With its mighty lungs, the blue whale is able to generate a profoundly deep rumble of sound that travels hundreds of miles underwater.

For some reason, the pitch of this otherworldly song, made only by the males, is getting even deeper.

That's the conclusion of scientists who have studied decades of measurements from the largest creature on earth.

And they think the reason for this shift might be good news: Thanks to international agreements to ban the hunting of blue whales, their numbers are slowly increasing.

Just like mammals, whales can "sing" louder at higher frequencies. But with more whales in the sea, the males don't need to be so loud to be heard, so they can sing at the lower frequencies that females - and perhaps Barry White fans - are thought to prefer.

This explanation is still a hypothesis at this point, but the scientists say the frequency data are solid, published this month in the journal Endangered Species Research.

The deeper sounds were recorded with underwater devices in blue whale populations around the world, from the North Atlantic to the Indian Ocean, though the change in pitch varied.

In the population with the best records, off California, the frequency dropped 31 percent since 1963, from 21.9 Hz to 15.2 Hz. That's below the lowest note on a piano, says physicist Mark McDonald, the paper's lead author and chief scientist of the company Whale Acoustics.

The scientists explored several ideas for why the song's pitch was lower, and decided the bigger population was the best answer. This may be controversial, says coauthor John Hildebrand, a professor at the Scripps Institution of Oceanography in California.

Still, the population does seem to be rebounding slightly, though far from its historic heights. Off California, Hildebrand says, the number of blues has risen from perhaps a few hundred at its lowest point to more than 2,000.

- Tom Avril