Acoustics BMW
In order to be able to drive comfortably and efficiently at the same time, the developers look for solutions in acoustic lightweight construction with acoustically effective components for increasing comfort and also yielding benefits in weight and installation space through intelligent material concepts. In this way, the customer gains a functional advantage with lower weight and more pleasure for lower fuel consumption and CO2 emissions. The minimisation of background noises normally requires the use of heavy insulating and dampening materials. This minimises surface vibrations. Nowadays engine parts are becoming lighter and lighter through new materials, while efficiency requirements rise, which means that crankcases, for example, are being made out of aluminium and there is a lack of material insulating against disturbing combustion noises. Intelligent measures are in demand for maintaining driving comfort. Acoustic engineers achieve this through systematic reinforcement of the crankcase, among other things. The procedure known as “ribbing” systematically minimises disturbing emanations and the crankcase remains, all in all, very lightweight. In addition, the engine is partially encompassed by an acoustic capsule. These absorber or insulation components require little installation space, are lightweight and at the same time quite effectively reduce noise emanation. Absorption and insulation directly at the engine has additional benefits to efficiency: if disturbances are reduced directly at the source, no costly, difficult insulation of the passenger compartment is necessary. This helps reduce material, weight, and fuel consumption.
Another method of acoustic lightweight construction for increasing comfort and efficiency is the integration of acoustic functions in the existing vehicle parts. In the undercarriage structure in use with current BMW models, which improves aerodynamics, a LWRT (lightweight reinforced thermoplast) replaces the former subframe made of polypropylene, which was heavier and fitted with more absorption material, and took up more space. With the new undercarriage structure, the absorption function is already integrated into the surface of the subframe. This reduces weight and installation space, while considerably enlarging the absorption surface at the same time. Only two to eight millimetres thick, as opposed to the previous maximum of 30 millimetres, the new structure is significantly thinner than before and only half as heavy as the previous structure of subframe plus additional shock absorber.
While the acoustic lightweight construction helps to fine-tune vehicle acoustics and increase comfort, active systems, such as Active Sound Design, ensure that the engine produces a dynamic sound during acceleration, because vehicle dynamics are an auditory phenomenon. With Active Sound Design, engineers can create the sound that best fits the vehicle character or even fulfil drivers’ individual auditory desires.
“To create the desired acoustic patterns, we’re refining the natural character of the engine with an electro-acoustic system so that acceleration becomes a special audio experience and provides even more pleasure.”
(Albert Kaltenhauser, Manager for Airborne Sound, Acoustics and Vibrations)
Presented in a MINI prototype for petrol engines in 2009, Active Sound Design now also helps diesel engines achieve a sporty sound that wasn’t possible for this type of engine until now. The sporty performance characteristics of modern diesel engines and their high torque, especially when starting up and accelerating, contradict the acoustic sensation of diesel vehicles. Harsh ignition impulses during combustion, which are inherent to diesel engines’ function, are responsible for their characteristic noises — commonly referred to as “knocking”. This undesirable acoustic characteristic of diesel engines has been brought to a very low level at the BMW Group through intelligent acoustic lightweight construction. This makes room for Active Sound Design, which supports a sporty sound quality. It makes outstanding driving performance audible.
In order to have a special audio experience and for a diesel vehicle to sound really sporty, sound designers have to optimally adjust the sound to the vehicle and its engine performance. Too much sound with too little engine power would make a negative impression. The engine sound must constantly provide for a harmonious driving experience. The particular challenge is allocating the right dosage of sound in all driving situations and creating an authentic audio character. An active system allows for significantly more systematic and finer adjustments than classic sound design, which is oriented on the intake or exhaust system.
“Even small changes to vehicle sound can have a big impact, since human hearing subconsciously evaluates acoustic surroundings like a high-performance analyser and all changes are continuously registered in the brain.”
