THE NETHERLANDS: Stopping windscreen noise

Windscreens are not just something to look through, say European university scientists: they also act as loudspeakers, attracting and magnifying noise created by a car or lorry and reflecting the racket back at drivers and passengers alike. This fact has made researchers in the European Union-funded and University of Twente, Netherlands-coordinated InMAR (Intelligent Materials for Active Noise Reduction) project consider how to change the materials making windshields, so they absorb noise rather than amplify it. The project is likely to spur technological developments in the automobile industry.

InMAR has already made significant strides since it started in 2004, working out ways of making gearboxes and air conditioners quieter. But it is with windshields that the project may have hit on a formula with potential for future development.

The key is piezoelectric ceramics. In standard cars, vibrations created by a vehicle's operation shudder through the body and reach the windshield, which reflects them as sound waves into the interior.

InMAR scientists have used piezoelectric ceramics to mount windshields so that when these materials are subject to vibrations, their stiff and brittle nature shakes at the same frequency - cancelling the original movement.

Project manager Dr Arthur Berkhoff of the University of Twente said: "If we can cancel the sound from the windshield, the noise in the interior can be reduced."

The principle is similar to that of noise-cancelling headphones that are now sold to air travellers. They emit noise at the same frequency as the ambient sounds outside and cancel them out. His colleague, Dr Joachim Bös of the Technical University of Darmstadt, Germany, said: "The volume of individual test signals can be reduced even up to 15 decibels."

Berkhoff said InMAR has used this technology to make gearboxes quieter, installing loudspeakers nearby that emit low frequency sounds at exactly the same frequency of noise created by a car's gears.

"The pressure goes down. The principle really works," he stressed. "That said, high frequency sounds are far more difficult to match accurately and so some sounds still get through."

Berkhoff's is team also working on using soft mounts for gearboxes, limiting the vibrations they pass on to the chassis. And they developed rubber mountings for air conditioner compressors for the same reason. These have also included piezoelectric ceramic patches and, as a result, additional oscillations in the passive vibration absorber can be induced, which actively change the natural frequency of the damper... making it more effective, said an InMAR report.

Despite these advances, some noise is still getting through and that is where the windshield comes in. Berkhoff and his team are experimenting with developing a thin, transparent film made of piezoelectric ceramics that could cover a windshield, making it even less susceptible to vibration and noise generation.

"If the transparency could be improved, you could integrate the ceramics into the windshield and this would make a much more powerful [noise dampener]," he said. "We have started research and we have already made improvements."

The more transparent that scientists can make the ceramics, the thicker a windscreen film could be, making it better able to cancel out vibrations from the rest of the car.

Funded by the EU's sixth framework programme for research, InMAR involves 41 partner organisations from 13 countries and has been coordinated by the Fraunhofer LBF institute, in Darmstadt, Germany, and has a EUR27 million budget.

Project coordinator Dr Thilo Bein, of Fraunhofer LBF, explained the project's key principles: "Sound waves must be decreased exactly in the frequency ranges, which are perceived as very stressful. Noise consists of many overlapping sound waves in different frequency ranges. Due to the adaptability of active structure systems, the vibration behaviour can be changed in those areas where they are most effective".

www.inmar.inf

keith.nuthall@uw-news.com