Press Release
Engine noise: Quiet performance and well-designed sound
Mercedes-Benz engine designers worked to avoid noise and design sound with state-of-the-art measuring technology and great meticulousness. Nearly all of the well over 200 separate parts of the powertrain were subjected to acoustic testing and assessed for volume and frequency, from crankcase to engine mount, from piston to injection valve.
The goal of the engine specialists was to obtain an agreeable sound pattern in every driving situation. To achieve this, the absolute values of the noise development were measured and acoustic interference sources eliminated, which in themselves are not loud but with load and speed changes can make their presence disagreeably felt at certain frequencies, and thus have a subjective effect on the sound pattern.
For example air intake: with its frequency spectrum, the engine intake noise spreads strongly in the direction of the vehicle interior and can thus impair pas-senger noise comfort. One result of the analyses using state-of-the-art measuring techniques aligned to the actual audible impressions is the newly developed intake pipe from nylon fabric, deployed for the V6 and V8 engines of the new S-Class. In contrast to the smooth plastic that was used previously, this material is sound-absorbing and considerably reduces noise intake.
The issue of noise alignment is increasingly become one of the main functions in engine development – mainly because engineers must solve the conflicting objectives of how to achieve lower passing noise and agreeable, sonorous engine sound. With the six and eight cylinders of the S-Class, the
With the newly developed V6 diesel engine in the S 320 CDI, the second-generation commonrail technology ensures quiet performance and permits double pilot injection. In less than a millisecond, small pilot volumes of fuel flow into the cylinders and effect an even better preheating of the combustion chambers before the main injection. This results in smoother combustion development and considerable lower operating noise of the diesel engine.
Vibrations and acoustics comfort: Audible progress in the Interior
It is not only the airstream which generates vibrations and noise. Axles, steering, tires, engine and powertrain also have a large impact on acoustics comfort. These components are also examined in the noise laboratory and optimised in meticulous detailed work. Examples:
In order to prevent the low-frequency vibrations generated by the road sur-face, Mercedes engineers examined not only the engine mount, spring bearing and shock-absorber mount, but also for the first time included the seats in their calculations. The result is an optimised seat structure and specially coordinated seat cushions which compensate for the disturbing low-frequency resonances with their dampening effect.
To avoid road and tire excited steering wheel vibrations, the
At the front the interior is shielded with two acoustically effective systems: with absorber made from high-quality resin foam at the firewall and with a “soundproof” dashboard, which is completely insulated in the front area, thus transferring no disturbing noises from the engine into the interior. The channelling areas for cables and wires were also sealed hermetically.
The Mercedes engineers also reduced the tire rolling noise which is perceptible primarily in the lower speed range (up to 100 km/h). This was done by the targeted deployment of high-quality insulation materials. Together with the high induction stiffness at the coupling points to the chassis, they reduced the noise level in the interior against the predecessor model by up to three decibels.
Overall some 170 fibre, fleece and foam elements are used for sound insulation in the new S-Class.
Test Centre: with up to 260 km/h through the Hall
Acoustic waves and vibrations are tracked in twelve state-of-the-art test chambers and measuring facilities in the Mercedes-Benz Technology Centre in
One of these is the external noise test chamber which is used for test cars making the “accelerated drive-by test” without moving. A total of 21 microphones set up in long rows to the left and right listen to every detail of the vehicle. As the car does not move, the engineers consecutively switch on the microphones in line with the vehicle speed in order to simulate the drive-by. This is an intelligent measurement technique which guarantees the highest level of repeat accuracy.
In order to meet the prescribed test conditions, the external noise test chamber must have an impressive size, even though the vehicle is stationary. Indeed, with a floor space of 18 times 25 metres, it is larger than a tennis court. Hundreds of metre thick, grey insulation elements made from rock wool cover the walls and the roof of the seven-metre high hall in order to exclude each and every sound reflection. Even possible external vibrations that could falsify a measurement were rigorously excluded: the hall floor which weighs well over 1,000 tons is placed on a huge air suspension system.
Unlike open air conditions, secondary noises or weather influences must not be allowed to impact the measurements in the state-of-the-art measuring hall. The simulation of actual driving conditions for the rolling noise measurements is guaranteed by a roll test chamber anchored in the floor. On its realistic road surface test vehicles reach top speeds of up to 260 km/h. As a supplementary drive, two 180 kW electric motors bring the large roll mass of the roll in movement. At the same time a 200 kW airstream blower provides the necessary cooling for the engine.
In their tests, Mercedes acoustics experts are interested not only in the volume passers-by perceive of an automobile passing by. They also intensively examine the noise world that the driver and passenger experiences, sometimes for hours without interruption. To do this, they also place passengers in the vehicles for measuring trips in the large hall: dummy heads that exactly match their human model. However, in the auditory channels there are not eardrums, bones and nerves, but highly sensitive capacitor microphones. They allow not only a spatial recording corresponding to the auditory perception of the ear, but also serve to obtain precise sound pressure and frequency measurements.
During the tests, the dummy head transfers data to a computer which reproduces the measuring results as coloured diagrams – more precisely designated spectro-grams. What appears to the layman as a colourful juxtaposition of curves and lines provides the engineers with information about causes of the disturbing resonances, which they can then eliminate in a targeted fashion. In the early eighties, Mercedes-Benz was in the vanguard in deploying dummy head stereo-phony. Using this technology, it has achieved notable progress in the area of acoustic comfort, which is audible in the new S-Class.
