The search teams looking for the wreckage of Malaysia Air Flight 370 believe they have heard a total of four pings from the “black boxes” in the last few days — two of which were received yesterday. The signals came from the narrowing search area off of Australia’s northwest coast, on frequencies consistent with those of commercial-aircraft flight recorders. After more than a month, though, time is running out on locating them through their beacon transmissions:

Since Saturday, the Australian navy ship Ocean Shield — equipped with a U.S. Navy black-box detection system — has picked up four separate transmissions, the two most recent of which came on Tuesday. The newest signals are significant, because the increased data could allow searchers to more accurately predict the location on the Indian Ocean floor where the sounds are originating.

Australian officials caution that they cannot yet determine whether the sounds are coming from the remains of Malaysia Airlines Flight MH370, which disappeared March 8 with 239 people on board. But Houston said that the signals are a “great lead,” one that has helped to drastically narrow a search field that once spanned much of the world’s largest continent and third-largest ocean. The area now being scoured by search teams is about the size of South Carolina.

Those acoustic signals, according to Australian analysis, match the kind emitted by black boxes and take the form of metronomic pings recurring just shy of once every second. Analysts looking at the first two transmissions determined that they were “not of natural origin,” Houston said, “and likely sourced from specific electronic equipment.”

It’s still like looking for a needle in a haystack, but the haystack has gotten much smaller in the last few days. The deployment of 14 ships and 15 aircraft still have not found wreckage from the aircraft on the surface, but this lead should draw them in more closely to where the remains of the aircraft eventually settled. As the Washington Post notes, it took 20 days of intensive searching for the flight recorders from the Air France that went down in 2009 off of South America, and searchers had a clearer idea of the search area in that case.

The man in charge of the search effort declared himself “optimistic” that this effort had finally reached that stage:

Angus Houston, the head of a joint agency coordinating the search in the southern Indian Ocean, said on Wednesday that the Australian navy’s Ocean Shield had picked up two more underwater signals that could be from Flight 370.

Houston said that the newly detected signals had not narrowed the search area enough to dispatch a submersible to search for possible wreckage, but did express his belief that searchers are finally “looking in the right area.”

“I’m now optimistic that we will find the aircraft, or what is left of the aircraft, in the not too distant future — but we haven’t found it yet, because this is a very challenging business,” he said. “We need to visually identify aircraft wreckage before we can confirm with certainty that this is the final resting place of MH370,” he said.

If you need a primer on flight-recorder transmissions, Time Magazine offers this video explaining the technology — and its limitations. The manufacturer’s best guess is that the signal heard by the search team is one of his products. They certainly hope he’s right.