WEAPONS that fire high-intensity 鈥渟onic bullets鈥� 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.
鈥淭hey wanted to know, could you use this without any destruction to fuselage
walls and windows? And the answer is yes,鈥� 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鈥檚 device, which he calls a 鈥渄irected stick radiator鈥�, 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鈥攁 sound
pulse鈥攁long the tube. The pulse soon reaches the second disc, which is
鈥渇ired鈥� 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. 鈥淚t shoots out a pulse of sound that鈥檚 almost like a bullet,鈥� says
Norris. 鈥淚t鈥檚 over 140 decibels for a second or two.鈥� 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. 鈥淚t looks right now like this would work over 100
yards,鈥� he says.
To test the system, Norris created a cutdown version and turned it on
himself. 鈥淚t almost knocked me on my butt. I wasn鈥檛 interested in anything for
quite a while afterwards,鈥� he says. 鈥淵ou could virtually knock a cow on its back
with this.鈥�
Acoustic weapons could hinder hijackers in two ways, according to a source at
QinetiQ, the British defence lab鈥攆ormerly the Defence Evaluation and
Research Agency. The main effect is to cause intense pain in the ear drums.
鈥淭his would be extremely painful and uncomfortable and you would probably lose
your hearing for a few hours,鈥� says the source. Acoustic pulses can also
disorientate people by shocking the balance system of the inner ear鈥攁n
effect known as the Tullio phenomenon. But this affects people differently and
can鈥檛 be relied upon.
Non-lethal acoustic weapons have yet to prove themselves in the field,
though. 鈥淎 lot has been written about their effects from tests in the 60s and
70s, and a lot of that is flatly wrong,鈥� says J眉rgen Altmann, an expert in
these weapons at the University of Dortmund. Inaudible, low-frequency sound
waves鈥攊nfrasound鈥攚ere claimed to induce nausea and even vomiting.
But Altmann says there鈥檚 no reliable evidence for this.
Using audible frequencies makes more sense, says the QinetiQ specialist.
鈥淚nfrasound takes too much energy to propagate and you can鈥檛 steer it, while
ultrasound is too easily absorbed and doesn鈥檛 do much anyway,鈥� he says. 鈥淭he
[American Technologies] system would be extremely painful, and you鈥檝e got a
definite risk of causing permanent hearing loss.鈥�
Altmann says there may be other problems. 鈥淭his beam won鈥檛 be fine enough to
hit just one person unless they鈥檙e very close. It could hit others, or reflect
around the aircraft cavity causing temporary hearing loss in other passengers.鈥�
