Physics tricks could create one-way soundproofing

April 28, 2011 10:11 PM

Physics tricks could create one-way soundproofing. Materials that genuinely discriminate between the direction of light or sound might be possible, according to a new study. That could make for true one-way mirrors or for directional soundproofing—imagine, for instance, a wall through which sound can enter but not escape.
posted by Leisure_Muffin (35 comments total)

More important than one way mirrors is optical diodes.
posted by empath at 10:15 PM on April 28, 2011 [2 favorites]

Is it wrong that the first thing I thought of was this?

Seriously though, cool tech.
posted by calamari kid at 10:26 PM on April 28, 2011 [1 favorite]

Isn't this a classic violation-of-conservation-of-energy problem? (Arrange a one-way material so that light or sound can flow from a cooler area to a hotter one; run a heat pump off the perpetually-replenished temperature difference; collect Nobel.)
posted by hattifattener at 10:33 PM on April 28, 2011

If it passes light through, then energy just goes straight through, and it's conserved. If it absorbs light, then it will get warmer and energy is conserved. If it reflects light, then light just bounces back in the original direction and energy is conserved.
posted by empath at 10:36 PM on April 28, 2011 [2 favorites]

Very interesting! The note at the end is important though:

the researchers note that each particular implementation would have a sweet spot of wave amplitudes and frequencies for which it would work best.

posted by Chuckles at 10:41 PM on April 28, 2011

Right, which isn't a problem for optical computers that are going to be using lasers.
posted by empath at 10:41 PM on April 28, 2011

I think Hatti was referring to the second law of thermodynamics - entropy always increases, meaning that it tends to flow into a less usable form.

I assume there would be something subtle about a physical realization of this technique, if it proves to even be realizable, would probably have some subtle thing that would prevent you from using it to spontaneously heat up a region.

Glancing over the paper, it looks like a given solution only works for a certain range of frequencies, so that might be part of it. Not entirely sure though.
posted by Zalzidrax at 10:42 PM on April 28, 2011

"It" in the first sentence meaning energy, not entropy. Should've taken more time to proofread that.

Energy always flows to a less useful form.
posted by Zalzidrax at 10:44 PM on April 28, 2011

empath: If light passes only in one direction, you can get free energy out of it. Like Zalzidrax says, it lets you reduce entropy with no energy input. A photonic version of Maxwell's demon.

The narrow-wavelength restriction wouldn't make it impossible to break thermodynamics, but the "only works for a certain range of intensities" might:

The one-way bias isn't universal, however—the researchers note that each particular implementation would have a sweet spot of wave amplitudes and frequencies for which it would work best

posted by hattifattener at 10:47 PM on April 28, 2011

1-way mirrors aren't really one way. They just have one room a lot darker then the other, so the reflection is more luminous then anything going on in the other room.
posted by delmoi at 10:48 PM on April 28, 2011

—imagine, for instance, a wall through which sound can enter but not escape.

I think I have one of those.
posted by Hoopo at 10:50 PM on April 28, 2011

The the entropy thing was the first thing I thought of, but obviously if these existed they would just end up converting sound waves into heat or something like that instead of reflecting them back into the room.
posted by delmoi at 10:52 PM on April 28, 2011 [1 favorite]

I don't see how entropy is a problem. All this would do is focus the energy on one side of the box. There will be waste heat, there will be physical decay, and through the whole system entropy will increase. I haven't studied physics in years, this is just intuition and maybe there's some factor I'm missing.

On preview: what delmoi said.
posted by polyhedron at 10:55 PM on April 28, 2011

This shit is going to blow ICP's minds.
posted by Uther Bentrazor at 11:03 PM on April 28, 2011 [10 favorites]

Activate the cone of silence!
posted by Jon_Evil at 11:19 PM on April 28, 2011

For the record, one-way optical propagation already exists, in the form of a Faraday isolator. It's used to prevent back-reflections from re-entering laser cavities.
posted by Upton O'Good at 11:21 PM on April 28, 2011

hati: Maxwell's Demon is a thought experiment in which no work is done by the demon and entropy is decreased. When you make a laser, the process is inefficient and a lot of energy is given off as heat. So while you may be able to decrease entropy locally if you stick a laser into a reflective cavity and only look between the mirrors, you're cheating because you're not looking at the whole system.

I'm curious what optical intensities they calculate are needed in order to see the effect and if on chip semiconductor diodes with simple lenses can provide a high enough power density.
posted by Quack at 11:28 PM on April 28, 2011

From the article:

By stacking layers of nonlinear materials along with ordinary linear layers in an asymmetric fashion, the researchers have calculated, a wave would be able to pass through in one direction but would almost completely bounce off when it arrives from the other direction.

This seems a lot like a variation on photonic crystals referenced in this SciAm article or this article in Wired.

"By selecting the proper width and distance between the bars, researchers can select the frequency of light that becomes trapped in this photonic "band gap."

The key difference in the research proposed in the FPP article is it's assymetric nature; letting sound or light through on one side and trapping it on the other.

We don't need to derail this with a discussion about entropy.
posted by lemuring at 11:39 PM on April 28, 2011

Let's imagine a box with this material on all 6 sides. Light goes in, doesn't come out. In what is that different from a black box?
posted by empath at 11:47 PM on April 28, 2011

Oh man, the first thing I thought of was how many TERRIBLE saw-esque movies are going to be made by people stuck inside such rooms.
posted by nile_red at 11:50 PM on April 28, 2011

I don't get how entropy is a problem either. Just because it can only go one way doesn't mean it HAS TO go that way. Like, a regular physical one-way trap-door doesn't violate the conservation of mass because "if it only goes one way you can get an INFINITE amount of mass in there!"

