The frequency used by aircraft flight recorders was chosen because no other devices use it, and because nothing in the natural world mimics it, said William Waldock, a search-and-rescue expert who teaches accident investigation at Embry-Riddle Aeronautical University in Prescott, Ariz.

"They picked that so there wouldn't be false alarms from other things in the ocean," he said.

But these signals are being detected by computer sweeps, and "not so much a guy with headphones on listening to pings," said U.S. Navy spokesman Chris Johnson. So until the signals are fully analyzed, it's too early to say what they are, he said.

"We'll hear lots of signals at different frequencies," he said. "Marine mammals. Our own ship systems. Scientific equipment, fishing equipment, things like that. And then of course there are lots of ships operating in the area that are all radiating certain signals into the ocean."

The Ocean Shield is dragging a ping locator at a depth of 3 kilometers (1.9 miles). It is designed to detect signals at a range of 1.8 kilometers (1.12 miles), meaning it would need to be almost on top of the recorders to detect them if they were on the ocean floor, which is about 4.5 kilometers (2.8 miles) deep.

U.S. Navy Capt. Mark Matthews said the signals picked up by the Ocean Shield were both 33.3 kilohertz, the manufacturer indicated the frequency can drift in older equipment.

He said the underwater vehicle, the Bluefin 21 autonomous sub, can create a sonar map of the area to chart any debris on the sea floor. If it maps out a debris field, the crew will replace the sonar system with a camera unit to photograph any wreckage.

The water depth there is right at the limits of the sub's capability.

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