Sound Waves May Be
Used To Generate Energy,
Produce Weapons And More
By Michael Fitzpatrick
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.
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.
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.
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.
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.
"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."
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.
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.
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.
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.
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.
Yoshiki Hashimoto, of Tokyo's Kaijo corporation, has developed a machine that lifts objects and moves them by acoustic levitation using supersonic waves.
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.
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.
"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."
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.
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.
"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."
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.
"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."
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."
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.
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".
"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.
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.
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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