Noise reduction in ducts achievable by feedforward point control

Noise control in a one-dimensional duct is analysed. It is shown that the optimal controller leads to an unstable closed loop. The noise reduction level achievable with a stable closed loop is calculated for arbitrary choices of sensor and actuator locations. This enables the best placement of sensors and actuators to be determined. Also, the analysis indicates that a “spatial waterbed” effect exists in some configurations of active noise control: i.e. that noise levels are increased for points outside of the region over which the design is done