Quiet please!

  • 26. November 2015
  • Mobility Concept
  • Text: Rüdiger Abele

Cities are inextricably linked with people and their cars. But what can be done when it gets too cramped for both? Creating separate spaces for vehicles and pedestrians used to be the standard response of urban planners. Today it is the lowering of noise emissions that is making city centres nicer places to be.

Noise is a nuisance. In your average city, vehicles, industry, commerce, roadworks and heavy machinery raise the decibels to unhealthy levels. To make its cars quieter, Mercedes-Benz meets and often even exceeds legal requirements.

For 2024 the upper limit for a pass-by noise test is already set at 68 decibels. When designing new vehicles, manufacturers are therefore careful to choose sound-absorbing materials and particularly quiet components, and to optimise aerodynamics with a view to reducing noise.

QUIET IS TRUE LUXURY

At the same time, sound is one of the factors that provide a sensation of superior comfort, an area in which Mercedes-Benz vehicles have always excelled. “Ultimately that is the most important focus of our work,” says Sperber, Head of Acoustics for Vehicle Concepts at Mercedes-Benz: “It’s about meeting customers’ expectations in terms of acoustics, right through to the stereo system. And a quiet car provides the optimum basis for high-end sound.”

“A new acoustic test rig is currently in the making at the Mercedes-Benz Technology Centre in Sindelfingen,” says Tobias Beitz, Head of Sound Quality and Sound Design. “Three core objectives are being worked on there: reducing noise, for example from the drive system, eliminating background noise and designing sounds, such as the ticking of the indicator.”

TURNING NOISE INTO SOUND

A change of perspectives can work wonders. In places where noise levels cannot be reduced, new urban planning concepts are helping to make city life more pleasant. Not far from Daimler headquarters at Sindelfingen is the town of Böblingen, where the company conducts its research and advance development projects. Here, a sound installation at a busy central intersection filters the noise from passing vehicles.

Fountains positioned along the intersection well up audibly whenever the traffic lights are green. The result is that the pass-by noise, which can reach up to 80 decibels, recedes into the background. If pedestrians have a green light, the fountains murmur softly and the town centre turns wonderfully quiet for a moment.

From a global standpoint, Böblingen’s Elbenplatz square may only be a tiny intersection. But vehicle noise can be a real problem for people living in towns and cities of all sizes. Böblingen therefore offers a good example of how noise perception can be actively influenced in busy urban centres.

ELECTRIC VEHICLES FOR QUIETER ROADS

As electric cars increase in popularity, there is growing hope that they will help to reduce noise. But according to the German Federal Environment Agency (UBA), that will only be the case up to a point: below 25 km/h, electric cars are indeed quieter than their internal-combustion counterparts. But at higher speeds, tyre noise and wind turbulence become responsible for the roadside concerto – just like with regular cars.

The UBA believes that other types of vehicle – from trucks, buses and waste collection vehicles to mopeds and motorcycles – have a bigger part to play in bringing down noise levels. If these have an electric drive, they run far more quietly than before. The critical factor, however, is the proportion of the fleet accounted for by electric vehicles – and it will be a long time yet before this becomes significant.

Ironically, electric vehicles can sometimes be too quiet: For safety reasons, some countries require them to be fitted with noise generators that are activated primarily when the car is travelling at low speeds – this is to warn pedestrians who might otherwise not hear them coming.

The thing with noise

The decibel is the most commonly used unit to quantify sound pressure level, measured in dB(A). The numerical value represents a relative figure based on logarithmic functions. Because it is logarithmic, and not linear, every increase on the scale is equivalent to a dramatic increase in loudness. As the decibel level rises, the corresponding sound pressure becomes increasingly harmful for one’s health. A whisper measures around 30 dB(A), while a bird singing or music playing quietly on the radio measure around 50 dB(A).

 

The World Health Organisation (WHO) classifies constant exposure to a noise level of 55 decibels or over as a health hazard, associated with a rising risk of cardiovascular disease. Examples include: vacuum cleaners (70 decibels), older cars (75 decibels) and heavy (truck) traffic (80 decibels). People who are continuously subjected to noises louder than these, and who don’t take protective measures, are at a high risk of suffering permanent damage to their hearing and health.

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