Engineers Invent Acoustic Equivalent of One-Way Glass

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.'"

Mechanical notch filter?

[Sertis]

seems like it only affects sound or wave functions in a specific frequency as determined by the speed of the air movement or electron migration rates or whatnot. Might not be very effective for general sound insulation unless it's fixed frequency, o (or else you'd hear the generated harmonics). Not too different than active noise cancellation.

This doesn't make any sense

[goombah99]

I don't see how this explanation is breaking the symmetry. Accoustic waves don'e have circular polarication and are axially symmetric.

Your example sort of proves the point, one should be able to make a silent CPU fan where the sounds go in but don't come out if this is true.

Here's my guess about what is REALLY going on here. if you phase delay two separated fans such that the sound that went through the open part of the first fan, reaches the the second fan just as the open part of it is inline with the dound direction it passes through. It would not work in the backwards direction. then layer on that the fact that air column is moving in one direction.

But I' don't see the symmetry breaking for a compression or rarefraction wave happening like it does in the E&M case.