No rattling of the cage
As the folk saying goes, “It’s not what you say but how you say it.” Applied to the sound of motion, we can say that the tone determines whether it’s perceived as a nuisance or perhaps even as enchanting. “Every noise inevitably conveys a message,” says Carsten Mohr, who leads the acoustics department at Schaeffler’s Automotive location Bühl. Whether sound is perceived as pleasant or disturbing also depends on a person’s expectations, says Mohr: “When I drive a car with a V8 engine I normally want to hear something of its sound, of the intake and exhaust noise. That’s totally different in an electric car, where low-noise gliding along the road is frequently regarded as a key selling point. Consequently, it’s important that all components be effectively coordinated with each other in terms of noise.”
Mohr has been with Schaeffler since 1996. He used to work alone and today his department has 23 people, which underscores the constantly growing importance of acoustics, a field that not only covers the issue of noise per se but, above all, its underlying mechanisms and propagation paths. That’s why in conversations with acoustics experts the abbreviation NVH can frequently be heard, which stands for noise, vibrations and harshness.
Compared to the low and therefore more pleasant sound of IC engines, electric cars, based on their design, cause a lot of whistling and whirring sounds. These higher frequencies can easily get on people’s nerves. To put a stop to that, Mohr and his colleagues take appropriate action as early as possible. “Especially for our customers it’s important to be able to estimate at an early development stage what impact electrical components are going to have on the subsequent sound pattern in the vehicle’s interior,” says Mohr. He knows that “complex troubleshooting in later development stages can only be avoided by early acoustic optimization.” Because sources of noise can overlap and therefore be amplified, weakened or masked, Schaeffler’s acoustics experts always run full simulations of interior vehicle noises in order to identify and evaluate interference factors. Comparisons have shown that those simulations very closely match the interior vehicle noise measured in the real world.
"As in the case of many developments, we need to find the best possible compromise between many factors to be considered in acoustics as well."
But why not just design every part to be as silent as possible? Mohr: “As in the case of many developments, we need to find the best possible compromise between many factors to be considered in acoustics as well. What good does the most silent e-axle do if it’s far too heavy, too expensive or delivers inadequate performance? Noise optimization must always be implemented in relation to assembly space, weight, performance and efficiency.”
Mohr can understand that manufacturers often roll out artificial sound carpets in their electric vehicles: “Cars are sold not only due to their technical characteristics but almost always also due to emotions, and sound can evoke very positive emotions. However, it turned out that a classic V8 rumble in an electric car suddenly sounds wrong and is rejected by customers.”
While Mohr thinks that it’s technically conceivable for consumers to download their own sound like a ring tone, he tends to assess that prospect as unrealistic: “The technical, signal-related effort required to achieve a sound pattern that’s satisfactory on all the seats of the vehicle is very high. Plus, to ensure a sound quality that’s typical of the brand, carmakers are already offering a variety of interior sound design options according to the selected equipment.” Like others, Schaeffler’s expert cannot predict in which direction the sound of electric vehicles will exactly be headed: “We’re still at the beginning of the powertrain transition in passenger cars. Not only we need to re-sort ourselves in that regard, but so do our ears.”