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Engineers Invent Acoustic Equivalent of One-Way Glass 114

Posted by Soulskill
from the sound-waves-aren't-welcome-here dept.
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.'"
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Engineers Invent Acoustic Equivalent of One-Way Glass

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  • by i kan reed (749298) on Friday January 31, 2014 @01:08PM (#46121677) Homepage Journal

    Can we put one between us and marketing. We don't have to hear their bad ideas to tell them they won't work for practical reasons.

  • Finally! (Score:4, Funny)

    by lord_mike (567148) on Friday January 31, 2014 @01:10PM (#46121691)

    When can I order my "Cone of Silence"? I can't wait to be finally able use my shoe phone in public knowing my conversation will be secure and private!

    • by sconeu (64226)

      The Cone of Silence never works, Max.

      • by ackthpt (218170)

        The Cone of Silence never works, Max.

        But this one, unlike all the others, will!

        Now we just need it compact.

      • by Anonymous Coward

        What did you say, sir?

  • by Anonymous Coward

    (sticks fingers in ears and yells 'I can't hear you')

  • by ZombieBraintrust (1685608) on Friday January 31, 2014 @01:12PM (#46121715)
    You can create the same effect with a microphone and a speaker behind a sound proof wall. Still pretty cool.
    • That was my first thought, but if this really can be used as the equivalent of soundproofing on freeways or whatnot, like the summary suggested, then it would mean that it provides something much more interesting than simply a wall, speaker, and mic combo.

      • by Anonymous Coward

        ", but if this really can be used as the equivalent of soundproofing on freeways or whatnot, like the summary suggested, "

        You fail to realize that the mentioned applications are complete bullshit.
        Soundproofing miles of freeways with active mechano-electrical devices? Sounds like a golden plan for the maintenance crew.
        A one way soundproofing for studio's? Why? So that random outside sounds can come into your microphones but you do not disturb the neighbours? Or so that everyone can hear you recording but you

  • Applications? (Score:4, Insightful)

    by badboy_tw2002 (524611) on Friday January 31, 2014 @01:14PM (#46121739)

    Music studios yes, but why would airplanes want to transmit sound from inside to outside, or vice-versa? Same with motorways. You definitely want to block traffic noise from adjacent buildings, but why would you care that folks in the cars can hear whats coming from the buildings? Just block it all.

    • by Kkloe (2751395)
      well for motorways because its probably cheaper to block it one way than both, say it costs 1000 to set up a concrete wall for blocking sound from both sides, but it costs 650 with this new type of wall, but doing it both sides would cost 1200, so its cheaper one way than two airplanes, did it even say where?, no, it could be around the engines or from the outside to the inside, was it that hard to understand?
      • Except this device is active, and a concrete wall is passive. Hard to believe the energy costs over, say, 20 years, would not more than make up the difference.
        • by Lumpy (12016)

          Or the fact that the concrete wall will last 20+ years without another penny spent on it, while the wall of fans will need constant repair and maintaince and end up costing $100,000 a year to keep working until they abandon it 3 years later.

      • by Dahamma (304068)

        Wait, so building a concrete wall is more expensive than building a wall filled with thousands (or millions?) of tiny fans? Not only is that not EVER likely to be true, but the latter will always require significant maintenance and electricity costs...

      • No I understand it I assure you. Again, why would you want to block the airplane sound coming in but not going out? Is there a specific reason you'd need to not hear the engines but hear what's going on outside form within the engine? Or from outside the cockpit but not vice versa? Neither of those are a good reason. And there's no way a sound damper wall or insulating material is more expensive than something filled with tiny fans.

        • by Lumpy (12016)

          So the man on the plane's wing cant hear that we are talking about him.

        • Imagine an area on the plane where the screaming kids will sit, able to hear what's going on, but not being heard. Or to separate first class from cattle class even better than it already is. Not the most necessary of solutions, but might gain them some extra pax.

    • by Ksevio (865461)
      All the applications listed want to absorb and get rid of the sound. Even in a music studio, the recording engineer wants the sound to be picked up by the microphones, he doesn't need or want to hear it directly. I can think of very few practical uses for this.
      • All the applications listed want to absorb and get rid of the sound. Even in a music studio, the recording engineer wants the sound to be picked up by the microphones, he doesn't need or want to hear it directly. I can think of very few practical uses for this.

        Especially considering that the fans in the device are going to generate both audible and electrical noise themselves.

        I was doing some recordings in my home studio a while back, and kept getting this buzzing sound in all my tracks. After an hour or so of sticking my ear against various surfaces in the room, I finally figured out that the noise was being generated by a device plugged into the same circuit as my recording gear, and was transmitting to the microphone through the wires.

    • So that you can hear noise from your kids bedrooms, but they can't hear the (ahem) noises from yours? Although, seriously, soundproof your walls and get a baby monitor.