One-way just means it won't (easily) come back out. Once the container is full, though, you also can't put any more in.
posted by DU at 2:25 AM on April 29, 2011

Can't I build a one-way sound wall today?

1. Build a well-insulated sound-proof wall
2. Arrange one side of the wall with microphones
3. Put speakers on the other side

Sure, sound has been converted to electricity and back but thats just "a property of the material."
And the wall requires energy of course underscoring hattifattener's point about energy creation.

Like, a regular physical one-way trap-door doesn't violate the conservation of mass because "if it only goes one way you can get an INFINITE amount of mass in there!"

You should read up on Maxwell's Demon.
posted by vacapinta at 2:35 AM on April 29, 2011

1. Build a well-insulated sound-proof wall

That is non-trivial.
posted by flaterik at 2:37 AM on April 29, 2011

Light goes in, doesn't come out. In what is that different from a black box?

The problem is the part "doesn't come out." It does. It probably won't come out in the same form. But nothing is PERFECTLY reflective. Eventually, light bouncing around in there will lose energy, most likely warming the box (by definition when you're dealing with nonlinear materials, you're dealing with forbidden quantum processes, so there will be some of that too, coupling of the wave to unusual electronic transitions in the material). It may take a long time, but in the end, it WILL happen.
posted by solotoro at 3:28 AM on April 29, 2011

Yes, I get Maxwell's Demon and that has nothing to do with this. Consider the original problem statement: "Arrange a one-way material so that light or sound can flow from a cooler area to a hotter one".

"One-way" is not synonymous from "can flow uphill". This is no more a problem than if the problem statement had been "Arrange a one-way material so that 1=2."

Maxwell's Demon has to do *three* things. 1) Identify high speed particles that should go from A to B, 2) Actually open the door and 3) Prevent high speed particles from B to A.

If the Demon is a trap door, then it can pass from A to B. But while the door is open, particles can go backwards. The more imbalance you've created by cramming from A to B, the more likely it is they will escape from B to A while the door is open.

More importantly for this discussion, the door is going to get harder and harder to open. All those particles on the high side are pressing on it. At some point, it's going to be impossible. This is the problem a one-way material will have that prevents a thermodynamics violation.
posted by DU at 4:25 AM on April 29, 2011 [1 favorite]

It may take a long time, but in the end, it WILL happen.

That was kind of my point. It would heat and give off black body radiation. There's absolutely 0 entropy problem here.
posted by empath at 5:08 AM on April 29, 2011

I don't see how the idea that you could build a box into which light could go in but not go out implies that there is some sort of problem (in the sense that the idea would be obviously false) due to entropy (or due to anything else). There are known objects in the universe into which light can go in but not go out.
posted by Flunkie at 5:51 AM on April 29, 2011 [1 favorite]

Once the container is full, though, you also can't put any more in.

BOSONS DO NOT WORK THAT WAY!
posted by Jpfed at 6:15 AM on April 29, 2011 [2 favorites]

Well, they kind of do. It isn't a fermi exclusion principle thing, but if you put too much energy in one place you definitely have a bunch coming back out.
posted by DU at 7:13 AM on April 29, 2011

"—imagine, for instance, a wall through which sound can enter but not escape."

-Former President George W. Bush?
-The Harper Government (TM)?
-My apartment next to the seven-day all-night rap-a-thon?
-Phil Spector's worst nightmare?
-Phil Spector's new digs?
posted by Hardcore Poser at 7:32 AM on April 29, 2011

Isn't the entire point just to prevent energy from transmitting the same way? For instance, light in, but only heat out, or sound in, but only heat (or low frequency vibrations) out? No one's saying that this has to violate any sort of laws of physics, but there are cool applications to this if it works.

Simple application: privacy windows that are transparent from the inside, dark (or at least obscured) from the outside.
posted by explosion at 7:39 AM on April 29, 2011

Oh man, the first thing I thought of was how many TERRIBLE saw-esque movies are going to be made by people stuck inside such rooms.

Right? I'm imagining a room with a wall that faces a public space, and the person locked inside can hear and see everyone just fine, but from the outside it's just a black, silent window.

And the person inside slowly goes crazy as no one acknowledges them, even though they are trapped only a foot or so away.

I kind of hate the fact that with as amazing as this technology could be, and what it could do to revolutionize different aspects of our world, that the first thing I came up with was a cheesy horror movie plot.
posted by quin at 8:06 AM on April 29, 2011 [2 favorites]

They should make this standard issue for anyone that installs ridiculously overpowered subwoofers in cars so I don't have to hear their shitty music at stop lights.

Seriously though, I want to develop one of those focused sound laser devices so that I can target these assholes with airhorn decibel level noise of my own.
posted by daHIFI at 8:36 AM on April 29, 2011

There are known objects in the universe into which light can go in but not go out.

Yes, and they required some serious rethinking of physics at the time of their discovery.
posted by overyield at 11:36 PM on April 29, 2011

There are known objects in the universe into which light can go in but not go out.

Not really, even black holes emit blackbody radiation in the form of Hawking Radiation and also they are maximally entropic to begin with, so if energy goes into them, they get larger.
posted by delmoi at 5:56 AM on April 30, 2011