Project Mogul
June 8, 2014 11:40 AM   Subscribe

You may have heard how sounds travel farther during a temperature inversion, when air near the ground is cooler than the air above. But do you know how this phenomenon is related to the 1947 UFO crash in Roswell, New Mexico?

In a temperature inversion, sound waves traveling upward are reflected back toward the cool air below, because sound waves bend toward regions where the speed of sound is slower.

Things get even weirder when a "slow" layer is sandwiched between two "fast" layers, trapping sound inside the slow layer like light inside an optical fiber. This occurs naturally about 1 kilometer beneath the surface of the ocean, creating a deep sound channel known to both submarine crews and baleen whales.

A similar sound channel in the upper atmosphere was the focus of a top-secret U.S. Air Force project during the Cold War, which according to the government was responsible for the famous Roswell "flying saucer" incident. To find out how, read a detailed explanation by UC Berkeley physicist Richard A. Muller, or watch a video of Muller's lecture on the same material.

(Previously on MetaFilter: declassified information on Area 51.)
posted by mbrubeck (14 comments total) 51 users marked this as a favorite
The Muller page is worth it just for the explanation of Sofar spheres for locating downed pilots in WWII. God that's clever.
posted by figurant at 12:02 PM on June 8, 2014 [5 favorites]

I wasn't aware of Sofar spheres despite knowing about SOSUS, made for interesting reading. One more ping for this article, Vasily.
posted by arcticseal at 4:10 PM on June 8, 2014 [2 favorites]

Oh, man, I really wish that I had had Richard Muller's page when I was teaching a "Science in Context" course three years ago -- the lab component had a lab on refraction/wave nature of light & sound that I could only do a rather poor job of incorporating in the lecture course, because I didn't have anything historical or pop-culture-ey that I really wanted to incorporate. Shit, I think I ended up showing the kids a video of the "Rainbows close to the ground!?!!?!" lady from YouTube. This would have been an AWESOME component of a lesson plan incorporating conspiracy theories and pathological reasoning.
posted by Made of Star Stuff at 4:48 PM on June 8, 2014

Holy shit, this is awesome.
posted by djeo at 5:38 PM on June 8, 2014

posted by Freen at 5:49 PM on June 8, 2014

At long last, an explanation that makes sense for what happened. No pictures of the actual microphones, of course... I wonder how big a diaphragm you need?
posted by MikeWarot at 6:02 PM on June 8, 2014

Ewing designed the spheres to be strong enough to withstand the pressure of water down to a depth of 1000 meters. But at that depth, the pressure finally was too much, and the spheres were suddenly crushed. (Like an egg, the round surface provides lots of strength, but when it breaks, it breaks suddenly.) The water and metal collapse, and bang against the material coming in from the other side. It's like a hammer hitting a hammer, and it generates a loud sound. The energy released from a sphere with radius 1-inch at a depth of 1 km, is approximately the same as in 60 milligrams of TNT. That doesn't sound like a lot -- but it is about the same you might find in a very large firecracker.

In the air, the sound of a firecracker doesn't go far, perhaps a few kilometers. But the sound channel is quiet. Only sounds generated in the sound channel itself are carried. So microphones placed within the sound channel can hear sounds that come from thousands of kilometers away.

The Navy had arranged for several such microphones placed at important locations, where they could pick up the ping of the imploding Ewing spheres. They could locate where the implosion had taken place by the time of arrival of the sound. If the sound arrived simultaneously at two microphones (for example), then they knew the sound had been generated somewhere on a line that is equally distant from the two microphones. With another set of microphones they could draw another line, and the intersection of the two lines gave the location of the downed pilot.

This doesn't make sense. The science makes absolute sense (if that IS how it works), and it's pretty awesome.

What doesn't make sense is how they were able to use this to locate downed pilots in the 1940s, and why this type of method can't be used to locate a downed plane that is filled with standard airline stuff which should act in the same way...
posted by hal_c_on at 7:51 PM on June 8, 2014

Because there probably weren't sealed pressurized canisters to spontaneously implode to make the sound at the right depth - many things would have crushed long before that. Also not like there are tons of people listening or being able to identify the sound of a 777's debris passing through that region, chances are it might not have made much noise at all considering all fuel and oxygen tanks would have been emptied before crashing and scattering on the surface.

If it was a valid form of plane wreckage finding technology it had probably been used for air France also, and the hundreds of other missing vessels as well.
posted by mrzarquon at 8:38 PM on June 8, 2014 [1 favorite]

So Roswell was the work of space whales... space whales ... Star Trek IV! It all makes sense
posted by fallingbadgers at 9:08 PM on June 8, 2014 [1 favorite]

posted by bruce at 9:55 PM on June 8, 2014

I was hoping the answer was "it is part of the ufo's warp drive," but nevermind.
posted by marienbad at 3:29 AM on June 9, 2014 [1 favorite]

The Muller page is worth it just for the explanation of Sofar spheres ...

D'ya think that's why they're called that? Like, as they're falling, they say to themselves, "so far so good, so far so good ..."

Excellent article. I also found the bit about sound curving towards the earth in the evening, and away from the earth in the morning, fascinating.
posted by iotic at 8:51 AM on June 9, 2014

I'm also curious why the SOFAR channel isn't used for civilian rescue work today. I assume that maintaining a global network of sensor stations deep beneath the ocean is extremely costly; maybe there's just no budget for it outside of military use in wartime. And it wouldn't have the secrecy benefits that it did in World War II. Most of the time, radio/satellite communication is going to be a better solution.

Also, according to this page, the SOFAR bomb used in production (the Mk 22) contained an adjustable fuse and four pounds of TNT as its "explosive sound source." You probably don't want to stuff that in the emergency kit in a civilian airplane. And any catastrophe that disabled electronic communications might also prevent deployment of SOFAR bombs.

On the other hand, the original hollow metal SOFAR spheres seem like a great fail-safe backup mechanism. And it's possible that new technology could make the underwater sensor network cheaper and/or more sensitive.
posted by mbrubeck at 11:10 AM on June 9, 2014

sound waves bend toward regions where the speed of sound is slower.

Conceivably a reason for the reports of mysterious, unreliable "hums" (and very loud noises) people on the ground have reported in various locations around the world.

Depending on the state of the atmosphere, they could be coming from higher altitudes ... or 'channeled' from a long distance before being bent ... much as radio waves are.
posted by Twang at 11:50 AM on June 9, 2014

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