      This sounds more to me like "let's invent this and see what we can do with it", than an invention with a current application.

  • Remember the old commercials for the roach motels? The roaches check in but they can't check out.
  • ...might become much easier to design.

    On the other hand, not sure how this will help with airplanes, where the entire cabin is simply "rumbling" including whatever one-way surface

    • by femtobyte (710429)

      Interrogation rooms ...might become much easier to design.

      Because sticking a microphone in a corner is obviously a gigantic difficulty in design, that has baffled engineers since the dawn of the electronic age.

  • by Vihai (668734) on Friday January 31, 2014 @01:22PM (#46121815) Homepage
    ...true one-way mirrors do not and can not exist: http://en.wikipedia.org/wiki/O... [wikipedia.org]
    • by jmv (93421)

      More generally, any device that lets energy (light, sound, heat, ...) only flow in one direction has to spend energy to avoid violating the laws of thermodynamics. That's true of this device just like for a heat pump. You could probably also create a real one-way mirror, but again it could not be a passive device and would require energy to operate.

      • by Anonymous Coward

        You could probably also create a real one-way mirror, but again it could not be a passive device and would require energy to operate.

        You mean like a TV?

      • by goombah99 (560566)

        More generally, any device that lets energy (light, sound, heat, ...) only flow in one direction has to spend energy to avoid violating the laws of thermodynamics. That's true of this device just like for a heat pump. You could probably also create a real one-way mirror, but again it could not be a passive device and would require energy to operate.

        Wrong. Circulators are able to perform this function without any input energy.

        http://en.wikipedia.org/wiki/C... [wikipedia.org]

        • by AdamHaun (43173)

          Wrong. Circulators are able to perform this function without any input energy.

          Technically, some of the energy from the transmitted signal is lost.

          Also, you're not going to be making glass out of these [oemarket.com].

      • by johndoe42 (179131)

        This is incorrect. You can't build a passive device that is 100% transmissive from one side and 100% reflective on the other side.

        You can, however (in theory), build a device that with these properties:

        1. Light from side A is transmitted and comes out side B
        2. Light from side B is absorbed and turns into heat
        3. Absorbed heat is re-radiated, mostly from side A

        This thing will be non-reciprocal [wikipedia.org]. I've never heard of anyone building one that looks like a sheet of glass, but they're common for microwaves -- they're cal

    • by iggymanz (596061)

      that depends only on definition. while a conventional silvered glass can't act as true one-way glass, powered devices certainly can. Some cameras, a computer with wares that left-right reverse image, a large screen, and presto a " true one way glass". or at least we'll have marketing tell people that.

      • by Dahamma (304068)

        Just polarize a window between two rooms and then make everyone in one room wear complementary polarized glasses :)

        • by iggymanz (596061)

          but they'll lean their heads when you're not looking!

          • by Dahamma (304068)

            Aha! Then make sure it's circularly polarized! (and I suppose, tape the glasses to their heads and tie their hands behind their backs)

            And since it's an absurd amount of effort and expense just to create a simple one-way window, the US DHS is sure to demand its large scale implementation right away. Time to patent it and make billions! And as a bonus, with everyone wearing these shades Guantanamo will be the new height of style in secret detention centers...

    • Yeah, this. The real "acoustic equivalent of one-way glass" would be to play some loud noise near the people who shouldn't be able to hear you - they have to yell really loudly to hear each other, which everybody else (including you) can hear over the noise. But they can't distinguish sounds you make from the noise because the noise is much closer and louder than you.

      • by Dahamma (304068)

        Actually, this was the first thing I thought of when I read "'if you can hear, you can also be heard" in the summary.

        It's also my explanation to my girlfriend as to why I can't hear her when she tries to talk to me through the bathroom door while I'm in the shower, but she can hear me ;)

    • by goombah99 (560566)

      Except that they do exist and are made all the time for waveguides. they are called circulators. Wikipedia is 100% wrong on that.

    • by ljw1004 (764174)

      ...true one-way mirrors do not and can not exist: http://en.wikipedia.org/wiki/O... [wikipedia.org]

      Citation needed. The wikipedia page contains your assertion ("true one-way mirrors do not and can not exist") but it backs it up with a now-dead link that, when it was active, never said anything of the kind...

      Two-Way Mirrors, copyright 1999, Jim Loy

      A two-way mirror is often called a "one-way mirror" by members of the general public. The misconception is that such a mirror acts as a mirror from one side, and acts as a window (letting light through) from the other side. Actually, the two-way mirror is letting about half of the light through, and reflecting the other half of the light, from both sides. it is also called a half-silvered surface, as just enough reflecting metal film (usually aluminum as far as I can determine) is deposited on the glass, so that about half of the light is reflected.

