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Leighton and a dolphin  (Source:
Twin inverted pulse sonar concept can detect targets through bubble clouds caused by breaking waves

University of Southampton researchers have created an underwater sonar device capable of detecting objects through bubble clouds. 

Professor Timothy Leighton, study leader from the University of Southampton's Institute of Sound and Vibration Research (ISVR), and his team, have developed a sonar device that can "see" through bubble clouds through the use of a new sonar concept. 

Standard sonar devices already in use utilize differences between sound pulses and their echoes to identify objects such as wrecks, reefs, fish and submarines. But when it comes to bubble clouds, which are created mainly from breaking waves, these conventional sonar devices' sound becomes scattered and the sonar image is blurred. 

"Cold war sonar was developed mainly for use in deep water where bubbles are not much of a problem, but many of today's applications involve shallow waters," said Leighton. "Better detection and classification of targets in bubbly waters are key goals of shallow-water sonar."

To remedy poor detection in bubbly waters, Leighton and his team created a new sonar concept called twin inverted pulse sonar (TWIPS). TWIPS dissects the way bubbles pulsate in sound fields, which is an act that influences the characteristics of sonar echoes. 

TWIPS' design consists of trains of twinned pairs of sound pulses where the first pulse of each pair has a waveform that is an inverted duplicate of that of its twin. The first pulse is sent exactly a fraction of a second before its inverted duplicate. According to Leighton, this concept was inspired by dolphins. 

"To catch prey, some dolphins make bubble nets in which the best man-made sonar would not work," said Leighton. "It occurred to me that either dolphins were blinding their sonar when making such nets, or else they have a better sonar system. There were no recordings of the type of sonar that dolphins use in bubble nets, so instead of producing a bio-inspired sonar by copying dolphin signals, I sat down and worked out what pulse I would use if I were a dolphin."

Leighton and his team have tested the TWIPS concept in a large testing tank as well as at sea on the University of Southampton's coastal research vessel, the RV Bill Conway. At sea, TWIPS was tested against conventional sonar when challenged to identify the seabed in Southampton Water, which ranges between 10 and 20 meters in depth. 

"TWIPS outperformed standard sonar in the wake of large vessels such as passenger ferries," said Dr. Justin Dix, co-author of the study from the University of Southampton's School of Ocean and Earth Science (SOES). 

Leighton and his team would like to see TWIPS used in harbor protection as well as an aid in the identification of marine sediments and manufacturing. Further research could lead to TWIPR (twin inverted pulse radar), which could be used to detect explosives or covert circuitry.

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RE: Why not just learn from nature?
By Solandri on 11/18/2010 5:19:12 PM , Rating: 5
Bubbles in themselves are pretty impenetrable by sonar. How much of the sound wave is absorbed or reflected by a medium change depends on the ratio of the density of the two media. Going from low density to high density results in fairly good transmission (if you dive into a swimming pool, you can still hear people above). But going from water to air has such a huge ratio of high to low density that it results in almost perfect reflection. (If you're standing above the pool, you can't hear what's going on in the water.)*

The image you're seeing through bubbles is because they're an imperfect screen - they have lots of gaps between individual bubbles. If it had been a solid sheet of air, you'd be getting no return beyond the bubbles. Concrete, metal, and stone are actually pretty easy to see through. Since their density is not that far off from water (typically 2:1 to 5:1 for stone to 10:1 for metal), they transmit a fairly good percentage of the sound wave when underwater.

I suspect by using twin inverted pulses, they're able to distinguish between the returns which come from the bubbles, and returns which come from something solid beyond. Then they can just subtract the bubble returns to get a clear(er) picture of what's beyond them.

*This is also the reason fish without swim bladders like tuna are harder to spot with fishfinders. Most of the signal return from a fish is when the sound wave hits the low-density swim bladder and has a perfect reflection. Fish without swim bladders are effectively stealth, and you need a more powerful transducer to get a readable signal off of smaller density changes like bone. (Muscle has almost the same density as water so generates very little return.)

RE: Why not just learn from nature?
By Solandri on 11/18/2010 5:20:22 PM , Rating: 2
I should add, for comparison, the density change from water to air is about 800:1.

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