Secret underwater messages could be camouflaged as the clicks and whistles of whale or dolphin calls, fooling eavesdroppers by making them believe they are hearing marine animals.
Marine mammal sounds can affect military sonar systems, so they are usually deemed ocean noise and filtered out. This makes these animal signals a stealthy solution for communicating secretly underwater.
To test the idea, and her colleagues at Tianjin University in China to hide code in the vocal calls of false killer whales (Pseudorca crassidens), which can then be transmitted and deciphered.
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Mimicking marine mammals to communicate underwater isn鈥檛 a new idea. But previous approaches mainly used artificial sounds that imitate animals and focused on just one type of call 鈥� either clicks or whistles 鈥� limiting their camouflage potential. Since many whales and dolphins live in groups, their calls naturally overlap. If someone heard only one kind of call in isolation it would raise suspicions.
In the latest work, the researchers scoured a database of clicks and whistles made by false killer whales, a species of dolphin, to find sounds with sufficiently different waveforms 鈥� the squiggly visualised shapes of sound 鈥� to use in a decipherable code.
Using the sounds, the team constructed two coded sequences 鈥� one with just clicks and one with just whistles. The codes were based on small differences in the time delay between the sounds in the sequence. The two sequences were then overlapped, forming one sequence that, it was hoped, sounded natural because it combined clicks and whistles.
Sure enough, the resulting waveform pattern closely resembled that of false killer whale chatter. What鈥檚 more, while the clicks and whistles could be separated and decoded by a recipient, they duped an artificial intelligence 鈥渆avesdropping鈥� program trained to identify codes in acoustic communications, suggesting the technique could be reliable enough to transmit information in a secure fashion.
However, the achieved communication rate was 76 bits per second over 5 kilometres, which is slow by current underwater communications standards, says underwater acoustics researcher at Curtin University in Perth, Australia.
鈥淭he performance of this system would certainly be poorer in the real world than in simulation [due to distortion and interference], but it could probably be made to work to some extent,鈥� says Duncan. 鈥淭here are situations in which the covert transmission of a small amount of data can still be crucial, so it should certainly not be discounted.鈥�
Applied Acoustics
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