The signals detected 1,645 kilometers (1,020 miles) northwest of Perth are the strongest indication yet that the plane crashed and is now lying at the bottom of the ocean in the area where the search is now focused. Still, Houston warned he could not yet conclude that searchers had pinpointed Flight 370's crash site.
"I think that we're looking in the right area, but I'm not prepared to say, to confirm, anything until such time as somebody lays eyes on the wreckage," he said.
Finding the black boxes quickly is urgent because their locator beacons have a battery life of about a month, and Tuesday marked one month since the plane vanished with 239 people on board. If the batteries fail before the black boxes are located, finding them in such deep water about 4,500 meters, or 15,000 feet would be immensely difficult, if not impossible.
The Ocean Shield is towing a pinger locator from a U.S. Navy that is designed to detect signals from a plane's two black boxes the flight data recorder and cockpit voice recorder.
A data analysis of the signals the ship heard Saturday determined they were distinct, man-made and pulsed consistently, Houston said.
"They believe the signals to be consistent with the specification and description of a flight data recorder," he said.
To assist the Ocean Shield, the Australian navy was using parachutes to drop buoys in a pattern near where the signals were last heard.
Royal Australian Navy Commodore Peter Leavy said each buoy will dangle a hydrophone listening device about 300 meters (1,000 feet) below the surface. The hope, he said, is the buoys will help better pinpoint the location of the signals.
Houston acknowledged searchers were running out of time, noting the last two signals were weaker and briefer than the first pair, suggesting the batteries are failing. One Saturday lasted two hours and 20 minutes and the second lasted 13 minutes; those heard Tuesday lasted just 5 and a half minutes and 7 minutes.
"So we need to, as we say in Australia, 'make hay while the sun shines,'" Houston said.
The weakening of the signals also could indicate the device was farther away, U.S. Navy Capt. Mark Matthews said. Temperature, water pressure or the saltiness of the sea could also be factors.
Leavy said thick silt on the ocean floor also could distort the sounds, and may hide wreckage from the eventual visual search.
Houston said a decision had not yet been made on how long to use the towed pinger locator while knowing the beacons' batteries will likely fail soon, saying only that a decision to deploy an unmanned submarine was "not far away."
"Hopefully in a matter of days, we will be able to find something on the bottom that might confirm that this is the last resting place of MH370," he said.
When the pinger locator's use is exhausted, the unmanned sub will be sent to create a sonar map of a potential debris field on the seafloor. The Bluefin 21 sub takes six times longer to cover the same area as the pinger locator.
Matthews said the detections indicate the beacon is within about a 20-kilometer (12-mile) radius, equal to a 1,300-square-kilometer (500-square-mile) chunk of the ocean floor.
That amounts to trying to find a desktop computer in a city the size of Los Angeles, and would take the sub about six weeks to two months to canvass. So it makes more sense to continue using the towed pinger locator to zero in on a more precise location, Matthews said.
"It's certainly a man-made device emitting that signal," Matthews said. "And I have no explanation for what other component could be there."
The Bluefin sub's sonar scans about to 100 meters and can "see" with lights and cameras only a few meters. Its maximum dive depth is 4,500 meters, and some areas of the search zone are deeper.
The disappearance of Malaysia Airlines Flight 370 on a trip from Kuala Lumpur to Beijing on March 8 is one of aviation's biggest mysteries. The black boxes' data may be the only way to explain why the plane lost communications and turned sharply off-course, before ending up far to the south in the remote southern Indian Ocean.
The audio search was narrowed to its current position after engineers predicted a flight path by analyzing signals between the plane and a satellite and investigators used radar data to determine the plane's speed and where it may have run out of fuel.
Houston noted that all four of the pings detected in recent days were near a final, partial "handshake" signal revealed earlier in the investigation.
He also noted the surface search for any floating debris has been adjusted and intensified based on where the four pings were heard and where ocean currents might have caused debris to drift. Fifteen planes and 14 ships searched a 75,400-square-kilometer area that extends from 2,250 kilometers northwest of Perth on Wednesday.
Despite the challenges still facing search crews, those involved in the hunt were buoyed by the Ocean Shield's findings.
"I'm an engineer so I don't talk emotions too much," Matthews said. "But certainly when I received word that they had another detection, you feel elated. You're hopeful that you can locate the final resting place of the aircraft and bring closure to all the families involved."
Gelineau reported from Sydney. Associated Press Writer Rod McGuirk in Canberra, Australia, contributed to this report.