SOFAR: The Ocean Layer That Carries Sound Across Whole Basins

In the middle of World War II, scientists set out to test a theory: that low-frequency sound could travel very long distances in the deep ocean. From a research vessel anchored off Woods Hole, Massachusetts, they lowered a hydrophone. Some 900 miles away, another ship lowered a four-pound explosive to a set depth and set it off. The pulses of sound crossed the whole distance — about as far as from Washington, DC, to Des Moines, Iowa.

The SOFAR channel lets sound travel through an ocean zone for hundreds, sometimes thousands, of miles. Credit: NOAA.

A sound no one could miss

The researchers had heard, for the first time, what they called SOFAR — a SOund Fixing And Ranging transmission. As they wrote, "The end of the sound channel transmission was so sharp that it was impossible for the most unskilled observer to miss it."

How the channel works

The ocean is layered, rather like oil-and-vinegar dressing before it is shaken, except that the layers come from differences in salinity and temperature: saltier, colder water lies beneath less salty, warmer water. Sound made at the right depth bends up and down between these layers and stays trapped between them. That bending lets low-frequency sound travel great distances without losing much energy — in some cases, across entire ocean basins.

Listening across an ocean

Researchers place hydrophones at the axis of the sound channel to record sounds from very far away: whale calls, earthquakes and human-made noise.

Sources

Based on "What is SOFAR?" from NOAA's National Ocean Service, a work of the United States government in the public domain.