Calling it the "most promising lead" so far, the leader of the search for a missing Malaysia Airlines jetliner says ships have again detected a signal on the same frequency used by "black box" emergency beacons. But Angus Houston also reiterated that it's too early to draw conclusions.

Houston, a retired Australian Air Chief Marshal who is heading the search effort based in Perth, says the signal "sounds to me just like an emergency locator beacon," according to Australia's ABC news agency.

The signal was picked up by Australian navy ship the Ocean Shield, which was investigating an "acoustic event" it detected over the weekend in the southern Indian Ocean. The ship is about 370 miles away from where a Chinese patrol ship reported a similar signal over the weekend. Officials say they aren't sure whether the signals are from the same source.

The Australian ship is using a towed pinger locator that's on loan from the U.S. Navy. A British ship, the HMS Echo, which CNN says "has state-of-the-art sonar and is capable of mapping the ocean floor," is investigating the area where the Chinese ship reported a signal.

While the signal detected by the Chinese ship lasted less than two minutes, the Ocean Shield picked up the signal twice, monitoring it for more than two hours in one span, and about 13 minutes in another.

Houston said ships are still searching for debris in the ocean that could help confirm the signal's potential import. In explaining his new optimism, he also said the pings heard clearly came from two sources.

"On this occasion, two distinct pinger returns were audible," Houston said. "Significantly, this would be consistent with transmissions from both the flight data recorder and the cockpit voice recorder,"

The ultrasonic signals detected over the weekend have a reported frequency of 37.5 kHz, the same used by emergency beacons. But it's also used by other marine equipment, as experts said when the first detections were reported over the weekend. And that's one reason officials are urging people not to jump to conclusions about the new report.

Adding a sense of urgency to the current search is the belief that the jetliner's emergency beacons would only have enough battery power to transmit their location for around 30 days.

It has now been more than four weeks since 239 people boarded flight MH370 in Kuala Lampur, hoping to travel to Beijing; the Boeing 777 disappeared from air traffic control systems shortly after takeoff. The search for the plane has been marked by confusion, adding to the anguish felt by the families of those aboard.

Australian officials warn that any recovery effort of the black boxes or any other parts of the plane would likely be a prolonged and difficult affair. The waters where the signal has been picked up are extremely deep — 4,500 meters, or 2.8 miles.

And the water itself can also make it hard to pinpoint a signal's location. Commodore Peter Levy, who leads Australia's military task force, explains that issue to ABC:

"Unlike in air, where sound travels in a straight line, acoustic energy, sound, through the water is greatly affected by temperature, pressure, and salinity. That has the effect of attenuating, bending, sometimes through 90 degrees, soundwaves."

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