London, June 26 : American and Italian researchers say that a technique called 'acoustic time reversal' can facilitate wireless broadband communication under the sea, a task that has been difficult to accomplish thus far because water molecules absorb radio waves.
The researchers say that this technique can clean up underwater sound signals, and thereby extend their range and capacity.
William Kuperman and colleagues at the Scripps Institution of Oceanography in La Jolla, California, has revealed that he and researchers from the NATO Undersea Research Centre in La Spezia, Italy, are testing the technique in the Mediterranean.
He says that the time reversal technique exploits the way undersea acoustic signals typically arrive clouded by echoes that travel at different speeds.
The researcher points out that a "ping" may arrive as three separate sounds - one that travel directly, an echo from the surface, and then an echo from the ocean floor.
He says that when the same sequence of sounds is transmitted backwards, they take the same routes back to the original source.
All three sounds arrive at about the same time at the original source because they are sent in such a manner that the one that took longest to travel goes first, the second slowest next, and the fastest last.
Consequently, they converge in time and reconstruct the original signal, say the researchers.
Kuperman says that the retransmitted sounds also create echoes of their own, but the original signal is strong enough to stand out.
He has revealed that, so far, his tema has managed to use the technique to transmit 15 kilobits a second at a range of 4 kilometres, and 5 kilobits per second at 20 km.
According to him, the technique even worked over 3,500 km, comparable to the distance some whales can communicate with song, though the data rate fell to only about 100 bits per second.
Geoffrey Edelmann, a physicist at the US Naval Research Laboratory in Washington, DC, says that time reversal is considered to be the best way to improve acoustic communications, and that Kuperman's team has achieved the best results.
"Their work is the best. I think they are leading the charge at the moment," New Scientist magazine quoted him as saying.
Kuperman will make a presentation on his team's work at the Acoustics 08 meeting in Paris on 1 July.