RUESSELSHEIM - Acoustic research and 'noise management' has become one of the most important areas of r&d for carmakers. Advances in acoustics will help to reduce model lead times by up to two months, say Adam Opel AG engineers.
Opel has just built a third, DM10 million ($5.9 million) powertrain sound chamber at its Dudenhofen Technical Development Center and proving ground south of Frankfurt.
The lab in Dudenhofen, Germany, plays a vital role in the development of all GM Europe cars. The laboratory has one of the world's largest anechoic chambers. The laboratory's main job is to refine the design of components to reduce noise-generating vibrations.
Dudenhofen also does longer-term research.
One project is Active Noise Control. This is a method of cancelling out a noise by generating sound waves that mirror those of the unwanted noise. Researchers are also studying the benefits of broadcasting false but soothing engine noises to the driver.
'Car acoustics have matured from a handicraft to a highly technical branch of science,' says Will Firnbacher, Opel director of vehicle testing and validation.
The purpose of acoustic research has also changed. It is not aimed solely at improving the driver's comfort by making everything as quiet as possible.
'On the contrary, a quiet car can also be a dangerous one,' says Firnbacher. 'It is not just car engineers who are dealing with car acoustics now, but psychologists and the medical profession.'
An improved understanding of how noise and vibration affect drivers - and everyone who is exposed to noise pollution - has led to environmental legislation. EU noise limits for cars have been tightened from 80dB in 1980 to 74dB today.
That means that one car today is as loud as four cars in 1980, says Bernd Kessner, manager of the noise and vibration department. Brussels legislators are currently negotiating for further restrictions with auto industry representatives.
Opel cars have achieved an average noise level of 72.9dB.
'Generally 1/8sound' describes a pleasant experience while 1/8noise' is understood as being a nuisance and something undesirable,' says Kessner. 'It is our aim to prevent irritating noises and vibrations, or at least to alter them by sound engineering so that the occupants find them pleasant. The subjective assessment of 1/8pleasant' is not necessarily linked to the objective property of 1/8quiet.''
Consumers' expectations make Kessner's job more difficult.
They expect a new car to have the acoustic pattern typical of its class. At the same time drivers want noise feedback that gives them important information about their car's behaviour even at high speed. And they want to be insulated from the noise of other vehicles in dense urban traffic.
All of which makes the creation of 'the optimum acoustic environment a challenging and varied task,' says Kessner.
Dudenhofen is Opel's worldwide sound engineering hub.
'So the global responsibility for acoustics is in our hands - or in our ears,' says Kessner. 'We have to define the demands made by the (class of the) individual vehicle. We also have to consider the specific national conditions of a market - its climate, its special driving habits, and the preferences of different customers.'
Dudenhofen's test facilities and programs include a number of specialized research facilities.
Powertrain test chamber
Dudenhofen now has three powertrain test chambers. The engine, transmission and exhaust system are assembled in working trim but without any bodywork and subframes. This allows engineers to position their sensors and microphones close to the working parts.
Front- and rear-drive units can be tested at every stage of their development. Traction and braking are provided by two 111kW, computer-controlled electric motors.
The chamber 'enables us to improve the basic engineering to make our powertrains quieter, instead of devising secondary measures,' says project engineer Gerhard Hellener. 'We needed the third facility because of the increasing workload. It is a reflection of the importance of acoustics.'
One example of this is Opel's current 2.0-liter engine. Research in the two older powertrain chambers led to modifications to the engine block, oil pan and crankshaft. These changes added DM100 ($59) to production costs and 3kg to the engine's weight. But they saved DM300 and 5kg by making shielding in the engine compartment unnecessary.
'This is typical of the trade-off that can be achieved by good modern acoustic engineering,' says Hellener.
The chamber is used for whole-car noise testing, including measurement of exterior noise to ensure it meets legal limits.
Cars can be accelerated up to 200kph on the four-roller test rig. Two rows of 14 microphones on each side of the chamber are set up to simulate the pass-by test.
For interior noise testing Opel uses a dummy head with microphones in each 'ear.' Engineers can build up an overall impression of all the noises in much the way a human occupant would.
Many of the tests in the anechoic chamber are on non-mechanical components, such as tires. The pass-by test of the 3.0-liter Omega in second gear showed that 49 percent of the noise came from the engine, and 27 percent from the tires. In third gear the engine contributed 21 percent, and the tires 65 percent.
'We are developing quieter tires with the suppliers who are working very hard on this issue,' says project engineer Helmut Heusler. 'But there is more potential in road surfaces than in tires. The real problem is money. Can governments afford to spend on quieter surfaces?'
At 22x16 meters the anechoic chamber is one of the largest in the world.
Holograms can reveal 'vibration hot spots.' Components, and even whole cars, are placed in 'shakers.' These units can be shaken very precisely under electromagnetic control to imitate realistic driving conditions.
The component being vibrated is photographed by a special camera. The surface of the component is illuminated by two laser flashes, each of 30 nanoseconds. The image is processed as a hologram.
A printout can be on the engineer's desk in two minutes. The same information took a week to gather using older techniques.
Dudenhofen has two holography facilities.
Active noise control
The principle of 'destructive interference' - the method of cancelling out a noise by generating sound waves of the same frequency and amplitude as the unwanted noise - is not new. It was patented in the 1930s, and Nissan tried - and abandoned - a system a few years ago.
But active noise control has emerged as a promising way to supplement conventional noise controls, say Opel researchers, especially in light weight designs.
They have developed a system of microphones, synthesizer and speakers which can reduce the boom of a four-cylinder engine at certain speeds by up to 10dB.
Six microphones mounted in the headlining detect noises at ear level.
A synthesizer produces a counter-wave pattern to eliminate some of these noises.
To match this success in cutting engine noise by mechanical means 'would have required the far more complicated use of balance shafts,' says Firnbacher.
Opel could introduce active noise control now.
However, the market is not considered ready for this option - it would cost around DM500. However, active noise control is regarded at Dudenhofen as a valuable future weapon.
'After all, noise that no longer exists will neither have to be taken into account in design modifications, nor be suffocated with insulating materials,' says Firnbacher.
Conventional sound insulation adds DM500 to the cost of a modern car. Active noise control could be an important part of future lightweight, fuel-efficient designs, says Firnbacher.
The intensity and quality of interior noise can influence driver behaviour - and therefore safety.
A droning engine can cause aggression.
It can also reduce concentration. Noise above 65dB can affect the autonomic nervous system, reducing the driver's overall performance.
'The goal of psycho-acoustics is to achieve an acoustic environment that offers pleasant sound properties and also has a clear feedback function,' says Rudolf Bisping. He is both an acoustic consultant for Opel and a professor at Duesseldorf University.
'To produce this combination we developed a method that translates the subjective perception of noise into controllable physical values,' he says.
The research team identified the elements of an engine's noise spectrum that were perceived as positive by drivers.
The team modified a Corsa so that when it accelerated the driver heard the sound of a sports car accelerating.
Sounds which have a soothing effect on the driver can be generated in response to changes in engine speed, or to pressure on the accelerator pedal.
The system 'locates the appropriate sound, and emits it into the car interior via the loudspeakers of the audio system, so that engine noise is overlaid by a synthetic acoustic pattern,' says Bisping.
The technique is unlikely to be used on production cars for several years.