THE sound of the ocean is being harnessed to create a covert message system for warning ports of intruders. The system cuts out chunks of the ocean鈥檚 natural background noise to create digital codes.
Dennis Jones and his colleagues at Defence R&D Canada, a naval research lab in Dartmouth, Nova Scotia, are developing the system to protect Canada鈥檚 naval bases.
The usual way to protect a base is to scatter microphones on the seabed at the entrance. The signals from these hydrophones are monitored for the sound of vessels that should not be there. But the signal used by the hydrophones to contact a central radio buoy can be detected easily. So intruders could flee, try a different route, or home in on the hydrophones and disable them.
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Jones wants a system in which the hydrophone array communicates as silently as possible with the radio buoy. And after two sets of Atlantic sea trials, he told the Acoustical Society of America meeting in Minneapolis on 21 October that he believes he has found it.
His answer is to use the ocean鈥檚 own background noise. Each sensor in a seafloor array records a sound clip, a few minutes long, of the natural ocean noise 鈥� caused by wave action, surface wind, rain and even snapping shrimp.
When the hydrophone senses a boat and wants to send a warning, it plays back the ocean noise clip through a loudspeaker on a loop, but with an important difference. To represent a digital 鈥�0鈥�, the system blocks out a 100-hertz-wide band of frequencies from the background noise. To represent a digital 鈥�1鈥�, it subtracts a 200-hertz-wide band. A receiver up to 500 metres away will detect the missing frequencies and decode the message.
鈥淥ur tests showed this noise-like signal is very hard to detect,鈥� says Jones. Because the signal is heavily disguised by ocean noise, and the precise frequencies that are chopped out are changed minute by minute, the signal will be imperceptible to all but the most persistent intruder. The sound level is also 20 decibels quieter than existing transducers.
Jones is also working with marine biologists to build a sensor network for tracking the endangered right whale, which has a migration route that crosses busy shipping lanes. In bad weather, whale-watchers cannot go out to warn ships of the whale鈥檚 whereabouts, but Jones says his system can track whale song in any weather, and without disturbing the animals. Because it replays ocean noise samples through a loudspeaker, the signal is always slightly louder than the ambient noise at the time, even in a storm.
