Much has been written about how quiet and refined Vauxhall’s new Astra is, and now the company can reveal how this has been achieved.
When the Astra arrives in the acoustic lab, it is still in the final phase of being made production-ready. Engineers, led by Bernd Justen, Vehicle Performance Manager for Vauxhall/Opel compact class cars, are responsible for ensuring that the compact class car meets the demanding noise requirements and test the Astra thoroughly in the Vauxhall/Opel acoustic lab in Rüsselsheim.
The Brit-built Astra was tested in the comfort and noise evaluation track at the Dudenhofen Test Centre, in Germany, where all new models developed by the carmaker have to come for final testing.
Reducing NVH (noise, vibration, harshness) in the new Astra’s cabin was always a high priority, especially from the latest generation turbo-charged petrol and diesel engines that power the Astra.
“That is only one aspect we check in the acoustic lab,” explains Justen. 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 behaviour, 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 analysed 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 unit from the same engine family as the innovative one-litre, three-cylinder engine, passes with flying colours while always remaining within the pre-defined tolerance curve for cabin noise. Just like the three-cylinder engine, the 125PS or 150PS unit was developed with setting new standards in noise and vibration characteristics in mind.
The outstanding refinement was achieved through the noise-optimised 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 reduce noise levels in the cabin.
“Dummy Head” Testing
“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 enables us to identify and analyse 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.
