Title: Sound Quality of Exhaust Pipes Through Semi-Active Noise Control
text: Due to the increasing demands on NVH levels Sound Quality has become a major issue for the vehicle development. This includes the care taken with the internal noise levels as well as with the way noise is perceived. A different issue relates to the sensation the vehicle leaves through the type of noise coming from the engine to the car exterior. A major part of this sensation comes from the exhaust system through the mufflers which also contribute to the interior noise to some extent. In situations involving one-dimensional wave propagation as in engine exhaust systems, the use of active noise control is a strategy with increasing interest. Nevertheless, considerable restrictions due to the actuators and to the computational power needed for an active control system reduce the possible practical applications. The design and implementation of a semi-active noise control system for ducts based on Helmholtz resonators are presented in this work, in order to overcome these shortcuts. The engine speed, related to predominant frequency components of the noise, is used as reference for an active tuning of the passive, dynamical characteristics of a Helmholtz resonator by varying parameters like resonator volume through a computer controlled mechanism. The practical implementation of the system and an experimental setup are discussed together with the necessary procedures for the system.