Far field active noise control of a vehicle: A proof of concept study

The active control of the free field noise radiated from a vehicle is demonstrated. Active noise control (ANC) has become a commercial proposition in recent years for several applications including duct noise cancellation and reducing interior car noise. Most current ANC applications use adaptive feedforward controllers that minimize the observed energy at one or more points in the noise field to be controlled. The main advantage of adaptation is the robustness of cancellation to environmental changes. Free field ANC is not as developed as the one dimensional case of ducts, or the modal control of enclosed sound fields because of the physical requirements of replicating the free field of a distributed source. Many demonstrations have attempted a quick fix with adaptive control while not adequately considering the physical acoustics. An additional difficulty in controlling the radiated field of a vehicle is that no measure of performance away from the vehicle is possible since it is assumed that a mobile vehicle source precludes error sensing microphones. In response to these problems some researchers have concentrated on reducing the noise radiated from structures by direct vibration control of the structure using an acoustic cost function. If the noise is radiated from an exhaust pipe control within the pipe can reduce radiated noise. This research concentrates on the more general approach of cancelling noise in a limited region with the use of secondary sources that are close to, but not co-located with, the primary sources. Insights are thus gained on the ANC of any distributed source where performance measurements in the region of desired cancellation are impractical. The primary source is a large land vehicle with a diesel engine. Reduction of far field engine noise directly in front of the vehicle is the chosen goal. The thesis is a proof of concept of ANC under these constraints. The physical acoustics are investigated through the use of simulations and the performance of the proposed system predicted. A simple demonstration system is implemented to investigate the principal assumption: that a fixed, feedforward controller is robust to environmental changes, since these affect both the original and cancelled sound almost equally. The prototype scheme successfully demonstrates: up to 15 dB of cancellation in a limited region, over 10 dB for a {dollar}pm{dollar}10{dollar}spcirc{dollar} arc, over 5 dB for a {dollar}pm{dollar}15{dollar}spcirc{dollar} arc. The frequency range of best cancellation is 60-140 Hz. Cancellation was shown to be robust to both atmospheric turbulence and to a change in physical location, without any changes made to the controller. The viability of a practical, non adaptive feedforward scheme for free field ANC was demonstrated.