Pitch perfect: Dummy heads help fine-tune cabin sounds and in-car engine noise
Coming soon: ADAM ROCKS available to order, in dealerships from late September
Rüsselsheim. The countdown has begun: in less than two months’ time the new ADAM ROCKS will start rolling into Opel dealerships. From late September, Opel’s mini crossover will offer a zest for adventure and open-air driving fun at prices starting from just 15,990 euros (RRP incl. VAT in Germany). To make the ROCKS experience as noise-friendly and vibration-free as possible, testing in the acoustic lab at Rüsselsheim has been an essential part of its development process. This is in addition, of course, to all the work carried out on the ride comfort and noise measurement track at the Opel Test Center Dudenhofen.
It all starts with the list of ‘must haves’
The development team must first fulfill a huge list of requirements under the umbrella of ‘noise and vibration’ – these range from ensuring a crisp engine note to achieving the quietest possible operation of the Swing Top canvas roof; and from giving the turn indicator a pleasant ‘click’ to assessing the closing sound of the doors. These elements, and many more, are then finally tested in a complete vehicle in Opel’s acoustic lab to ensure all the team’s calculations and the car’s engineering functions meet Opel’s demanding noise requirements.
The walls and roof of the acoustic lab are totally sound insulated. It’s like a radio studio, with noise-absorbing materials giving it an unnatural quietness. The room is packed with strategically placed microphones and loudspeakers. During testing, the ADAM ROCKS 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 throughout a range of road-like speeds. Under controlled conditions, independent of weather, engineers test various scenarios such as idle behavior, the Start/Stop function and driving under part or full engine load.
A typical procedure on the roller test bench is ‘full-load, rev-up’ in third gear. It’s a demanding test the ADAM ROCKS’ all-new 1.0 ECOTEC Direct Injection Turbo passed easily by always remaining within the pre-defined tolerance curve for cabin noise. Opel’s pocket powerhouse, which is configured to deliver 66 kW/90 hp or 85 kW/115 hp, comes with an all-new six-speed manual transmission and is not only economical and climate-friendly, but also silky smooth to drive. Its noise and vibration characteristics, as well as its running smoothness, are superior to many four-cylinder engines. Numerous sound engineering measures contribute to the engine’s benchmark refinement, such as an acoustically-developed cylinder block, a sump-mounted balancer shaft and structural isolation of the fuel injection system and crankshaft. They have all demonstrated their effectiveness in the acoustic lab. Engine covers optimized for acoustic attenuation also contribute to the ROCKS’ pleasant soundtrack, and its Swing Top canvas roof, fitted as standard, keeps unwanted secondary noise at bay.
Dummy heads and ears help deliver a pleasant aural experience
Apart from running quietly and sonorously, the ADAM ROCKS should also sound powerful and meaty, particularly during off-road-like drives. Here Opel engineers have been assisted by using artificial heads, plastic recreations of human head and shoulder geometry reminiscent of dummies or mannequins. They are fitted with a precise reproduction of all the acoustically relevant parts of the outer human ear. Microphones are then placed within these dummy ears to receive and record noises just like a real person 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’s 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. Engineers can therefore identify and analyze different sound sources and vehicle noise paths. In the ADAM ROCKS, this analysis includes noise from the engine, the engine mountings, the exhaust system, chassis components, the body, and even the car’s cabling.
Using a noise transfer path model, engineers can identify, for example, which screw needs to be tightened on Opel’s urban crossover to reach agreed target values. When improvements are needed, the change is first made virtually and the transfer path model is configured accordingly. Engineers listen to the result on the computer and, if they are satisfied, they then make adjustments on the actual vehicle and test the desired sound in real life. If this test is also passed, they can be sure the ADAM ROCKS sounds as good as it looks.
