Researchers from Magdeburg dampen electric car noise with granulate from old tires

„Weil das gewohnte Motorgeräusch fehlt, treten andere Töne deutlich hervor", the researchers explain as the starting point of their work. At low speeds, electric cars must even generate artificial sounds so that pedestrians or cyclists can perceive them at all.

What the researchers are working on

Instead of using damping mats to minimize the vibration of the power electronics, the cavities in which the electronics are located are filled with a granulate. It is obtained from old car tires. The particles move in the component similar to sand in a rattle. This allows them to dampen vibrations right at their point of origin – before any audible sound is even produced.

Hermann Rottengruber, head of the Institute for Mobile Systems at Otto von Guericke University Magdeburg, summarizes the approach as follows: „Wir zeigen eine zusätzliche Anwendung dieses Materials in der Fahrzeugtechnik und verbinden akustischen Komfort, Leichtbau und Kreislaufwirtschaft". In contrast to damping mats and other soundproofing devices, the granulate is lightweight, requires no additional space, and is inexpensive.

What the measurements show

In that frequency range, the research team was able to achieve a significant reduction in sound pressure level. The range between 800 and 1,100 hertz is particularly critical. In the frequency range especially sensitive to humans, the researchers were able to reduce the noises by about 6.5 decibels. For individual components, almost 10 decibels less was achieved, a halving of the original volume. This corresponds to an assumption of the perceived loudness to just over 60 percent of the original value.

„Das klingt wenig, ist aber deutlich hörbar. Es kann den Unterschied machen zwischen einem auffälligen Sirren und einem unauffälligen Hintergrundgeräusch", says Rottengruber about the measured values. In the cavities, the granulate distributes itself like sand and prevents the vibrations that can become intense as power increases. The granulate prevents transmission to other components.

Project leader Braj Bhushan Prasad puts the results in context: „Die Partikel entziehen der Struktur Energie, bevor daraus störende Geräusche werden". However, its density and the grain size must be precisely calculated for each component. Only then can the particles be used with maximum effectiveness. Prasad formulates the open research question: „Wir müssen genau bestimmen, welche Kombination unter welchen Bedingungen am besten funktioniert".

From wind turbine to electric car

The technology is not entirely new: it was first used in wind turbines to dampen the vibrations of the components. Now the Magdeburg researchers are transferring the principle to the power electronics of electric cars. In Germany alone, around 50 million old tires are generated annually. Only a small portion of the old ones is reused or retreaded. Most are burned, for example, in cement plants, or end up as mats on sports or playgrounds. With the new process, another application would be added.

In addition to use in electric cars, future use in inverters or control units is also conceivable, wherever disruptive vibrations occur in electronics. In the long term, the approach could find application far beyond electric cars – wherever electrical systems are exposed to vibrations.

Next steps toward series readiness

However, it is still a long way to an application in series-production vehicles. „Noch steht die Technik am Anfang", the research team says. Before the granulate could be used in a production model, long-term durability, long-term behavior under temperature fluctuations, and the effects on other components would have to be thoroughly tested.