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- Sound, as any deafened rocker will tell you, can be surprisingly
powerful. So powerful, in fact, that scientists are discovering just what
a little bit of pumped-up volume is capable of - from levitating objects
in space to rearranging our internal organs.
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- To prove how potent sound waves can be, a selection of
unfortunate laboratory animals were placed on the receiving end of this
more sinister use of acoustic technology. When high-powered infrasound
was directed at the subjects, it caused internal bleeding and even destroyed
body tissue. Good news if you happen to be a military scientist looking
to develop a sound-wave weapon.
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- Encouraged by the lab results, militarists around the
world are working on such sound arms, which they hope to put to use in
conflicts. On the battlefield, they say, an array of loudspeakers could
be directed at the enemy, who would be blasted by very powerful infrasound
- very low-frequency sound waves below the hearing threshold of the human
ear. With such a weapon, devastating attacks could be made without ever
firing a conventional shot.
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- Fortunately, sound is also being used more constructively
- promising to defeat noise pollution and even power engines in the future.
Researchers working at the Los Alamos National Laboratory in New Mexico,
for example, have developed what they claim is an environmentally friendly
engine with no moving parts that is powered by sound waves.
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- Built on the principle of a 19th century pump invented
by Robert Stirling, who discovered that cooling and heating gases could
drive a piston, the thermoacoustic Stirling heat engine produces sound
from heated helium within the engine. The sound waves then drive a piston
which in turn generates electricity.
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- "Small low-cost engines like this could be used
in homes for cogeneration," says Greg Swift, one of the creators of
the experimental engine. "That is, they could be used to generate
electricity while at the same time producing heat to warm water or the
home."
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- Cars create a great deal of the sound pollution to which
we are exposed each day. A group of Japanese scientists believe that the
solution to this is to fight noise with noise.
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- Japan's Public Works Research Institute has unveiled
a new kind of wall fitted with a device that ameliorates highway and railway
noise by overlaying its own sounds. The idea came from the technology used
in aeroplane headsets, which also cancel out unwanted noise using noise.
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- The institute's cylindrical Active Noise Control (ANC)
measures the sound waves generated by traffic and then spouts its own sound
waves, whose peaks and troughs cancel out the peaks and troughs of the
sound waves coming from the traffic. The result is a lessening of the racket
on our tormented ears, although the reduction achieved so far is only in
the region of five per cent.
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- Acoustic research is not only coming up with some answers
to problems caused by transport but has discovered that sound can do a
little transportation of its own - using the magic of levitation.
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- Acoustic levitation was first experimented with successfully
in the 1940s. Now, the use of high-powered sounds is sophisticated enough
to suspend objects in the air and move them along as though on an invisible
conveyor belt.
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- Yoshiki Hashimoto, of Tokyo's Kaijo corporation, has
developed a machine that lifts objects and moves them by acoustic levitation
using supersonic waves.
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- Firing off sound waves which vibrate 20,000 times per
second, the Kaijo acoustic levitator can keep a small silicon wafer hovering
one millimetre above the surface. The direction and speed of the supersonic
waves can also be controlled.
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- It could revolutionise the semiconductor industry, according
to Hashimoto, as super-sensitive microchips don't stand up to a lot of
handling. Other methods of "no contact" transportation for the
industry have been experimented with before - using gas jets or \electromagnetic
force, for example - but acoustic levitation offers the greatest advantage,
says Hashimoto.
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- "Any material can be levitated, regardless. It has
high controllability and good positioning and manipulation capabilities.
Also, in comparison with other systems, the suspension mechanism is compact
and uses very little power."
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- Further experiments with acoustic levitation methods
are being conducted in space. Because of the absence of terrestrial gravity,
many experiments involving acoustic levitation are better observed in zero
gravity. The weightless conditions in space labs orbiting the Earth also
gave rise to the discovery of another sound-related phenomenon which has
excited a lot of American scientists.
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- What caught their interest was the presence of so-called
acoustic luminescence; that is, light being emitted from liquid under high-intensity
sound. The phenomenon, known as sonoluminescence, had been observed on
Earth as far back as the 1930s, but in space a very different type of luminescence
was produced.
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- "Experiments using a single bubble, such as can
be conducted in space, reveal that the pulses of luminescence are very
short," says Tim Leighton, professor of ultrasonics and underwater
acoustics at the University of Southampton. "This is something which
is very difficult to explain with standard physics."
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- Around the world there have been claims that this new
type of acoustic luminescence is a discovery that could lead to hundreds
of different applications, from nuclear fusion to sewage treatment. Leighton,
however, remains cold-blooded.
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- "Nobody has actually proved an application for it,"
he says. "The excitement over the single bubble experiments rely on
having one bubble in a perfectly symmetrical, controlled environment. Of
course the moment you throw some sewage in there, for example, you've ruined
that."
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- Southampton's acoustics department is also hot on the
trail of another ground-breaking use of sound - a virtual sound system
to complement virtual reality (VR) entertainment. The department calls
its system the Stereo Dipole, and it has won an accolade from the Design
Council as a "Millennium Product."
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- The secret of realising virtual sound is through virtual
sound fields. Adjusting these means you can make a sound source appear
to be anywhere: behind your head, above or to the side, or you can move
it around. All of this, of course, would be very useful for entertainment
systems using VR, which is at the moment basically a visual experience
enhanced by earphones.
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- Led by Philip Nelson at Southampton's Institute of Sound
and Vibration Research, and by Hareo Hamada at Tokyo's Denki University,
the team has produced a system for creating what it calls "virtual
acoustic images".
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- "Crucially, two loudspeakers are placed very close
together, which produces a sound field that radiates," says Prof Nelson.
"This creates a particular property where you can produce a null in
the sound field in one or other of the listener's ears." This enables
the sound produced at the listener's ears to be a very close replica of
the sound that would be produced by the virtual source.
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- The signal processing involved can either be executed
on a computer at the time a sound recording is made, or it can be implemented
on a special purpose chip which can be embedded into commonplace devices,
such as television sets, personal computers and portable stereo systems.
Yamaha Corporation in Japan has licensed the rights to use this technology
and is now making chips for its implementation.
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- Ultimately, it will make VR capable of very closely simulating
our "take" on reality. Nostalgic headbangers could even immerse
themselves in replicas of bygone concerts - if their hearing is up to it,
that is.
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