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 Technologies 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.
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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.”
