HONG KONG — Scientists plan to release detailed information Wednesday about a mysterious noise, possibly that of an ocean impact, recorded by two undersea receivers in the Indian Ocean about the time Malaysia Airlines Flight 370 ceased satellite transmissions and vanished March 8.
The low-frequency noise, which was outside the normal range of hearing and had to be sped up to be made audible, appeared to have traveled halfway across the Indian Ocean to the receivers off the coast of Australia.
“It’s not very exciting. It’s not even really a thump sort of a sound — it’s more of a dull oomph,” said Alec Duncan, a senior marine science research fellow at Curtin University near Perth, who has led the research.
“If you ask me what’s the probability this is related to the flight, without the satellite data it’s 25 or 30 percent, but that’s certainly worth taking a very close look at,” Duncan said.
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The general vicinity from which the noise emanated is a large area of the central Indian Ocean off the southern tip of India and about 3,000 miles northwest of Australia. But that is not consistent with calculations of an arc of possible locations in the southeastern Indian Ocean where the plane, carrying 239 people, might have run out of fuel. Those calculations were from Inmarsat, the global satellite communications company, and scientists have struggled to figure out the origin of the noise.
A very small section of the arc that was searched by a deep-sea submersible this spring, without success, was about 600 miles off the Australian coast.
Even without the Inmarsat data, there are other possible explanations for the noise besides an aircraft impact, Duncan said. One would be a very small undersea earthquake that would produce a similar noise audible across great distances but not strong enough to show up on the nearest seismometers on land, which did not record a tremor at the time.
The direction from which the noise arrived does produce some small earthquakes every year, although there had not been one in the days preceding Flight 370’s disappearance, Duncan added.
Two receivers — one operated by Duncan’s team and the other by the Comprehensive Nuclear Test Ban Treaty Organization in Vienna — produced enough data to establish the direction from which the noise arrived. But the distance to the source of the noise was less clear. The result, according to Duncan, is an area of ocean roughly the size of Texas.
Mark Prior, an acoustics expert at the test ban organization headquarters, said the sound might be consistent with an ocean impact or with some kind of a sealed, air-filled container that sank into the depths until the exterior water pressure caused it to crumple.
Prior said the sound might have been reflected off a seamount, or underwater mountain, before traveling to the two receivers, so the origin of the noise could have been somewhere else.
The Australian government has funded the research at Curtin University, and additional experts in Canberra, the country’s capital, have also collaborated. The Australian authorities have been skeptical so far about ordering a new search thousands of miles away based on the noise.
Martin Dolan, chief commissioner of the Australian Transport Safety Bureau, said last week that search operations would continue to focus on the narrow arc defined by the Inmarsat calculations.
Duncan and his team operate an undersea device off the coast of Perth that is mainly used to monitor the low-frequency cries of blue whales, the world’s largest mammals and one of its most endangered species. The team’s receiver, or hydrophone, records noises on an undersea data device, which is hauled up to the ocean surface every six months and its contents downloaded.
The team sailed out for an extra visit to pull up the recorder and recover the data after the search for Flight 370 shifted to the Indian Ocean. The noise was faint but detectable, prompting Duncan to request and obtain the test ban treaty organization’s data for March 8 from its receiver off Cape Leeuwin, 220 miles to the south.
The staff did not initially detect any noise, Prior said, because it was using a high threshold for confirming a noise, based on its experience with underwater explosions. It checked the records of its other operating hydrophone in the Indian Ocean, near Diego Garcia, but the noise was not detected there. Its undersea receivers continuously relay data by cable to shore, where it is uploaded by satellite to Vienna.
Noises travel through water at a speed of almost a mile per second. That is five times faster than they travel through the air, and there is less attenuation of the sound over very long distances through water as well.
Prior and Duncan said in separate telephone interviews that the time it took for the noise to reach the receivers was consistent with the last partial electronic “handshake” between the missing Boeing 777-200 and the Inmarsat satellite, plus the time it would have taken for the sound to have traveled several thousand miles to the hydrophones.