New technique allows sound to pass through walls

Scientists have developed a new technique to allow sound to pass through walls.
The technique developed by South Korean scientists involves drilling very small holes in a wall and then tightly covering them with a thin sheet of plastic.
The new study extended on earlier work by other researchers in 1998 where it was discovered that holes, made in a metal sheet that were smaller than the wavelength of light shone on it, allowed more light to pass through than expected - a property that has come to be known as extraordinary optical transmission.
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.

Jong Jin Park of Yonsei University in Seoul, South Korea, and colleagues suspected that altering certain aspects of the barrier might allow for the property to hold for sound after all, 'phys.Org' reported.
They began by drilling several holes (10 millimetres in diameter) in a 5-millimetre 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 per cent more - almost as if the wall weren't there at all.
The membrane allows for "zero resistance" as the sound encounters the holes.
At the resonance frequency of the membrane (1200 hertz), air moved in the holes as if it had no mass at all. That in turn allowed sound waves 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.

The study was published in the journal Physical Review Letters.