post by lutusp »

kruczek wrote: post #60
sillysaurus3 wrote: post #57

Earlier quoted context omitted.

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 c…

"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.

Re: The Sound So Loud That It Circled the Earth Four Times

#64

Post by EGreg »

sillysaurus3 wrote: post #59
dkhar wrote: post #54

Earlier quoted context omitted.

Sound intensity actually falls off as 1/r², and here's a source: http://hyperphysics.phy-astr.gsu.edu/hbase/acoustic/invsqs.h...

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 Ea…

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?

Re: The Sound So Loud That It Circled the Earth Four Times

#65

Post by sillysaurus3 »

EGreg wrote: post #64
sillysaurus3 wrote: post #59

Earlier quoted context omitted.

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 Ea…

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.

Re: The Sound So Loud That It Circled the Earth Four Times

#67

Post by lifeformed »

ivansavz wrote: post #49

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 lin…

> ...somewhere diametrically opposite to Krakatoa...

Dang, this was your chance to say the word "antipode" in everyday conversation!

Re: The Sound So Loud That It Circled the Earth Four Times

#68

Post by jonsen »

sillysaurus3 wrote: post #65
EGreg wrote: post #64

Earlier quoted context omitted.

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 followi…

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

Re: The Sound So Loud That It Circled the Earth Four Times

#69

Post by acqq »

dingaling wrote: post #50
Arjuna wrote: post #38

Earlier quoted context omitted.

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 techn…

> Speaking of the Saturn V... it's one of the most powerful machines that humans have ever created. In the interest of completeness, here are the two most powerful launchers even flown in terms of first-stage lift-off thrust N-1: 4,600 tonnes / 10 million lb Energia: 3,600 tonnes / 7.9 million lb Saturn V is currently in third place but will be pushed down if / when Falcon Heavy flies.

If somebody is, like I was, interested to start reading about the N-1 and Energia:

http://en.wikipedia.org/wiki/N1_%28rocket%29

http://en.wikipedia.org/wiki/Energia

Re: The Sound So Loud That It Circled the Earth Four Times

#70

Post by agumonkey »

dandelany wrote: post #28
jbert wrote: post #5

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... "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" aga…

As if, at that level, the air bangs against itself """like""" a solid ?
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