Sound solution

The world is getting noisier. Aircraft and traffic din, burglar and car alarms and loud music are all intrusive and impossible to block out completely. Even double glazing is not a perfect solution; it will cut out high frequencies, but the low-frequency thrum at the bass end of the sonic spectrum will still get through.

Now help for sleepless residents and perpetually disturbed office workers could be on the way. Researchers working on materials at the

in Darmstadt have devised a window that not only blocks out high and low-frequency noises but also actively cancels them out.

Sound waves are patterns of pressure transmitted through the air. When the pressure waves hit a building they cause it to vibrate and these waves are transmitted through the fabric of the building into the air inside.

The most efficient materials for transmitting the sound are glass and metal; curtain-wall buildings, where the outer skin is composed entirely of glass attached to a steel frame, are particularly vulnerable. Passive vibration dampers can be attached to the buildings but, as with the double glazing, this is not a perfect solution.

Increased thickness

The only other way to prevent the transmission of bass frequencies is to increase the thickness of the glass but this is heavy, and the support framework of curtain-wall constructions cannot support it.

The Fraunhofer team, working with researchers from the Darmstadt University of Technology, has developed a method that can prevent both window vibration and the transmission of sound vibration from steel structures to glass. Effectively an active soundproof window, the equipment is particularly effective at low frequencies of 50-1,000Hz, typical of the low-frequency noise of jet engines, traffic or bass-heavy music.

The key to the active soundproofing is piezoelectric crystals, which flex when an electric current is applied to them.

The team, led by Thilo Bein, attached sensitive accelerometers to the window frames and the glass, which measure the vibrations generated by noise. These are connected to an electronic oscillator which vibrates a piezoelectric strip placed close to where the window meets the frame. The oscillations are phased so they cancel out the vibrations caused by the incoming noise. The result - in theory  - is that the glass remains completely still, preventing the transmission of the noise.

In practice, not all of the noise is blocked. But the results are still impressive. 'Tests have shown that our windows are capable of lowering noise levels by an average of 6dB,' said Bein.

'We're concentrating on the frequency range of 50-500Hz, which is particularly troublesome. The perceived noise inside is only half as loud.' For some frequencies, he added, the noise reduction is as high as 15dB.

Bein's team has also adapted the system to prevent transmission via the frame of the building. the method is the same, but the frame incorporates stacks of piezoelectric chips that impart a bending movement to the steel, again preventing the vibration from reaching the outer cladding.

'These systems are designed mainly for apartment blocks, hotels and offices near airports and main roads,' said Bein. 'In principle it works for any sort of windows; it's embedded in the frame, near to the glass, and it's almost invisible. We're working on using transparent ceramics that could be embedded in the glass itself. They could be used in normal residential buildings - it's simply a matter of cost.'

He added that the systems could be on the market within four years.