The Seabourn Spirit had a Long Range Acoustic Device, or LRAD, installed as a part of its defense systems, said Bruce Good, a spokesman for Miami-based Seabourn Cruise Line. The Spirit was about 100 miles off Somalia when pirates fired rocket-propelled grenades and machine guns as they tried to get onboard.

The subsidiary of Carnival Corp. was investigating whether the weapon was successful in warding off the pirates, he said. The ship's captain also changed its course, shifted into high speed and headed out into the open sea to elude the pirates, who were in two small boats, he said. He had no further details.

Device maker American Technology Corp. said earsplitting "bangs" were directed by trained security personnel toward the pirates. That, combined with ship maneuvers, caused the attackers to leave the area, the company said.

The LRAD is a so-called "non-lethal weapon" developed for the U.S. military after the 2000 attack on the USS Cole off Yemen as a way to keep operators of small boats from approaching U.S. warships.

The military version is a 45-pound, dish-shaped device that can direct a high-pitched, piercing tone with a tight beam. Neither the LRAD's operators or others in the immediate area are affected.

American Technology, based in San Diego, compares its shrill tone to that of smoke detectors, only much louder. It can be as loud as about 150 decibels, while smoke alarms are about 80 to 90 decibels.