By measuring the differences between emitted sound pulses and their echoes sonar is able to detect and identify targets such as reefs, wrecks, submarines and fish shoals. However, standard sonar has limitations in shallow water because bubble clouds, which result from breaking waves or other causes, can scatter sound and clutter the sonar image. Inspired by the exceptional sonar capabilities of dolphins, scientists have now developed a new underwater device that can outperform standard sonar and detect objects through bubble clouds.
If I were a dolphin...Like many scientific breakthroughs, Professor Timothy Leighton of the University of Southampton’s Institute of Sound and Vibration Research (ISVR) took inspiration from nature to develop the new sonar concept called twin inverted pulse sonar (TWIPS).
“To catch prey, some dolphins make bubble nets in which the best man-made sonar would not work. It occurred to me that either dolphins were blinding their sonar when making such nets, or else they have a better sonar system,” said Leighton.
However, because there were no recordings of the type of sonar that dolphins use in bubble nets, Leighton wasn’t able to produce a bio-inspired sonar simply by copying dolphin signals. Instead, he sat down and worked out what pulse he would use if he were a dolphin.
Inverted twinned pairs of sound pulsesThe TWIPS system he and his colleagues devised exploits the way that bubbles pulsate in sound fields, which affects the characteristics of sonar echoes. It does this by using twinned pairs of sound pulses. The first pulse of each pair has a waveform that is an inverted replica of that of its twin and is emitted a fraction of a second before its inverted twin.
First, Leighton’s team showed that theoretically, TWIPS might be able to enhance scatter from the target, while simultaneously suppressing clutter from bubbles. Therefore, in principle, it could be used to distinguish echoes from bubble clouds and objects that would otherwise remain hidden.
Put to the testThe team then used a large test tank to test the concept and found that TWIPS outperformed standard sonar at detecting a small steel disc under bubbly conditions resembling those found under oceanic breaking waves.
Encouraged by their findings, the team then headed to sea to conduct more trials. On Southampton Water, a tidal estuary with a seabed varying in depth between 10 and 20m (33-66 ft) that handles seven percent of the UK’s entire seaborne trade, they compared the ability of TWIPS and standard sonar to discern the seabed.
“TWIPS outperformed standard sonar in the wake of large vessels such as passenger ferries,” said co-author of the study, Dr Justin Dix of the University of Southampton’s School of Ocean and Earth Science (SOES).
Interestingly, even though dolphins were the inspiration for TWIPS, it’s still not known whether they actually use such a system.
Key ingredients of a TWIPS system appear in separate species but they have never been found all together in a single species,” said Leighton. “There is currently no evidence that dolphins use TWIPS processing, although no-one has yet taken recordings of the signals from animals hunting with bubble nets in the wild. How they successfully detect prey in bubbly water remains a mystery that we are working to solve.”