The US Navy "towed pinger locator" connected to the Australian ship Ocean Shield picked up the signals in an area some 1680 km (1040 miles) northwest of Perth, which analysis of sporadic satellite data has determined as the most likely place the Boeing 777 went down.
"I'm much more optimistic than I was a week ago," Angus Houston, head of the Australian agency coordinating the search, told a news conference in Perth in western Australia, while cautioning that wreckage needed to be found for a confirmation.
"We are now in a very well defined search area, which hopefully will eventually yield the information that we need to say that MH370 might have entered the water just here."
If the signals can be narrowed further, an autonomous underwater vehicle called a Bluefin 21, will be sent to find wreckage on the sea floor to verify the signals, said Houston, who noted that the potential search area was 4.5 km (2.8 miles) deep, the same as the Bluefin range.
The black boxes record cockpit data and may reveal what happened to the plane, which was carrying 227 passengers and 12 crew when it vanished on March 8 and flew thousands of kilometres off its Kuala Lumpur-to-Beijing route.
Authorities have not ruled out mechanical problems as a cause of the plane's disappearance but say evidence, including loss of communications, suggests it was deliberately diverted.