Researchers discover way to allow 80 percent of sound to pass through walls

Credit: Oula Lehtinen/Wikipedia

( —A team of researchers in Korea has discovered a way to allow sound to pass through walls almost as if they were not there at all. As the group describes in their paper published in the journal Physical Review Letters, the technique involves drilling very small holes in a wall and then tightly covering them with a thin sheet of plastic.

In this new effort, the researchers sought to extend prior research done by Thomas Ebbesen and colleagues in 1998 where it was discovered that holes, made in a metal sheet that were smaller than the shone on it, allowed more light to pass through than expected—a property that has come to be known as extraordinary . Subsequent research found the principle did not apply to sound waves due to rigid parts of the barrier reflecting back most of the applied sound. The researchers on this new team suspected that altering certain aspects of the barrier might allow for the property to hold for sound after all.

They began by drilling several holes (10 millimeters in diameter) in a 5-millimeter -thick piece of metal. Next, they placed a speaker on one side of the "wall" and a microphone on the other. With just the holes, they found the wall blocked sound almost as effectively as if there were no holes drilled in it. Next, they covered one side of the wall with a thin tensioned membrane (plastic wrap). After playing the sound again, the researchers discovered that the addition of the membrane allowed much more sound to pass through the wall—on average 80 percent more—almost as if the wall weren't there at all.

The membrane, the team explains, allows for "zero resistance" as the sound encounters the holes. At the of the membrane (1200 hertz), air moved in the holes as if it had no mass at all. That in turn allowed to move through very quickly. The sound in the holes was actually concentrated as it passed through, suggesting that the technique might be used as a way to magnify small signals. One application of this discovery could be walls that serve as security barriers.

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New experiment helps explain extraordinary optical transmission

More information: Giant Acoustic Concentration by Extraordinary Transmission in Zero-Mass Metamaterials, Phys. Rev. Lett. 110, 244302 (2013)

We demonstrate 97%, 89%, and 76% transmission of sound amplitude in air through walls perforated with subwavelength holes of areal coverage fractions 0.10, 0.03, and 0.01, respectively, producing 94-, 950-, and 5700-fold intensity enhancements therein. This remarkable level of extraordinary acoustic transmission is achieved with thin tensioned circular membranes, making the mass of the air in the holes effectively vanish. Imaging the pressure field confirms incident-angle independent transmission, thus realizing a bona fide invisible wall. Applications include high-resolution acoustic sensing.

Journal information: Physical Review Letters

© 2013

Citation: Researchers discover way to allow 80 percent of sound to pass through walls (2013, June 21) retrieved 22 September 2019 from
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User comments

Jun 21, 2013
I think that some motel builders have already bettered this number.

Jun 21, 2013
I think they have been using this technology in US for decades... espeically in motels and apartment buildings.

Jun 21, 2013
Why would this be a good thing? Walls are for keeping this space separate from that space.

Jun 22, 2013
Does the sound travel just as easily (with as little attenuation) in the other direction? One-way sound transmission?

The article mentioned one frequency (1200 Hz). Less effect or no effect at other frequencies? At frequencies where it is more transparent it's presumably less reflective. Seems like someone should be able to come up with ways to leverage these attributes.

Jun 22, 2013
A 10mm hole is huge, almost .4 inches. Your cellphone has a speaker smaller than that and the holes for the microphone are in the 1mm range. I am not really sure what they have proven here.

Jun 24, 2013
How is this beneficial to anybody? Oh right, it isn't.

Another waste of perfectly good money. How about finding ways to make walls thinner and block more sound?

Jun 24, 2013
How is this beneficial to anybody?

Sometimes you do not want the energy of sound to be dissipated in a layer (e.g. a sound system cover that protects from environmental influences).
Or you want the reverse: a covered/protected sensor that nonetheless can register sound.

Never underestimate the creativity of engineers to put such findings to use.

And maybe these findings can be transferred to plasmonics in general which would open up all kinds of uses in energy/heat transmission.

Jun 24, 2013
Vandalproof speakers need to be boxed to metal but still need the sound to come out.

Jun 24, 2013
How is this beneficial to anybody? Oh right, it isn't.

Another waste of perfectly good money. How about finding ways to make walls thinner and block more sound?

I love how every idiot thinks that every single scientific research project is from a federal grant. Private research?! from a corporation? Preposterous! And of course if you were to entertain that ignorant concept, these idiots I speak of are concerned about the allocation of funds no matter what country it is; as if it makes a difference in their life, across the globe. :

"Another waste of perfectly good money". Oh right, money which is of course the key ingredient in the cure for cancer. Just turn the cash into a concentrated paste and inject into the bloodstream. Complete remission.

Jun 24, 2013
And in our upcoming press release: Korean researchers discover how to reduce noise transmission through walls by 80%.

Jun 24, 2013
The easiest way to study this is to study the walls in any cheap apartment complex or hotel/motel. I'd say that in any sleazy motel, the transmission rate is more like 90%... at least, it's enough to keep a weary traveller awake all night... especially if a local hooker is using the place as her 'office'...

Jun 27, 2013
this is actually not that unexpected, as I suspect the plastic wrap is acting as a membrane vibration transmission speaker situated at the geometrically optimal location of each hole.. think cans and string without the string

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