Reduction of the inner vehicle noise level by active vibration damping of the rear axle drive

For rear-wheel and four-wheel drive vehicles the so-called rear axle gear whine is a well-known acoustic phenomenon. The focus of the research project conducted by Volkswagen AG and the Fraunhofer Institute for Machine Tools and Forming Technology IWU which is presented here was a marked reduction of the noise phenomenon for vehicle interiors by means of an active regulatory system for absorbing vibrations. Because this noise, caused by the gear mesh in the rear axle gear units, is dependent on the driving speed and load state, a permanent frequency and phase adjustment in relevant speed ranges is necessary. Regulatory concepts that execute real-time frequency detection and also continually determine the energy content of the excitation signal were developed for this purpose. Within the framework of the project, two actuator concepts were implemented and tested on the vehicle. These included a piezo stack actuator as well as an inertial mass actuator. Both actuator systems achieved a clear reduction of the interior noise pressure level in the relevant operating state. The effect is clearly (and subjectively) perceptible to vehicle occupants and has been assessed by experienced acousticians as an improvement of 2 BI.