(CNN) -- Crews looking for Malaysia Airlines Flight 370 are narrowing the search down to an even smaller area.
They're trying to find the source of sounds picked up by underwater listening equipment--sounds that are said to be like the signals sent out by an airliner's flight recorders. The signals haven't been heard again since the weekend.
And it's now a race against the clock. The batteries that power the beacon on a flight recorder typically die after thirty days-- although they may last longer. The plane disappeared more than a month ago.