One of the many things that are forming our common sense is that fact that sound is symmetrical. If in a room I can hear you talking, I take for granted that you can hear me talking. Of course a different story would be when you and I are talking over a phone line, but in this case the reason is that the communication is mediated via two separate channels, one you use to talk to me and the other I use to talk to you. One may work well, the other may not.
Anyhow, in a room where the communication media is in common, the air, sound waves move symmetrically from A to B and from B to A. Well, until now!
Researchers at University of Texas at Austin have managed to create a device that can block sound in one direction whilst letting sound waves in the other direction to be undisturbed.
What they did was to apply principles used in microwave circulators, devices used in radar and telecommunications, to sound waves demonstrating that also in the case of sound waves it is possible to have sound transmission in one direction and to block it in the opposite one.
They constructed a device that contains a resonant ring cavity (as shown in the figure) where they have inserted three computer fans to circulate the air at a specific velocity. There are three ports connected to the cavity, each one with a microphone to detect sound. By producing sound in port 1 the microphones in port 2 and 3 detects it. However, if the fans spin at a certain speed the sound from port one reaches only the microphone in port 2 and not the one in port 3. By changing the fan velocity it is possible to make the sound reach port 3 and to skip port 2.
According to the scientists this opens the door to better sound insulation materials and to the containment of sound in specific areas with possibile application to wireless devices, like cell phones. Indeed I would love a system blocking the sound of my voice with the exception of the part reaching my cell phone microphone when I am in an open space, and I would love even more such a system in a train blocking other people voices as they blah blah in their phone through the entire trip!