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# Large Neutral Buoyancy Parabolic Dish

Since sounds obey the Inverse Square Law underwater as well as in air
 (+4) [vote for, against]

At the time of this writing there is an ongoing search for a missing aircraft and its passengers. Most searchers are convinced it crashed (or was deliberately crashed) into an ocean. But the ocean is a big place, and apparently special equipment has to be within a kilometer or two of the aircraft before it can detect "Find Me!" signals from the plane's "black box" flight-recorder equipment. Obviously, that's a problem that must be solved!

So, start with a parabolic microphone. Anyone interested in picking up distant sounds is aware that the shape of the dish can focus a faint sound until it becomes loud enough to be noticed and identified. And the bigger the dish, the longer the range at which it can detect faint sounds.

In principle a parabolic dish should work just fine underwater, so long as it is constructed of materials such that it reflects sound better than it absorbs or transmits sound. I have no doubt that such materials exist.

So, what if we had a parabolic dish that was 100 meters in diameter? We want the "facing" of the dish, the concave side that does sound-reflection, to be constructed of appropriate material. However, if the whole dish was constructed of that stuff, it would probably be very heavy and very akward.

So, behind that facing we can employ other materials, in constructing our overall Large Parabolic Dish. We want the overall thing to be "neutrally buoyant", with zero tendency to either sink or float. If we add some extra hardware, such as the support strut that holds the microphone at the focal point of the parabola, we ensure that that weight is balanced, too.

So that takes care of "very heavy". "Very awkward" can be addressed, also. Around the rim of this Large Parabolic Dish we place a number of "MHD thrusters" (see link). These can be computer-controlled and operate totally silently, and can be used to efficiently maneuver the overall dish into pointing in various directions. It should be possible to poke a zillion pinholes throughout the Large Parabolic Dish, to let water flow through it as the dish is re-oriented (while not much affecting the ability of the dish to reflect and focus sound waves).

In using the Large Parabolic Dish, we first "sink" it to maybe 50 meters below the surface of the ocean, so as to become unaffected by ocean waves. With its concave side downward, even if the ocean is more than 4 kilometers deep, the dish will be able to easily hear anything making noise on the ocean bottom. We can now simply tilt the dish to scan the bottom, out to the maximum range (and a 100-meter dish will have quite a range!). Then we tow it to a new spot (faster than using those onboard thrusters), and repeat.

 — Vernon, Apr 06 2014

MHD thruster http://www.rmcybern...c_thruster_mhdt.htm
As mentioned in the main text. [Vernon, Apr 06 2014]

Sound absorption in seawater versus frequency http://www.dosits.o...oceanacidification/
see second chart [xaviergisz, Apr 07 2014]

 The wavelength of 37.5kHz sound (the frequency of the black-box signal) is 4cm. And small variations in temperature or salinity can alter the speed of sound in water.

Therefore, is it realistic to expect to focus sound from a 100m dish? Isn't it more likely that the sound waves will be out of phase, and therefore won't add up at the focal point?
 — MaxwellBuchanan, Apr 06 2014

I'm kind of surprised that a system like SOSUS isn't in place for just this sort of thing. If a submarine can be triangulated from hundreds of miles away, you'd think that a sinking aircraft could be detected the same way.
 — 2 fries shy of a happy meal, Apr 06 2014

 That's because the military are very interested in submarines, and substantially less interested in civil aircraft.

The parabolic shape woud be appropriate, but should be elipsoid in outline, giving a "slot" of reception sensetivity rather than a point. On detection of a signal, triangulation using a second dish will rapidly yield a location for a more precise search.
 — 8th of 7, Apr 06 2014

 [8th of 7], since parabolic systems are inherently directional, if this thing helps to detect the black- box clicks at all, then at least you will know which direction to go to get closer to a sunken aircraft.

