Synopsis: Wall Filters out the Sound You Want to Hear

Sound at a specific frequency passes through a wall with membrane-covered holes, as if the wall weren’t there.
Synopsis figure
Courtesy J. J. Park/Yonsei University

Researchers have created the opposite of a soundproof wall by punching small holes in a metal sheet and then covering the holes with tightly stretched plastic wrap. At the membrane’s resonance frequency, incoming sound waves pass through the wall nearly unimpeded. The design, described in Physical Review Letters, was inspired by composite structures called metamaterials, which can manipulate light and have been used to make invisibility cloaks and other strange devices. A wall “invisible” to particular sound waves might be used as a filter or even an audio microscope.

Naively, light or sound waves should not be able to pass through holes smaller than their wavelength. But “extraordinary” transmission through walls with subwavelength holes is made possible by various resonant effects. Recently, experiments showed that light transmission could also be enhanced through channels filled with metamaterials that have a near-zero permittivity (equivalent to having “zero resistance” to electric fields).

Jong Jin Park of Yonsei University in Seoul, South Korea, and his colleagues have now explored enhanced metamaterial transmission in an audio setting. They began with 5-millimeter-thick aluminum walls perforated with one or more 10-millimeter-diameter holes. To create a similar “zero-resistance” effect, they covered the holes with 10-micron-thick plastic film. At the resonance frequency of this membrane (1200 hertz), air moves in the holes as if its mass were zero, which speeds up the corresponding sound waves. This acceleration results in 80% of the incoming sound transmitting through the wall, no matter the incident angle. The concentration of sound inside the holes (as much as a 5000-fold increase) might be used to magnify a small signal, as is done in near-field optical microscopes. – Michael Schirber


More Features »


More Announcements »

Subject Areas


Previous Synopsis

Biological Physics

How Plants Do Their Math

Read More »

Next Synopsis


Cosmology Near and Far

Read More »

Related Articles

Focus: Making Shear Waves for Ultrasonic Imaging

Focus: Making Shear Waves for Ultrasonic Imaging

New filter allows shear waves to be produced efficiently, which could lead to higher resolution ultrasound images. Read More »

Synopsis: Jiggles that Help Bat Biosonar
Biological Physics

Synopsis: Jiggles that Help Bat Biosonar

The directional sensitivity of bat biosonar is improved by the wiggling of structures on the bat’s nose and ears. Read More »

Focus: <i>Image</i>—Sound Waves Guided Along Curvy Path
Condensed Matter Physics

Focus: Image—Sound Waves Guided Along Curvy Path

A new image from 3D computer simulations demonstrates that tiny, randomly arranged pillars can allow an acoustic wave to be efficiently guided through an arbitrarily shaped channel. Read More »

More Articles