Soundproofing Drowns Out Deep Noise

If the booming bass of your neighbor's music keeps you awake at night, a new soundproofing method may be the stuff that dreams are made of. A material made of tiny, vibrating spheres that pulse copies of a sound back toward its origin can block noise that passes through current soundproofing. The lab demo may be developed into thin, silencing sheets that bring peace of mind. One of the material's inventors says another application might protect buildings from seismic shocks.

Current soundproofing uses materials that resist vibration and transform sound waves into heat. It doesn't work well at low frequencies, however, because the longer wavelengths pass relatively unimpeded through the thickness of the material. It takes about five times as much padding to shield a 200 Hz sound, such as a church bell, as it does a 1000 Hertz sound, which is about the frequency of a whistle.

To quiet deeper disturbances, a team led by physicist Ping Sheng of the Hong Kong University of Science and Technology coated 1-centimeter-diameter lead spheres with a thin layer of silicone rubber, then encased them in a hard matrix of epoxy. The spheres vibrate inside the soft silicone as though they were suspended by springs. When a sound with a frequency close to that of the resonant frequency of the spheres enters the material, the spheres vibrate in their soft silicone sheaths, sending out sound waves in all directions. Some of those waves interfere with the forward movement of the offending noise, quieting it. Testing the material, the researchers could attenuate sound by 20 decibels at both 400 Hz, the resonance frequency of the spheres, and at 1400 Hz, the resonant frequency of the silicone layer. By tinkering with the materials, they reduced noise at other frequencies as well, they report in the 8 September issue of Science. By stacking layers of differently tuned spheres, they say, they should be able to soundproof the entire sound spectrum.

The method achieves significant noise reduction with relatively small structures, says theoretical physicist Eleftherios Economou at the Foundation for Research and Technology Hellas in Heraklion, Greece. "This is a simple, but clever idea," he says. The principle behind the new material may work on a much vaster scale as well. The team suggests that suspending somewhat larger spheres with springs would reflect seismic waves in much the same way that smaller spheres disrupt sound waves--an interesting application, Economou agrees.