Of course, the more such dishes are employed in doing a search, the more ocean floor can be scanned quickly.
 — Vernon, Apr 06 2014

I'm not sure attaching thrusters to it would work - they would create noise and vibration sufficient to drown out the weak signal from the black box.
 — hippo, Apr 06 2014

[hippo], how much do you know about MHD thrusters? They can be VERY low-power, and when they cause "laminar flow" they will be totally silent.
 — Vernon, Apr 07 2014

 I'm not sure how useful a parabolic collector will be for significantly increasing the maximum listening range.

Sound waves disperse (following the inverse square law), but they are also absorbed (a function of distance only). From my very quick search, it looks like sound in seawater at 37.5kHz is absorbed/attenuated at about 10dB/km.
 — xaviergisz, Apr 07 2014

 //That's because the military are very interested in submarines, and substantially less interested in civil aircraft. //

So, shouldn't the event still be registered on somebody's radar? Or is that the sort of thing which would be on a need to know basis? The tech 'is' pretty old after all.
 — 2 fries shy of a happy meal, Apr 07 2014

 "the dish will be able to easily hear anything making noise on the ocean bottom."

I believe you are missing the point of reflectors for they do not, as you say "let you hear everything" but instead limit to limit what you hear to sources in a specific direction. If you are looking for something making a specific noise over a wide area but in a specific orientation, you would do better to drag a string of microphones behind you on a miles long array. Then you have very wide field, very high sensitivity, and the ability to gauge the distance and position of the source.
 — WcW, Apr 07 2014

 [WcW], the main text here DOES mention something about tilting the dish to scan the bottom. I'm well aware that parabolic systems are directional.

As for the string of microphones, fine. Post it as another Idea.
 — Vernon, Apr 07 2014

Vernon, what purpose does the large drag inducing cone serve? What if the array is aimed away from the source when the sound occurs? Focusing reflectors are only helpful to the extent that you know generally where the source is.
 — WcW, Apr 07 2014

[WcW], who said anything about a cone? It is quite possible for a parabolic dish to be wide and shallow, much like a "dish". Yes, I'm aware that the location of the focal-point microphone could be rather far from the surface of the dish, but the struts holding it will have lots less drag than a big more-conical dish.
 — Vernon, Apr 08 2014

 Some very rough calculations (i.e. ± an order of magnitude) indicate that the parabolic dish needs to be about 20 times bigger (i.e. 20 times more surface area) for each additional kilometre of range. Reason: factor of 2 to account for the dispersion, a factor of 10 to account for the absorption; total factor of 20.

 So if the currently used hydrophones have a 1m² surface area parabolic dish and have a range of 3km, then a 20m² parabolic dish would have a range of 4km; 400m² would have a range of 5km; 8000m² would have a range of 6km.

In summary, a 100m dish would have a range 3km greater than a 1m dish.
 — xaviergisz, Apr 08 2014

Am I crazy to think that several militaries have the tech to find the plane, but don't want to tell us where it is, as that would give away too much info on it's tech?
 — sophocles, Apr 08 2014

Only if by 'crazy' you mean 'correct', because if that's what you mean then yes, you are definitely crazy. It's not the first time such a dilemma has occurred.
 — Alterother, Apr 08 2014

 Yes, there has been a lot of speculation about that, sophocles. There has also been speculation that the secret military technology has been used to tip-off the searchers to the location. Most of this speculation has been about the satellite photos and inmarsat/ACARS radio signal.

I haven't heard any similar speculation about the secret military underwater capabilities, but I suppose this speculation is sure to occur. I imagine the underwater sound listening capabilities of military/navy submarines is fairly advanced (and better than towed pinger locators being used), but I doubt any of their submarines have a maximum depth of 4.5km that would allow visual confirmation of the MH370.
 — xaviergisz, Apr 08 2014

Oil companies have really good ways of telling what's under the water too. And, under the rock under the water. They share just a bit of this tech publicly, but the rest is competitive trade secret.
 — sophocles, Apr 08 2014

 //Am I crazy to think...//

 A lot of the conspiracy theories over MH370 arise from the belief that nobody could have fucked up so badly as to lose track of a plane carrying 239 people.

These theories are ill-founded.
 — MaxwellBuchanan, Apr 08 2014

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