Driving is a sensory experience. Even the most pragmatic driver doesn’t think of their vehicle as a way of getting them from A to B - the feel of the car and particularly the sound are important. This poses a problem for car makers. How can they ‘design-in’ acoustic performance in their new cars? Germany’s Fraunhofer Institute is trying to find out.
Acoustic performance is determined on a variety of rolling road-test rigs. The cars sit with their wheels on revolving drums, whose surfaces are covered with strips of different road surfacing materials. This allows the makers to test performance at different speeds. However, the systems are bulky, costly and can be dangerous at high speeds.
Also, they can only test perfect road surfaces. If you want to find out what happens if the car goes over a pot-hole or any other source of vibration, you need a different rig that tests the car’s suspension with hydraulic systems.
The Fraunhofer team, from the Institute for Structural Durability and System Reliability in Darmstadt, have combined the performance of the rolling roads and the hydraulic systems and use sound-processing techniques to show what someone inside the car would hear. The vertical jolts on a road surface are overlaid by a high-frequency vibration to simulate the interaction between the surface texture of the road and the tyres.
‘We expanded the hydraulic test rigs by adding a highly-dynamic stress interface, so we can now simulate the structural loads in frequency ranges up to 50Hz, as well as vibro-acoustic stress in the 50-1000Hz range,’ said Michael Matthias, research director and deputy director of the Mechatronics and Adaptronics Competence Centre at the Institute. This is almost a reversal of their speciality, which uses piezoceramic materials to remove vibrations.
‘We use smart-structure technologies, such as piezoceramics, which vibrate in high-frequency ranges, to reduce vibrations in larger structures,’ said Matthias. ‘These could be cars, but also submarines, ships -anything that experiences high-frequency vibrations, from any source.’
The rig is a far less ‘realistic’ system than the rolling road; the wheels are replaced by electric motors that work against the acceleration and simulate the resistance to forward movement caused by gradients and friction.
The road vibrations are provided by piezoceramic actuators, which vibrate at a frequency proportional to an oscillating electrical signal and sit between the hydraulics and the car. To determine the character of the vibrations, the team attaches sensors to the tyres and interior of a car, which is driven over different road surfaces at varying speeds and with a range of tyre profiles. ‘This gives us the high dynamics, the high-frequency disturbances that come from the interaction between the street and the wheels,’ said Matthias.
The different vibration profiles are stored in a database and can be used to modify the tests on the car. ‘One major advantage of this is that we can change very quickly from a good road to a bad road,’ said Matthias. ‘This is something that can’t be done with the large, rolling-road cylinders.’
The piezoceramics provide a small amplitude vibration, around 5-100m for a frequency of 40-50Hz, but enough to simulate road vibrations. ‘We use an algorithm to modify the vibration signal in the same way that it would change on the way to the ear during real-world driving,’ said Matthias. ‘That way, we get a realistic sound.’
The rig and simulation system is still under development. At present it cannot simulate lateral vibrations, such as those that occur when rounding a curve. But once complete, it will help the team to develop piezoceramic vibration dampers.
‘We use these on the mountings of large marine engines to cancel out high-frequency vibrations,’ said Matthias, ‘and we could just as easily mount them on the chassis of a car to damp out the vibration of the road.’