Active Noise Control of Enclosed Sound Fields Optimising the Performance

Doctoral thesis, 2003

Active noise control is the art of reducing a primary unwanted sound field by interference with an actively created secondary sound field. To achieve high reduction, it is crucial to obtain both temporal and spatial matching between the primary and the secondary sound fields. Temporal matching is maintained by using a fast, powerful control system that is capable of tracking any changes in the input characteristics without delay. Spatial matching requires that the positions for the actuators and sensors in the control system are carefully selected. In Active vibration control, the primary objective is to minimise a vibration field. This is done using structural actuators and sensors. Active structural acoustic control reduces the sound radiation from a vibrating surface. In this case, structural actuators are used to minimise the sound field as measured by a number of acoustic sensors. This thesis is primarily dedicated to the active control of harmonic, enclosed sound fields. A number of multiple-error, multiple-output control algorithms, derived from the complex LMS algorithm are presented. Among the virtues of these algorithms are fast convergence and low computational complexity. Performance results are presented from off-line computer evaluations using in-flight data from a Dornier 328 twin-propeller aircraft. Results are also presented from measurements in a Mock-Up of a SAAB 340 aircraft. In most practical applications of active noise control, finding the optimal positions for the actuators and sensors in the control system is a difficult task that goes well beyond acoustic intuition. This is especially true in applications where the control sensors must be placed away from the desired ''zone of quiet''. A method is presented for searching for a sub-optimal configuration of actuators and sensors, along with a discussion on evaluation methods. The search algorithm is based on a model for simulated annealing.