The sound specialists at Mercedes-Benz achieved the remarkable acoustic qualities of the new A-Class through a process of rigorous and meticulous development work underpinned by the services of cutting-edge computer and test-bench technology. Even before the first prototype was built, powerful computers explored how the car’s individual body components would respond to vibrations caused by engine or suspension movements. The next step involved the bodyshell – fresh from a process of virtual optimisation – being subjected to various critical frequencies on a structural dynamics test bench in order to analyse and enhance its responses to vibrations in practice.
Although the initial test results for the new A-Class were extremely impressive, the acoustic specialists were not satisfied and pushed for further improvements. This involved gluing in metal plates to provide additional reinforcement in three areas of the main floor and the spare wheel recess. The A-Class is also fitted with a firewall made from composite metal, which significantly improves soundproofing to the tune of an impressive five dB (A).
On the exterior noise test rig, each individual engine/transmission combination had to complete a variety of tests with a row of 21 microphones set up on either side. The aim of these exercises was to establish how loud the noise generated by the A-Class when accelerating and driving past was perceived to be by passers-by. And here, the compact Mercedes dips significantly below the limits stipulated by law.
The Mercedes-Benz sound experts used the interior noise test bench to explore the measures available for optimising the acoustic surroundings for the driver and passengers. One of the solutions they opted for was a glued-in roof lining, which significantly reduced the level of noise emissions compared with the conventional attachment method, whilst at the same time enhancing air-borne sound absorption. Further progress was achieved through the preference for innovative insulating materials such as melamine resin foam – in place of the mixed-fibre material previously used – for absorbing the noise emitted by the powertrain. The new material may be 25 percent more expensive, but it weighs around 40 percent less and is just as efficient. For other areas, the experts favoured a single-type microfibre material over a similarly effective mixed-fibre alternative.
Acoustic fine tuning eliminates obtrusive noise
In addition to the aim of reducing overall noise levels, the Mercedes acoustics experts also devoted close attention to filtering out annoying disturbances caused by shaking and vibration. To this end, they used sensitive measuring instruments to identify components generating irritating background noise and introduced the optimisation measures necessary to eliminate the source of the disturbance.
In the fine-tuning process for the vehicle acoustics, the engineers employed a total of 19 sophisticated measuring devices to optimise the car’s sound engineering. These included an echo chamber, in which the sound permeability of individual components – the doors and floor panel, for example – was measured and improvements made. Another was the small-assemblies test bench, which helped the engineers to develop a pleasant operating sound for specific components such as the motors for the power windows.
Optimising the car’s aero-acoustics was particularly high on the priority list, as wind noise becomes the single loudest sound inside the car between 100 and 110 km/h. In the “whispering” wind tunnel, where wind speeds of up to 260 km/h can be reproduced without the installation itself creating any significant amounts of noise, the aero-acoustics experts searched meticulously for sources of obtrusive noise and subsequently eliminated them.
Having said all that, the declared aim in the development of the new A-Class’s sound characteristics was in no way to create a totally silent vehicle. After all, a variety of sounds give the driver valuable information on the status of the vehicle and also have an emotional effect. For this reason, the acoustics specialists defined a desired target sound for each engine and each driving situation as part of their overall focus on sound engineering. The result is a sonorous tone for the new A-Class which exudes power and strength but never becomes overbearing. At the end of the day, the compact Mercedes-Benz still has to pack the vocal punch of its bigger brothers.