Holes Provide Acoustic Screening

Scientists in Spain recently made an intriguing discovery while performing experiments on transmitting sound through metal plates. They placed a series of millimeter-thick metal plates in a tank of water and transmitted ultrasonic waves of different frequencies through the material, measuring how much of the sound made it to the other side. The normal rule for how sound transmits is the "mass law," stating that doubling the mass per unit area of a solid reduces the sound passing by -5 decibels.

However, when holes are placed in the metal plates, the scientists noticed a big drop in the amount of sound transmitted. The team found that the sound attenuation (the drop in sound) was greatest when the spacing between the holes was approximately equal to the wavelength of the sound being transmitted.

This is in contrast to a previous paper which showed that a different array of holes in plates actually increased transmission. This process was dubbed by the discoverers Yan-Feng Chen and Ming-Hui Lu as "extraordinary acoustic transmission" or EAT.

This new behavior has similarly been dubbed "extraordinary acoustic screening" (or EAS). Francisco Meseguer, one of the physicists who wrote the paper, believes that the dampening is a result of wave interference created by the holes in the plates. The next step would be to perform the experiment in air, which would require a much larger experimental set-up. Hopefully this process can be used to create more effective acoustic screening, such as that used in machine housing. The major benefit of this screening is that it would still allow light and air to enter freely, while blocking sound.

Related Articles:

• Physicsworld.org - Holes prevent sound from passing through plate
• Physics Review Letters - Extraordinary Sound Screening in Perforated Plates (abstract available for free, full paper requires subscription ... or you can get it from Cornell University)
• Physics Review Letter - Extraordinary Acoustic Transmission through a 1D Grating with Very Narrow Apertures (abstract available for free, full paper requires subscription - paper is from October 2007)
• Interference, Diffraction, and the Principle of Superposition
• Huygens' Principle
• Mathematical Properties of Waves