Nano-Biotechnology

Soundproof Windows

New windows reduce outside noise to half

By Lucian Dorneanu, Science Editor

4th of April 2007, 12:46 GMT

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You finally raised the money to build your dream house, or to buy an apartment, and you're trying to find the lowest price of a piece of land. Please take into account one easily neglectable aspect:

When an aircraft takes off over your roof every five minutes, the noise makes it impossible to hold a conversation. Road traffic or a booming discotheque often push inhabitants of the neighborhood to the edge of nervous frenzy breakdown. What can be done about it?

A double or triple layer of glass will absorb the high frequencies, but can do nothing against low-frequency noise such as that produced by aircrafts or thrumming bass tones of disco music. This would normally

call for panes of glass so thick and heavy that their use would be almost prohibitive in lightweight constructions or extensive curtain-wall facades.

When noise waves meet the walls of a building, they can be propagated to the interior by various routes. One is by causing the windows to vibrate, thus carrying the noise into the building. The other is by transmitting sound waves to the interior via the bridges in the structure where the curtain-wall elements are attached to the frame of the building.

Researchers at the Fraunhofer Institute for Structural Durability and System Reliability LBF and Darmstadt University of Technology have found a solution, namely a new type of soundproof window with active sound insulation.

"Tests have shown that our windows are capable of lowering noise levels by an average of six decibels at frequencies between 50 and 1000 hertz. The perceived noise indoors is only half as loud," says Dr. Thilo Bein, who manages the institute's department of energy, environment and health. "We have even been able to reduce the volume of certain test signals by up to 15 decibels." The experts have predicted a reduction of up to 10 dB for the engine noise of passenger aircraft in the frequency range below 1000 Hz.

Their innovative solution consists in acceleration sensors attached to the window panes that measure the vibrations generated by the noise, and a thin chip of piezoelectric material also attached to the window that counteracts the vibration by generating an oscillation at the same pitch but in the opposite sense to that measured by the sensor - causing the pane to move in the opposing direction.

"We have devised a similar solution for the points where the outer cladding is attached to the frame of the building. In this case, a stack of piezoelectric chips, rather than a single piezoelectric strip, counteracts the impinging force," says Bein.

The new active noise-reducing windows could find marketable applications in about four years' time.

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