A sound so loud that it circled the Earth four times (2014) | Hacker News Reader

A sound so loud that it circled the Earth four times (2014)

nautil.us

94 pointssinghkays36 comments

I learned something the other day that blew my mind. We are now able to take pictures of the far side of the Sun.

How? We could put a telescope in Earth orbit 180 degrees opposite from us. But we don't have one there yet.

We use sound waves.

Sound? In the Sun? Don't you need air for that?

No, any gas will do (solids or liquids too). The Sun is a big ball of very dense gas, perfect for propagating sound waves.

So we image the near side and analyze distortions caused by the sound waves traveling through the Sun. The resulting images of the far side are low resolution, but they are good enough to "see" things like coronal mass ejections and the giant sunspot that rotated into view last week.

https://www.google.com/search?q=solar+sound+waves

https://www.cora.nwra.com/~werne/eos/text/solar_waves_main.h...

https://www.cora.nwra.com/~werne/eos/text/farside.html

https://www.space.com/thanksgiving-sun-sunspot-treat-2020

That blew my mind, too, when one of the astrophysicists at the university I was attending grad school at told me this. He also had a telescope set up with a solar filter so people could look at the sun. It was moderately active at the time, and I saw this cluster of sunspots that looked quite a lot like the Hawaiian islands. He told me the biggest one was much, much larger than the diameter of the Earth, which also blew my mind.

We're actually kind of lucky to have a fairly typical, main sequence star right in our back yard to study.

There is the anthropic principle regarding that last: if the star wasn't typical and stable, life wouldn't have developed here to observe it.

The anthropic principle doesn't need it to be typically stable though, right? It could be exceptionally stable, in a particularly evil universe, and we'd most likely crop up in a nice little stable oasis.

If it were unstable during the 4 billion year history of life it could wipe life out and we wouldn't be here to observe it. But it depends on the magnitude of "unstable".

Our sun is exceptionally stable - it has no close encounters with other stars that might disrupt planetary orbits, which does happen in galactic nuclei or globular clusters. We are in that nice little stable oasis.

Wow, that's definitely amazing.

This reminded me that the way humanity knows about the interior structure of the Earth is also amazing -- and one important source of information is looking at propagation and reflections of earthquake vibrations!

Physics is the language that never ceases to blow my mind.

Physics is amazing. Reminds me of non-line-of-sight optical imaging which makes perfect sense scientifically but still boggles my mind. [1]

[1]: https://news.ycombinator.com/item?id=20813557

We may not have a satellite at 180 degrees, but we have STEREO A and B at 120 and 240, used along with the SDO to give us a pretty full view of what’s going on

Can we take pictures of the sun at night?

No you have to wait for the sun to come up.

The sum total effect of small quips, when people upvote them, is the degradation of forums to reddit as people seek easy points.

While not "directly", we can look at the sun through the Earth: http://strangepaths.com/the-sun-seen-through-the-earth-in-ne...

I wonder if there was a local loudness maximum on the globe diametrically opposed to the volcano. As the advancing wave makes it half-way around the globe, it seems like it would converge simultaneously on a single point on the opposite side.

https://news.ycombinator.com/item?id=8386018

I was interested to learn that the loudest possible sound on Earth would be 194 db. [0] Krakatoa apparently "only" clocked in at 180 db, meaning the loudest possible sound is about 5x louder than said eruption.

The article states that the sound from Krakatoa was well over the 194dB limit.

If a reader were skimming the article, they might have missed that the 172 decibel "reading" was measured at a gasworks 100 miles from the eruption!

IIRC A 10 Db increase in SPL is perceived by most people as a doubling of "volume".

thr article mentions this too... its hard to get a feel for log scales lol.

Yeah, it's funny how humans don't really comprehend logarithmic or exponential growth, yet, the whole reason we use a log scale for stuff like sound pressure is that our nervous system has a logarithmic response curve.

TIL how the decibel scale works.

Genuine question, how's the difference between 194db and 180db only 5x? Shouldn't it be more than 10x?

The decibel is defined as a tenth of the log10 of power, so power doubles every ~3dB. Amplitude and pressure are proportional to the root of power, so amplitude doubles every ~6db. Loudness is roughly proportional to the root of amplitude so it doubles every ~9dB.

This leads to confusion when people don't specify which quantity they are comparing.

Thank you for that explanation. Based on that, the original poster meant 194db is 5x the power of 180db but only ~1.6x loud?

Increase of 14db would be an increase in perceived loudness of ~2.6x

194 db is the loudest possible sound, where sound is an oscillating pressure wave. Pressure above 194 db become Shock or blast waves, which are a pressure pulse.

https://waitbutwhy.com/2016/03/sound.html

It wouldn't be simultaneous. The difference in great-circle lengths based on nonspherical oblateness (tens of miles) would account for up to a minute difference in arrival times at the antipode. At most it would be a sustained rumbling.

And on a global scale, there are many differences in pressure and wind speeds and temperature that do slightly alter the speed of sound. I don't know the magnitude of that, but it would further disperse the arrival times at the antipode to at least some extent.

There would still have to be some place that the sound waves met, right? Is there any way of figuring out where that would be?

No, there doesn't need to be such a place (even on a surface that's much less irregular than the Earth is).

They all cross the antipode, but not at the same time. There is no point where they all meet at the same time, if the planet is oblate rather than spherical, or has other irregularities in pressure or temperature or surface terrain.

Given that it started at a "point", it would go outwards in a circle, so you would end up with a "line" where sound from each direction crossed over. In theory, you could end up with a point where they all met again, however due to the focusing effects of the various landscape, and ambient pressure differences, this would rather be a diffuse area than a concentrated point, just like the lines where the sound met wouldn't be straight either.

This, combined with that the fact that on the on the "other" side of the world it was really only detectable by barometric readings means that you would be hard pressed to notice it, even along those lines and or point.

FWIW the antipode of Krakatoa is near Medellin, Columbia.

The RaspberryShake team makes a high-quality "Raspberry Boom" infrasound detector for hobbyists. Next Krakatoa, you can be prepared to detect it yourself. :)

The "RBOOM" sensor alone is $300+, but you can also buy a kit and integrate it into a Raspberry Shake home seismometer setup. I run a RS seismometerat home, but not the infrasound sensor.

"From incoming harsh weather, planes flying overhead, gun fire and nuclear testing, to volcanoes erupting on remote islands in the Pacific Ocean and meteors exploding over the Atlantic… See all these and more. Discover the untold secrets that are out there with your own RBOOM!"

For sale: https://raspberryshake.org/products/raspberry-boom/

Lots more info in the user manual: https://manual.raspberryshake.org/boom.html

(2014)!

previous discussion:

https://news.ycombinator.com/item?id=8383158

https://news.ycombinator.com/item?id=18746225

(2014)

Added. Thanks!

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