Engineers Invent Acoustic Equivalent of One-Way Glass 114
Hugh Pickens DOT Com writes: "Up until now, acoustic waves traveling between two points in space always exhibited a basic symmetry summed up with the phrase, 'if you can hear, you can also be heard.' Not anymore; Tia Ghose reports at Live Science that a team at UT Austin has created a 'nonreciprocal acoustic circulator,' the first step that could lead to the sound equivalent of a one-way mirror. All waves — whether visible light, sound, radio or otherwise — have a physical property known as time reversal symmetry — a wave sent one way can always be sent back. For radio waves, researchers figured out how to break this rule using magnetic materials that set electrons spinning in one direction. The resulting radio waves detect the difference in the material in one direction versus the other, preventing reverse transmission. To accomplish the feat with sound waves, the team created a cavity loaded with tiny CPU fans that spin the air with a specific velocity. The air is spinning in one direction, so the flow of air 'feels' different to the wave in one direction versus the other, preventing backward transmission. As a result, sound waves can go in, but they can't go the other way. The result is one-directional sound. With such a device, people can hear someone talking, but they themselves cannot be heard. The findings will likely lead to many useful applications, says Sebastien Guenneau. 'I would be surprised if sound industries do not pick up this idea. This could have great applications in sound insulation of motorways, music studios, submarines and airplanes.'"
Microphone and Speaker (Score:4, Informative)
Except for the fact that... (Score:3, Informative)
Time Reversal Violation (Score:5, Informative)
All waves — whether visible light, sound, radio or otherwise — have a physical property known as time reversal symmetry — a wave sent one way can always be sent back.
No, not all waves. Kaon and B-meson waves violate time reversal symmetry. We have known about this for almost 20 years since the first CPLEAR paper on the evidence of this and the more recent papers from Babar have confirmed it beyond any reasonable doubt. I'm always amazed how such a fundamental result as the laws of physics defining a direction of time (even when you take account of phase space/entropy effects) seems to be forgotten by many physicists.
There are other ways to do it (Score:5, Informative)
You can do the same with impedance changes if A and B are in different mediums. The impedance difference due to the density change causes asymmetrical transmission to reflection ratios [psu.edu] (bottom two animations). Consequently, if you're underwater in a swimming pool, you can hear all the noise from people talking in the air. But if you're outside the water, you can't hear sound originating in the water. (You can hear it a little, but nowhere near as well as sound from the air transmitted into the water.)
You can also do it with refraction changes if sound is allowed to propagate along two or more axes. The ocean creates a natural acoustic waveguide [wikipedia.org] this way. If you're in the middle of the waveguide, you can easily hear things at the edge of the waveguide. Sound from the thing at the edge of the waveguide spreads radially, and consequently about half of it captured by the waveguide. Whereas sound from the middle of the waveguide reaches that point at the edge only at a very specific angle. Consequently the listener inside the waveguide gets greater amplification. (A conceptually easier example is a megaphone if you use it to try to communicate with someone standing far away. If you speak through it, all your acoustic energy is directed in one direction, before it reaches the end of the megaphone and is allowed to spread radially. Most of it continues in the direction you pointed the megaphone. If you listen through it though, the acoustic energy from the other person spreads radially first, then the tiny bit captured by the broad end of the megaphone is concentrated. Consequently the megaphone is much more effective as a speaking amplifier than it is as a listening amplifier.)
I don't think any of these methods allow for a perfect "one-way mirror" though, where someone at A can hear B, but B cannot hear anything from A.. I can see the device in TFA getting close. It uses moving air to guide sound one way - move the air faster than the speed of sound and in theory it can't go backwards. But I have to think there will be some sound transmission back along the stationary frame used to contain the moving air (not to mention in their device the air is moving in a circle).