BOSTON (AP) — Researchers say increasing amounts of underwater noise, largely from shipping traffic, are enveloping rare right whales in "acoustic smog" that makes it harder for them to communicate.

The endangered North Atlantic right whale relies far more on sound than sight, using distinctive noises to maintain contact.

A paper by federal scientists and Cornell University researchers published Wednesday estimates that in the last 50 years, the area where the whales can effectively communicate in Stellwagen Bank and surrounding waters off Massachusetts has fallen by two-thirds because of the noise.

The researchers say the racket is cutting down on the animals' ability to gather and share vital information that helps them find food, avoid predators, reproduce and protect their young.

"Basically, the whales off Boston now find themselves living in a world full of our acoustic smog," said Christopher Clark, director of Cornell's bioacoustics research program and a study co-author.

The paper's lead author, acoustics expert Leila Hatch of the National Oceanic and Atmospheric Administration, compared the whales' plight to a person at a crowded cocktail party who must either speak up or leave the room to be heard.

In the whales' case, the animals might change the frequency or volume of their calls, which can limit the effectiveness of the communication and put them under physiological stress, Hatch said.

The slow-moving whale, which can grow to 55 feet and 70 tons, was nearly hunted to extinction in the 18th century. With only 350 to 550 North Atlantic right whales remaining, it's crucial to better understand specifically how the noise affects each one of them, Hatch said.

But the increasing noise demonstrated in the study has implications for other struggling whales who rely on sound to communicate.

"What we're trying to do is get these endangered species into better condition," she said. "There's some 'i's to dot and 't's to cross, to put it mildly."

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The potential effect of underwater noise on marine animals has been studied for years, including how manmade noises can mask the low-frequency sound whales make.

In the latest study, researchers sought to better measure the natural and man-caused noise levels in a particular area over a long period, as a crucial step toward figuring out how that noise level affects the animals.

The scientists used computer models, vessel tracking information and data from underwater microphones in Massachusetts Bay to study the noise in about 3,900 square miles of ocean, including the Stellwagen Bank marine sanctuary.

The study focused on April 2008, a month when federal scientists documented more than 22,000 contact calls between right whales. It found that during most of the time the background noise, combined with the intermittent sounds of the ship traffic, left the whales with an area where they could communicate that was 62 percent smaller than the area 50 years ago.

Actually reducing the ocean noise is complicated, given that the background noise is often generated outside U.S. waters and most of the vessels that transit into U.S. ports are based in foreign countries with various regulations.

Kathy Metcalf of the Chamber of Shipping of America said her group has no doubt noise is increasing and affecting the whales. But measures such as slowing down ships or retrofitting them with new, more efficient propellers are costly and may not even work, she said.

A better remedy would be devising and incorporating quieter designs in the hulls and propellers of new vessels.

"We're kind of a slow industry," Metcalf said. "But the bottom line is if we could do something now that can be used as guidelines for new construction, 15 years from now, half the world's fleet would have already been built that way."