Just when you thought you needed a lot of money to do physics experiments, along comes a team of scientists in Japan and Korea making a wall sonically invisible with plastic wrap. We'll tell you why a little barrier is more sonically transparent than no barrier at all.
Putting a wall between you and a sound will cut down on the amount of sound that you hear. Putting holes in that wall will make the sound a little louder. And putting a barrier over those holes will, it turns out, make it like the wall was never there in the first place.
Not all kinds of barriers will turn a wall sonically invisible. The material of the barrier has to be something fancy, like cling wrap. This is what a team of Japanese and Korean scientists found, when they tested out different substances. They started with a thin layer of rubber over holes, but found that ordinary kitchen wrap works better and is easier to use.
How does a layer of plastic help transmit sound rather than muffle it? It helps make the air pressure on both sides of the wall equal. Empty holes let bits of the sound through, but the wall reflects a lot, and there is a difference in pressure between one side of the wall and the other, inhibiting sound. When the plastic wrap vibrates, it actively pushes the air on the far side of the wall around, and helps make the pressure on both sides of the wall equal. If the pressure waves of sound are the same on one side of the wall as they are on the other, the wall, acoustically, doesn't exist.
Some experiments found that the plastic wrap transmitted only three-quarters of the original sound, but with the right size holes in the wall, about ninety-seven percent of the sound gets through. Is it wrong to automatically think of putting blindfolds on concert goers and watching them walk into walls? It would make a great music video.
Top Image: Ildar Sagdejev
Via Physics Central