– How could a Canadian P3 aircraft, while flying over the Atlantic Ocean, possibly detect ‘banging noise’ attributed to a small submersible vessel potentially thousands of feet below the surface?
In: 3042
Hey! Another question I can answer from on the job experience! [Related to my other answer to an ELI5 from yesterday](https://reddit.com/r/explainlikeimfive/comments/14edr8k/eli5_how_does_a_submarine_in_this_day_and_age_not/jow553j). Hopefully I don’t stay up till 3 am like yesterday replying to genuine curiosity.
Got no time for a tome? Okay. So much for the short and sweet ELI5. The TL;DR on top: Planes can deploy sonobuoys that transmit sound they hear over radio and can be sensitive enough to detect the clunk sounds from 4km away. With many buoys, math, and luck one could generate a smaller area for a ship with a remotely operated sub to search, than the, “well they’re down there somewhere below me” that the mother ship could give since there was no acoustic beacon on this thing to start with.
Have time and are interested? Read on.
So others have mentioned the aircraft is a CP140, which is a Canadian derivative of the P3. Similar, yet different. Different hardware on board, and a bunch dedicated to underwater acoustics. MAD really isn’t a factor because this sub is tiny and very far underwater Moving on!
History:
As the cold war kicked off, it was figured out quickly that you want to be able to detect the “other guy’s sub” far outside the range that they could fire a torpedo at a ship. What can get far away fast and cover great distance? An airplane. Wait, how can they listen to the sub like the sonar dudes on the ship?
Well, how about we stick a radio to a hydrophone so it broadcasts what it hears to the plane, and put it in a package the plane can punt out the back then tune into the sounds of the sea (maybe a sub, too)? Sounds crazy, let’s do that!
And from that point on, the Sonobuoy (sonar-buoy) was born.
So basic concept here, we’ve solved the problem of having a plane that can’t park on the water keep an ear out for sounds of a sub trying to pull a sneaky (okay, there’s helicopters now, too but nevermind that right now). Nuclear subs from the cold war weren’t very quiet, so passive sonobuoys was a good option to keep tabs and figure out who was in the neighborhood. However, diesel subs can be very quiet, so you need to ping on them with active sonobuoys, and either time the echo with a watch, or use something fancy like a computer to display a range when those shrank to a size that could be carried on board.
Enough history. Fast forward to our CP140 on station.
‘Buoys have advanced from just a single microphone in the water to arrays that can either improve sensitivity to sound in any direction (like a vertical array of several microphones), or be able to give a direction to the sound (with at least four microphones in the corners of a horizonal square array). You can set these buoys to deploy to a preset depth once they hit the water. That can help improve detecting something that is trying to sneak like a sub with quiet humming motors.
However, the “sub” in question really wants to be found, so every 30 min someone is banging on a surface that’s got water on the other side. The bigger the clang, the more sound that will go out into the ocean.
In that novel of an answer in the link above, I mentioned that sound in sea water travels about 1500m/s and if you’re directing as much energy into it as possible, you’ll benefit from the good conductivity of sound that water has. So even at 4km from floor to surface, you won’t loose too much that a sonobuoy won’t be able to detect that clunk.
Problem is, if a clunk can go the distance, then so can the big honking diesel engines that drive propulsion or power generation on board any ships in the area. So noise could potentially mask clunks. But if you can figure out a part of the sound spectrum that contains most of the clunk sound, you will be able to isolate and detect it better, leading to other wizardry through the magic of more buoys, geometry and computer power.
Fixing the position of sound:
So you detect the clunk. That’s good -someone’s home and wants company in the form of a pickup. But where did this clunk come from? Hopefully the surface. But of course they painted this thing white, instead of something that really stands out like neon green or orange. Let’s assume they’re at or near the surface but radar or infrared/optical sensors aren’t getting anything.
How do you pinpoint the sub? With more buoys. If you drop an assortment of buoys at various locations, and several of them hear the clunk, then the difference of time between the sound reaching the first buoy and the others can mean you can plot curved lines which are calculated by that time difference. Do that many times between at least three -but like gps more is better- buoys and if these curves all intersect in one area, then “x” marks the spot – like Indiana Jones once begrudgingly said in a library.
Did I mention buoys can find the direction of a sound? That would be too easy – plot lines from each buoy get your technicolor “x” and leave the fancy mathemagics at home. Only one problem, short sounds might generate too much ambiguity that the processor will point a different direction each time for each buoy hearing the clunk. So back to the time differences and nerd tools. BTW, this is called hyperbolic fixing, because the shape of the curve is a hyperbola – not very ELI5, but good for a Google search for those who would like to know more. Steve Mould on YouTube featured a system that was a “sound camera” and it used a bunch of microphones around a camera. Very neat, same principle, and worth a watch because it’s a video.
But what if they’re on the bottom and the sonobuoys are near the surface?
That’s certainly a complicating factor. Ideally, you’d want the sensors at the same depth as the source, as it means you’re only worried about horizonal distance. However, adding up almost 4km of vertical distance to the few hundred meters of horizontal distance can really create some error. But, if they consistently result in an overlap covering an area where folks could get an ROV with a higher resolution sonar to look, that’s better than going in blind.
I skipped over bending of sound rays, or propagation loses, attenuation of higher frequencies and a lot of other things that are important considerations, but will leave someone wanting an intro cross-eyed.
Hope this helped introduce a “whole new world” (don’t sue me, Disney).
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