      So, why does a two-way mirror seem to behave like the two sides are different? It behaves this way when one side is in the dark. Then almost no light goes from the dark side to the light side, and almost no light is reflected back from the dark side to the dark side. Most of the light comes from the bright side. Plenty of light travels through the mirror, and plenty of light is reflected back. To people on both sides of the mirror, the light from the bright side overwhelms the light from the dark side. So, people on the bright side see a mirror, and people on the dark side see a window. See the above diagram.

      Robin Williams told a joke about policemen in the South having mirrors on the inside of their glasses. Good joke, but such glasses are two-way mirrors, and are shaped so that your eyes are always in the dark.

  • Bah (Score:5, Funny)

    by Jeremi (14640) on Friday January 31, 2014 @01:22PM (#46121821) Homepage

    My cell phone has been doing this for years.

  • We have that in the midwest already
    its called the wind
    also useful for generating power

  • by MiniMike (234881) on Friday January 31, 2014 @01:46PM (#46122065)

    the team created a cavity loaded with tiny CPU fans

    And I was hoping I could use this to reduce fan noise in my computer.

  • by Sertis (2789687) on Friday January 31, 2014 @01:54PM (#46122121)
    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.
  • by Gordo_1 (256312) on Friday January 31, 2014 @02:06PM (#46122223)

    > "This could have great applications in sound insulation of motorways"

    Last I checked, the screams of children within neighborhoods protected by motorway sound barriers were not a major nuisance or safety hazard for motorists.

    Or are we just brainstorming ideas without any thought behind them?

    A one way sound barrier makes sense if you have an application where you actually need sound to go through in one direction. If you don't, then a wall is a better and almost certainly cheaper solution.

    • > "This could have great applications in sound insulation of motorways"

      Last I checked, the screams of children within neighborhoods protected by motorway sound barriers were not a major nuisance or safety hazard for motorists.

      but the children can barely hear the loud roaring traffic as it goes by. think of the children!

  • by Roger W Moore (538166) on Friday January 31, 2014 @02:17PM (#46122363) Journal

    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.

  • by Solandri (704621) on Friday January 31, 2014 @02:20PM (#46122391)
    The gizmo they're describing is for acoustic transmission along a single axis. i.e. you have a pipe between points A and B, and A can hear B but B can't hear A.

    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).
  • How many times have you asked a user to do something, but they end up doing something completely different?

    Obviously, they can't hear you because of the CPU fans...

  • I need this for my wife. She can hear me, I can't hear her!
  • This has the potential to save many marriages!

    Oh, my bad...this is /.
    What was I thinking? *scurries back to basement

    • Is that because the wife can hear you screaming for a beer but you can't hear her screaming at you?

  • My bathroom fan already does this! If I'm standing in the bathroom talking, my wife can hear me from another room. If she talks, however, I can't hear her at all.

    I guess they just built a higher resolution version of my bathroom?

    • by nashv (1479253)

      Unless your bathroom fan is creating hurricane level winds in your home, it is unlikely that it is functional as an acoustic circulator. More likely , it's too noisy.

  • Of course you can eliminate all sounds, duh. You just have to make sure the fans are blowing supersonic air. Sound can't travel upstream supersonically.
  • I've read Professor R.V. Jones "The Wizard War" (originally published in Britain as "Most Secret War"), and this reminds me very much of the story of the electronic warfare between England and Germany in WWII. This device reminds me very much of the cavity magnetron oscillator, sort of an "acoustic-frequency "magnetron"", not the best descriptor perhaps, but that's what immediately comes to my mind.

  • Any application I can think of for this can already be better served by the amazing technology of microphones and speakers plus sound absorbent materials. You know, like we already use in interogation rooms etc..

  • Really drowns out the reverse transmission, doesn't it? Filter error: Don't use so many caps. It's like YELLING.
  • i'm 15 yrs old dating a 12 yr old grrl. i'm allowed to decide the evenings activity, we are going to listen to a radio mystery from the 1940's as an experiment in psychology (i'm Mach more than Jung). i set more than several candles of various highs and lows on a table between mother/father and myself/daughter. i light the candles turn off all other light sources and say to here mother can you see us, she reply's no, i inform her it works both ways, lets listen. upon the conclusion she( the mother ask's) di
  • I am unable to think of any problem of this that would not be solved by a good both ways sound blocker along with microphone-speaker combo
  • There's another well-known effect in which sound propagates asymmetrically - up- and down-wind at ground level. Viscosity effects produce a wind speed gradient (wind shear), with air nearest the ground travelling slowest. The result is that sound waves travelling downwind are refracted downwards (making them easier to hear), and waves travelling upwind are refracted upwards (making them harder to hear). If the wind's at your back, you can be quite close to a noise source yet unable to hear it - whereas some

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