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Anchorage - Glaciologist Erin Pettit began a research project to find out what humpback whales heard when a big piece of ice falls from a glacier and crashes into the ocean. But the sound generated by ice drifting in the water turned out to be just as interesting.
Acoustic research in Alaska's Icy Bay and other glacier ice-filled waters found that the fizz created by the release of pressurized air bubbles within glacier ice makes fjords the noisiest places in the ocean.
"The glacier fjord sound on a typical day for Icy Bay is louder than being in the water beneath a torrential downpour, which really surprised me," said Pettit, a researcher at the University of Alaska Fairbanks.
In a paper Pettit and fellow researchers speculate that one reason harbour seals flock to fjords with tidewater glaciers is because noisy icebergs provide acoustic camouflage, protecting seals from transient killer whales that hunt by sound.
In July 2009, the researchers deployed underwater microphones 70m deep in Icy Bay, a fjord near the top of the Alaska Panhandle just 6.5km from 5 500m Mount St Elias. They also sampled sound at nearby Yakutat Bay and at Andvord Bay in Antarctica.
Researcher Jeff Nystuen of the University of Washington's Applied Physics Laboratory, who had had used hydrophones to measure the underwater sound of rainfall, quickly realised the significance of the sound collected in the fjords, Pettit said.
"He was kind of blown away when I showed him the results of our data set," she said. "He's like, 'This is really, really loud.'"
Colleagues at the University of Texas at Austin conducted laboratory tests in acoustic tanks with Alaska glacier ice to find out how bubbles make noise.
They recorded air bubbles making a bloop, tick or pop sound as they separated from ice, a sound that lasts 10 milliseconds or less.
"The bubble rising through the water is relatively silent," Pettit said. "The bubble hitting the surface is also relatively silent."
The snow compressing to ice on a glacier creates bubbles of nearly the same size under the same pressure, Pettit said. That makes for consistent sound underwater.
"In terms of our hearing, the range of those notes is kind of that area that's on the upper half of the piano," she said. "It's centred about three-quarters of the way up to the high end of a piano."
In general, "It's kind of a combination of a babbling brook and a hissing sound," she said.
