The
LRAD is the round black device
on top of the New York City police
Hummer.
The long range acoustic device (LRAD) is a crowd-control and hailing device developed by American Technology Corporation.
The equipment weighs 45 pounds (20 kg) and can emit sound in a 15 to 30° beam (only at high frequency) from a device 33 inches (83 cm) in diameter. At maximum volume, it can emit a warning tone that is 150 dBSPL (1000 W/m²) at 1 metre, a level that is capable of permanently damaging hearing, and 50 times the normal human threshold of pain (120 – 140 dB). The design range extends 300 to 500 metres. At 300 metres, the warning tone is 105 dB. The warning tone is a high-pitched shrill tone similar to that of a smoke detector, only somewhat louder.
It's instructive to note that any loudspeaker of equal size will generate a beam of the same directivity as LRAD. The parameter "ka", which is the wave number multiplied by the speaker radius, is often used to characterize sound source directivity. For this source, ka=19 at 2.5kHz, and according to the LRAD data sheet, the beam angle of about 30 degrees total - precisely what is predicted for a regular loudspeaker [1]. Contrary to some beliefs, the device does not use ultrasound; it uses an array of conventional acoustic tweeters, the same as those used in many professional audio applications.
Carl Gruenler, (former) vice president of military and government operations for American Technology Corp. (and who now runs a company making a competing device), says that being within 100 yards (90 m) of the device is extremely painful, but its use should be limited to 300 yards (270 m) to be effectively used. He concedes that the device is powerful enough to cause permanent auditory damage, but that it is only meant to be used for a few seconds at a time.
Countermeasures may include the use of passive hearing protection (earplugs, headsets), which may bring the sound down to ineffective levels. In addition, sound could be reflected from a solid surface, and redirected back to the originator.
Small spherical "point-source" acoustic devices follow the known inverse square law, which predicts the loss of 6db per doubling of distance from the source. Large speakers (or large arrays), such as these mentioned above or those commonly used in concert halls, etc., produce less loss with distance in the nearfield, typically 3-4db per doubling of distance from the source. The larger the speaker, and the higher the frequency, the longer the effective nearfield is (see Beranek). Devices like this generally have nearfields of only a few meters.
Usage
The LRAD in use on a U.S. Navy
patrol ship.
LRAD on a cruise ship
The device was originally intended to be used by American warships to warn incoming vessels approaching without permission, but has now diversified as a more general-use weapon in America's non-lethal force arsenal. Other devices are being developed by the military as non-lethal weapons, such as the Active Denial System, which uses a painful energy beam to discourage would-be attackers or simply any individual the possessor of the weapon might wish to control.
The device is currently used extensively at Camp Bucca Iraq and is being tested in regions of Baghdad, Fallujah, along with other regions of Iraq. The device was also used by police in New York City during protests of the 2004 Republican National Convention.
The luxury cruise ship "Seabourn Spirit" employed an LRAD while repelling pirates who attacked the vessel with RPGs about 160 km off the coast of Somalia in early November 2005.[1][2] The effectiveness of this device during the attack is not completely clear, however the pirates did not succeed in boarding the vessel and eventually fled.
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