Sound defence

November 14, 2001

WEAPONS that fire high-intensity "sonic bullets" could be used by sky marshals to incapacitate terrorists who try to hijack passenger aircraft. The US Department of Defense is assessing the technology following the attacks on 11 September.

Elwood Norris, chairman of American Technology Corporation in San Diego, California, says the Department of Defense approached him about a device the firm has patented that produces narrow but high-power beams of sound. Norris says the device could be used on hijackers to inflict pain and possibly disorientation. "They wanted to know, could you use this without any destruction to fuselage walls and windows? And the answer is yes," he says. A key defence contractor, cruise missile maker General Dynamics of Falls Church, Virginia, is funding development of the system and is helping AT to brief the Army and the Pentagon on its capabilities.

Norris's device, which he calls a "directed stick radiator", is encased in a tube made of a polymer composite, around a metre long and 4 centimetres in diameter. Inside the tube are a series of piezoelectric discs, each of which acts like a small speaker. Sending an electrical signal to the first disc at the rear end of the tube makes it expand, sending a pressure wave-a sound pulse-along the tube. The pulse soon reaches the second disc, which is "fired" at precisely the right time so that the sound pulse it produces magnifies the pressure wave. By firing each disc in sequence, the amplitude of the sound pulse increases along the length of the tube until it reaches the exit nozzle. "It shoots out a pulse of sound that's almost like a bullet," says Norris. "It's over 140 decibels for a second or two." Sounds become painful between 120 to 130 decibels.

Norris says the final version is likely to fire audible pulses at a frequency of between 6 and 10 kilohertz. "It looks right now like this would work over 100 yards," he says.

To test the system, Norris created a cutdown version and turned it on himself. "It almost knocked me on my butt. I wasn't interested in anything for quite a while afterwards," he says. "You could virtually knock a cow on its back with this."

Acoustic weapons could hinder hijackers in two ways, according to a source at QinetiQ, the British defence lab-formerly the Defence Evaluation and Research Agency. The main effect is to cause intense pain in the ear drums. "This would be extremely painful and uncomfortable and you would probably lose your hearing for a few hours," says the source. Acoustic pulses can also disorientate people by shocking the balance system of the inner ear-an effect known as the Tullio phenomenon. But this affects people differently and can't be relied upon.

Non-lethal acoustic weapons have yet to prove themselves in the field, though. "A lot has been written about their effects from tests in the 60s and 70s, and a lot of that is flatly wrong," says Jürgen Altmann, an expert in these weapons at the University of Dortmund. Inaudible, low-frequency sound waves-infrasound-were claimed to induce nausea and even vomiting. But Altmann says there's no reliable evidence for this.

Using audible frequencies makes more sense, says the QinetiQ specialist. "Infrasound takes too much energy to propagate and you can't steer it, while ultrasound is too easily absorbed and doesn't do much anyway," he says. "The [American Technologies] system would be extremely painful, and you've got a definite risk of causing permanent hearing loss."

Altmann says there may be other problems. "This beam won't be fine enough to hit just one person unless they're very close. It could hit others, or reflect around the aircraft cavity causing temporary hearing loss in other passengers."
Author: Ian Sample

New Scientist issue: 17th November 2001


New Scientist

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