Nov 19, 2015
Rüsselsheim. Silence. It is so quiet in the unusual looking room that you could literally hear a pin drop. The new Astra is positioned in the center of the room and is surrounded by numerous microphones. Is the engine sound of the new compact class star being recorded for the next advertising campaign? Not quite. When the Astra arrives in the acoustic lab, it is still in the final phase of being made ready for series production. The engineers around Bernd Justen are partly responsible for making sure that the new Astra really is a quantum leap. They ensure that the compact class car meets the demanding noise requirements and test the Astra thoroughly in the Opel acoustic lab in Rüsselsheim. Apart from being tested on the comfort and noise evaluation track at the Opel Test Center in Rodgau-Dudenhofen, all new models developed by the carmaker have to come here for final testing.
The dynamic sound of the vehicle while simultaneously delivering excellent running smoothness was always high on the list of specifications of the new Astra. Primarily, the turbo-charged gasoline and diesel engines of the latest generation that power the Astra and make its athletic appearance audible ensure this. “That is only one aspect we check in the acoustic lab,” explains Bernd Justen, Vehicle Performance Manager for compact class cars. Moreover, the Astra is put through an extensive and precisely defined requirements list. “Apart from the appropriate sound of the engine or rolling noise, this includes the sound made by closing a door or the clicking of the indicator. We initially check the simulation-based basic settings. Based on this we keep on working until the Astra meets our noise requirements.”
The walls and the ceiling of the lab are completely soundproof thanks to noise absorbing materials. During testing, the Astra is driven on large floor rollers, which produce the same resistance the car would experience on the road from wind and rolling friction. “This enables driving with road-like loads. It does not depend on the weather and we can test various scenarios under consistent conditions,” said Justen. These tests include idle behavior, the Start/Stop function and driving under part or full engine load for example. The highly sensitive microphones record all noises so they can be analyzed afterwards.
A typical procedure on the roller test bench is ‘full-load, rev-up’ in third gear – a demanding test that the all-new 1.4 ECOTEC Direct Injection Turbo, a four-cylinder Opel unit from the same engine family as the innovative one-liter, three-cylinder engine, passes with flying colors while always remaining within the pre-defined tolerance curve for cabin noise. Just like the three-cylinder engine, the 92 kW/125 hp or 110 kW/150 hp unit was developed with setting new standards in noise and vibration characteristics in mind (fuel consumption combined: 5.5-4.9 l/100 km, CO2 emissions combined: 128-114 g/km). The outstanding refinement was achieved through the noise-optimized engine block design. The implemented measures to reduce the noise level included splitting the oil pan, noise protecting the integration of the cylinder head into the exhaust manifold, designing a sound-absorbing cam cover, decoupling the high-pressure injection valves and trimming the timing chain for quiet concentricity. The test bench quickly reveals that the engineers did an outstanding job: The engines offer a pleasant and dynamic sound experience – and reduces noise levels in the cabin.
“Our dummy heads confirm the exemplary noise level,” explains Justen, referring to the weird looking figures in the Astra cabin that are reminiscent of mannequins. Plastic effigies of the human head and shoulder area that possess a perfect replica of all acoustically relevant elements of the human outer ear adorn the seats. “There are highly sensitive microphones fitted in the ears of dummy heads too. They allow us to record and replay noises in the same way that a ‘real’ human would perceive them.” This enables stereo recording and replaying, as well as differentiation between the upward and downward direction of noise, making the dummy’s hearing virtually three-dimensional.
To achieve the desired vehicle sound that is pleasant for occupants, a binaural transfer path analysis and synthesis is conducted using so-called binaural measurements. This enables a distinction between airborne and structure-borne noise source paths. “This allows us to identify and analyze the different sound sources,” explains Justen. Using a noise transfer path model, engineers can identify, for example, which screw needs to be turned to achieve the agreed target value – first virtually and then in reality. If both cases produce satisfactory results, the Astra has taken its next hurdle on the way to production readiness.
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