Jan 17th 2008
From The Economist print edition
How to stop echoes giving you away
IN GREEK mythology, Echo was a mountain nymph who lost her voice and was condemned to repeat only the words of others. Now science is poised to silence the sprite completely. A group of physicists, led by Steven Cummer of Duke University in North Carolina, has devised plans for a cloak that would shield objects from sound, preventing its reflection. Such a device could be used to hide submarines.
Sonar, the technique employed to detect subs, uses a transmitter to emit a pulse of sound-usually a distinctive "ping"-and a receiver to listen for its reflection. That reflection indicates the presence of an object and the time that elapses between the sound's being emitted and its being detected indicates how far away it is. A second ping allows the object's direction, speed and location to be calculated.
Dr Cummer, however, has devised a plan to surround a submarine with a shell that directs sound waves to flow around it as though the vessel were not there. The proposal relies on two properties of the material used to make the shield-its density and its "bulk modulus", a measure of its springiness. It should be possible to tailor these so that sound waves are bent such that no echo results. The design would also avoid absorbing sound, ensuring no acoustic "shadows" were cast.
Dr Cummer's method, reported in the current issue of Physical Review Letters, is akin to an existing design for an invisibility cloak that would work for light waves, proposed by Sir John Pendry of Imperial College, London. (Sir John is also one of the authors of the new paper.) Yet the acoustic version has a distinct advantage over its optical counterpart. Making an invisibility cloak would be tricky because the device would work only at certain wavelengths. An aeroplane shrouded in such kit might be invisible to the human eye, for example, but would be picked up readily by radar, which works at radio wavelengths.
An acoustic cloak, however, would work for a wider range of wavelengths, making it far harder to spot. That is possible because light and sound are rather different sorts of waves. As Einstein observed, light in a vacuum travels at the greatest speed possible, around 300m metres a second. Even when it is slowed by air and water, its progress usually remains close to this limit. That means light must obey the rules of Einstein's special theory of relativity. When light is bent by an invisibility cloak, certain components of the wave are allowed to stretch the laws of physics and travel faster than the nominal speed of light, but only under strict conditions. The energy and the information that the wave carries, for example, cannot exceed the speed of light. The effect is to narrow the range of wavelengths that can be bent by an optical shroud.
Sound, meanwhile, travels at a sedate 300 metres a second. Because this is a million times shy of the relativistic limit, the behaviour of sound waves is not restricted in the same way. Under non-relativistic conditions, many different wavelengths can be bent simultaneously by the same acoustic shield, making it far more effective at concealing an object.
It was unrequited love that made the Echo of Greek mythology fade away until only her voice remained. Although Dr Cummer and his colleagues are still some way from transforming their design into a working device, they reckon precisely engineered materials may soon erase her final utterances.
From The Economist print edition
How to stop echoes giving you away
IN GREEK mythology, Echo was a mountain nymph who lost her voice and was condemned to repeat only the words of others. Now science is poised to silence the sprite completely. A group of physicists, led by Steven Cummer of Duke University in North Carolina, has devised plans for a cloak that would shield objects from sound, preventing its reflection. Such a device could be used to hide submarines.
Sonar, the technique employed to detect subs, uses a transmitter to emit a pulse of sound-usually a distinctive "ping"-and a receiver to listen for its reflection. That reflection indicates the presence of an object and the time that elapses between the sound's being emitted and its being detected indicates how far away it is. A second ping allows the object's direction, speed and location to be calculated.
Dr Cummer, however, has devised a plan to surround a submarine with a shell that directs sound waves to flow around it as though the vessel were not there. The proposal relies on two properties of the material used to make the shield-its density and its "bulk modulus", a measure of its springiness. It should be possible to tailor these so that sound waves are bent such that no echo results. The design would also avoid absorbing sound, ensuring no acoustic "shadows" were cast.
Dr Cummer's method, reported in the current issue of Physical Review Letters, is akin to an existing design for an invisibility cloak that would work for light waves, proposed by Sir John Pendry of Imperial College, London. (Sir John is also one of the authors of the new paper.) Yet the acoustic version has a distinct advantage over its optical counterpart. Making an invisibility cloak would be tricky because the device would work only at certain wavelengths. An aeroplane shrouded in such kit might be invisible to the human eye, for example, but would be picked up readily by radar, which works at radio wavelengths.
An acoustic cloak, however, would work for a wider range of wavelengths, making it far harder to spot. That is possible because light and sound are rather different sorts of waves. As Einstein observed, light in a vacuum travels at the greatest speed possible, around 300m metres a second. Even when it is slowed by air and water, its progress usually remains close to this limit. That means light must obey the rules of Einstein's special theory of relativity. When light is bent by an invisibility cloak, certain components of the wave are allowed to stretch the laws of physics and travel faster than the nominal speed of light, but only under strict conditions. The energy and the information that the wave carries, for example, cannot exceed the speed of light. The effect is to narrow the range of wavelengths that can be bent by an optical shroud.
Sound, meanwhile, travels at a sedate 300 metres a second. Because this is a million times shy of the relativistic limit, the behaviour of sound waves is not restricted in the same way. Under non-relativistic conditions, many different wavelengths can be bent simultaneously by the same acoustic shield, making it far more effective at concealing an object.
It was unrequited love that made the Echo of Greek mythology fade away until only her voice remained. Although Dr Cummer and his colleagues are still some way from transforming their design into a working device, they reckon precisely engineered materials may soon erase her final utterances.
