The noise emission of cruising vehicles essentially consists of tire/road noise, drivetrain noise (engine with intake and exhaust system, transmission and driving axle) and aerodynamic noise due to the flow around bodywork, chassis, wheels and cooling air flow (fan). Engines and drivetrains have become quieter due to many man-years of engineering attention and tire noise has also been reduced - at least the noise reaching the passenger compartment. Consequently, the aerodynamic noise of ground vehicles has become dominant at driving speeds above 100 kph both in interior and exterior noise.
In order to determine the contribution of aerodynamic noise to the overall noise, measurements are carried out more and more in specially equipped automotive wind tunnels. Besides wind tunnels in which aeroacoustic testing was already envisaged in the specification and design phase, existing wind tunnels can be upgraded to acoustic testing by a suitable treatment of the tunnel airpath and the plenum chamber.
The existing 22.45 m2 Open-Jet Automotive Wind Tunnel of Stuttgart University - Institute of Internal Combustion Engines and Automotive Engineering (IVK) - built in 1988 was converted into an aeroacoustic wind tunnel in 1993 by a novel silencing concept. In the return duct of this Göttingen-type wind-tunnel two U-bend silencers were installed upstream and downstream of the fan, using membrane absorbers as thin and smooth vertical splitters (for the low frequency range) in conjunction with profiling the turning vanes with coated porous Polyesterfoam (for the middle and high frequency range). An anechoic plenum chamber was achieved with a new type of broadband absorbers, mounted on the plenum walls and ceiling. Due to its very low self-noise level the IVK Aero-Acoustic Wind Tunnel provides a high potential for the acoustic rating of design optimizations in the development of future vehicles.