Right of way for vehicle acoustics

Research in focus January 2014

First of all, cars need to be fixed before the acoustic engineers of the Fraunhofer Institute for Building Physics IBP can start to carry out multiple tests and acoustic measurements at the 4WD roller test bench. As these preparations may be quite time-consuming, the scientists have come up with some innovative ideas. After having been fixed, the vehicle will be "lifted" on a mobile pallet onto the test bench by means of air cushions. These removable coverings enable test engineers to rapidly replace test vehicles, which allows the test facility to be used very efficiently: as soon as the tests have been completed or need to be interrupted, another test vehicle can be placed on the roll by means of the pallet. Eye catchers in the vehicle acoustics lab building not only include prestigious cars like Porsches or electric cars such as Teslas, visitors are also impressed by many meters of long microphone rows placed opposite each other. Accurately placed in a long straight line, they give a clue that this laboratory hosts high-level research in acoustics. Under the direction of Dr. Peter Brandstätt the 4WD roller test bench is used to perform precision measurements and comparative tests (which are required for the approval of a new vehicle, for instance) and to investigate new developments in vehicle acoustics. As early as in the developing stage it can be verified whether compliance with regulations is ensured. Taking appropriate countermeasures at such an early stage will possibly save car manufacturers high costs. Depending on the respective require¬ments, various surface layers can be used in these tests to cover the rolls: for instance, a "safety walk" – this is a smooth surface – or a rough-textured asphalt cover.

Reducing and optimising the (inside) noise levels of vehicles remains a constant challenge for developers. For example, how to achieve a low noise level in the interior of the vehicle and how to improve speech intelligibility when talking (also on a mobile phone)? Research work also focuses on rolling noise, which can be heard at lower velocities from 40 km/h on. The interior surface lining of the walls, the ceiling and the doors of the large hall reveals the many sophisticated features of this special laboratory building. So-called broadband compact absorbers, compound panel absorbers and asymmetrically structured absorbers absorb the sound so that conditions comparable to free-field conditions are achieved. The absorbers are particularly distinguished by their low depth: in spite of the lean construction there are certain absorber combinations which allow measuring even deep-frequency sounds with a lower limit of 40 Hz, which are necessary for simulating the driving performance of vehicles under real-life conditions. To allow exact simulation of the exterior noise generated by a vehicle when passing by a specified measuring point, a hall needs to have a length of at least 25 m and to house two microphone arrays at a distance of 15 m in width. "It was not easy to find the space to raise a laboratory building of these dimensions on the densely-built institute centre. With the renowned car manufacturer Porsche as our partner we were eventually able to realize this construction," Brandstätt says with some pride. For the simulated pass-by, thus the technical term for these tests, a completely neutral (in terms of acoustics) pallet or spectrum of exterior noise is used, which automatically vanishes below the hall's roof when not in use. The scientists, too, appreciate the fact that they are now able to run their tests inside the laboratory building irrespective of the weather conditions outside.
Appropriately, "We drive ahead" was the motto the Fraunhofer IBP's acousticians chose in 2008 when they inaugurated their new test bench, thus adding another unique test facility to the more than 20 specialized acoustic labs operated so far. A closer look at the technical data of this test facility explains its uniqueness. "Our 4WD-roller allows us to achieve an acceleration of 1 g, i.e. 10 m/s²; when using real vehicles, we can achieve just half of this value," Brandstätt explains. "We still have room for improvement and we are also optimally prepared to perform tests on hybrid or electric cars. In contrast to vehicles with conventional drives, an electric car, for instance, starts at full power, which our test facility can handle without problems. We want to provide our partners and customers with sophisticated acoustic system solutions in the automotive sector. The passenger space of a vehicle is subject to physical requirements we know from spaces in buildings. An interior space shall provide a comfortable environment, be free of disturbing noises and must not impair the driver's performance," Brandstätt explains. Using cutting-edge methods of sound analysis and sound design, Fraunhofer IBP scientists detect and analyse even minute details of noised generated by single vehicle parts. In this way, undesired rattling or buzzing noises inside the vehicle can be prevented, whereas a solid sound when a car's doors are opened or closed can be emphasized. Referring to sound quality, experts use terms like "rolling sofa" when describing luxury limousines or 'powerful sound character' in the case of sports cars.
A whole generation of micro-perforated acoustic components has been developed and marketed by the Stuttgart scientists in collaboration with partners from industry, and now these products are also used for optimising vehicle acoustics. Reductions in formerly predominant motor and flow noises allowed other noise components to be perceived that previously were hardly ever noticed. Here, the focus is particularly on rolling noise at low velocities from 40 km/h onwards. The contact between the wheels and the road causes resonances in the air-filled cavities of the tyres. For common tyre sizes, this causes a measurable sound level increase inside the vehicle at a frequency of about 200 Hz. This noise is perceived to be annoying and uncomfortable. A sound absorber, which acts on the air cavity inside the tyre, could lead to improvements. Here, however, fibrous materials reach their limits, as these materials would probably not remain undamaged after a certain period of regular tyre changes. Besides, the sound absorber must neither affect driving behaviour nor driving safety.
Scientists of the vehicle acoustics group developed the idea to integrate a robust, all-metal resonator with micro-perforation in a rim. In addition, the researchers placed a micro-perforated sound absorber directly inside the cavity of the tyre. An air chamber inside the rim ensures the acoustically indispensable volume. The vehicle measurements were carried out at the acoustic roller test bench of the Fraunhofer IBP using roller surface coverings to simulate rough-textured asphalt with increased sound excitation. In all cases, the noise emission could be reduced by 5 dB compared to wheels provided with a standard rim. Now that the principle works the next task will be tackled, namely to find appropriate ways to manufacture the micro-perforated elements. "In this case we could benefit from our long-standing expertise in the area of absorber development. Experience has repeatedly shown that fundamental scientific findings can often be practically implemented in various fields," Brandstätt is convinced.
Peter Brandstätt is quite optimistic about the future. Presently, comparative field tests involving various types of test tracks are being carried out. The goal is to carry out future tests using this type of test benches. Insights obtained from these tests are included in the work of the ISO TC 43/SC1 standardization group. As the example of the rim shows, research and development continue to be in high demand. He also sees good chances to get more engaged in automotive technology. Together with partners, Brandstätt would like to tackle more projects bilaterally and work on solutions that provide additional know-how which is beneficial to all parties involved.

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