Naval sound defense (weapons) New Scientist 8th Oct 05

Shock tactics to destroy torpedoes

* 06 October 2005
* NewScientist.com news service
* Paul Marks

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* Defense Advanced Research Projects Agency
* Anteon Technologies
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Torpedo Zapper
Enlarge image
Torpedo Zapper

THE US navy wants to protect its warships with a system that will
destroy incoming torpedoes by firing massive underwater shock waves
at them.

The ships would be equipped with arrays of 360 transducers each 1
metre square - effectively big flat-panel loudspeakers - running
along either side of the hull below the waterline. When the ship's
sonar detects an incoming torpedo, the transducers simultaneously
fire an acoustic shock wave of such intensity that the torpedo either
detonates early or is disabled by the pulse's crushing force,
according to the Defence Advanced Research Projects Agency (DARPA),
which is funding the project.

But these are no ordinary loudspeakers: instead of having a
membranous diaphragm that can vibrate in response to a range of audio
frequencies, each of the devices has a ram-like cylindrical metal
armature at its centre. This is projected outwards by electromagnets
at very high speed, producing a shock wave. The array can be fired as
many times as needed.

When the six rows of 60 transducers on each side of the ship fire at
once, the cumulative action should generate a "destructive pressure
pulse capable of disabling an enemy's torpedo", according to DARPA.

Exactly how the system works is shrouded in military secrecy. But by
making a speaker several times larger than the wavelength of the
sound wave required, a tightly focused beam can be produced in frontof the speaker. This is because beam width is partly determined by
the aperture of the source - a bigger loudspeaker focuses sound in a
smaller area. (New Scientist, 9 September 2000, p 38). And the
combined size of the array makes for a very large speaker indeed.
This focusing would allow the array to precisely target incoming
torpedoes. In addition, the beam can be steered in different
directions - probably by slightly altering the phase of the applied
signals - a technique that is widely used to steer radio waves using
side-by-side antennas. So torpedoes homing in on the ship's wake from
many directions can be targeted (see Diagram).

So far, the system's developers, Anteon Technologies of Fairfax,
Virginia, and BAE Systems of Farnborough, UK, have only built one
transducer. But encouraged by software simulations that show the
array should work, they plan to press ahead with a one-quarter-scale
test rig.

If it reaches the stage of testing in the open ocean, however, the
developers are likely to come into conflict with marine biologists.
They have evidence that whales blasted by frequent acoustic signals
from submarine or ship sonar appear to develop symptoms of
decompression sickness, and die. (New Scientist, 11 October 2003, p 10).
“The torpedo detonates early or is disabled by the pulse's crushing
force”

But neither DARPA, Anteon, nor BAE Systems was willing to respond to
questions about the array's proposed energy levels and any threat to
marine mammals they might pose.
From issue 2520 of New Scientist magazine, 06 October 2005, page 32
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