(CNN) -- The optimism that bubbled up over the weekend when an Australian navy ship detected pulses that appeared to signal the nearby presence of Malaysia Airlines Flight 370's so-called black boxes had subsided somewhat by Tuesday, as continuing listening efforts yielded nothing.
Searchers are still scouring the waters, but their optimism is "more cautious," said U.S. Navy Cmdr. William Marks. "As hours pass," he said, "our optimism is fading away, ever so slightly."
His restraint contrasted with the cheers that erupted Saturday when the team aboard Australia's Ocean Shield detected a possible signal from the plane's flight data recorder or its cockpit voice recorder. A second possible signal was heard soon after.
Investigators hope the signals were from locator beacons that were attached to the data and voice recorders that were stored in the tail of the Boeing 777-200ER when it disappeared from radar screens on March 8.
"The audible signal sounds to me just like an emergency locator beacon," Houston said. "We are encouraged that we are very close to where we need to be."
If the signals are heard again, searchers could deploy an underwater drone to take photos to determine whether they do mean the discovery of the black boxes. That process could take more than a week.
"Until we have stopped the pinger search, we will not deploy the submersible," Houston said. "We will not deploy it unless we get another transmission in which we'll probably have a better idea of what's down there."
First signal continued for more than 2 hours
The first signal, detected by the towed pinger locator dragged behind the Australian ship, continued for more than two hours; the second for about 13 minutes. But since then, there's been silence.
"That the Ocean Shield has not reacquired the ping is a genuine setback," said Peter Goelz, former managing director of the National Transportation Safety Board. "We're talking now an investigation that could go on months, if not years."
"I think the pings that they did hear were tremendously helpful," said Mary Schiavo, former inspector general for the U.S. Department of Transportation. But more pings would have been more helpful, she said, noting that those that were identified could have emanated from anywhere within a 5-mile radius. "If they had been able to pick up more pings, they could really have reduced that size and then given the submersibles just a small area to search."
Still, she expressed hope that -- even if the batteries powering the locator beacons have died -- the flight data recorder and cockpit voice recorder can be found and, with them, clues to what happened.
"It just means that the submersibles are going to have a really long -- as they say -- mowing the ocean patch" over an area the size of Houston, Texas, she said.
Tuesday marks day 32
But searchers, mindful that the batteries could work for days more, have kept listening.
"We need to continue ... for several days right up to when the point at which there's absolutely no doubt that the pinger batteries will have expired," said Houston.
"We know that the batteries can last up to 40 days," retired Royal Air Force Lt. Col. Michael Kay told CNN. "If I was Angus Houston, I would be putting the search out to at least 42, 43 (days) to make absolutely sure that the batteries had failed."
"I think they're going to keep towing for the sound, probably another week or 10 days," Goelz said. "Then they're going to have to reassess."
That reassessment could lead officials to begin using side-scanning sonar, a painstaking process of searching the depths for signs of the missing airliner.
"I think we're in a very critical stage of the search, and some big decisions are going to have to be made in the next week or so," Goelz said.
Searchers are looking for debris over 30,000 square miles (more than 77,500 square kilometers) of the Indian Ocean, about 1,400 miles (2,250 kilometers) northwest of Perth. That's about a third of the size of the previous search zone and is based largely on Inmarsat data about where the plane may have traveled, said CNN analyst David Gallo.
The search area presents major challenges
The area of ocean is deep, and a cyclone that packed wind speeds of more than 160 mph churned through two weeks ago, when crews were focused elsewhere in the Indian Ocean. The storm could have further spread any debris.
"This was an area that looked like a washing machine in the first place, but now we know it was even worse than that," CNN meteorologist Chad Myers said.
But Tuesday's weather was calm, with no major systems near where the pings were detected, said CNN meteorologist Sherri Pugh.
Chinese ship detected pulses
Teams are also still investigating pulses detected Friday and Saturday by a Chinese ship about 600 kilometers (375 miles) southwest of where the Ocean Shield is searching.
The signals detected by the Chinese weren't as sustained as those picked up by the Ocean Shield, and the Chinese vessel's detection gear isn't as advanced as the U.S. pinger locator.
Houston said Monday that they were probably separate events.
Some friends and relatives of passengers said they were keeping their hopes in check.
"Until they physically locate the bulk of the plane with the black box intact and passenger bodies, I won't believe it," said Sarah Bajc, the partner of American passenger Philip Wood.
At a candlelight vigil in Beijing on Monday night, some relatives sobbed and others bowed their heads.
"If the plane is there, it's there. We can't change it," the husband of one passenger said. "But I am still hoping for a miracle to happen."
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