An active noise reduction system using adaptive filters. A method of operation the active noise reduction system includes smoothing a stream of leakage factors. The frequency of a noise reduction signal may be related to the engine speed of an engine associated with the system within which the active noise reduction system is operated. The engine speed signal may be a high latency signal and may be obtained by the active noise reduction system over audio entertainment circuitry.

Active noise reduction adaptive filter leakage adjusting

A method for determining leakage factors or adaptation rates, or both, for adaptive filters in an active noise reduction system. The leakage factor or adaptation rate, or both, may vary depending on a parameter of an input reference signal. The parameter may include one or more of reference signal input frequency, rate of change of reference input signal frequency, if a predetermined triggering condition exits, or if a predetermined event has occurred.

Active noise reduction adaptive filter adaptation rate adjusting

A single input, single output active noise control system using the time-domain Filtered-X LMS algorithm with output constraint is investigated. The constraint on the output of the control filter is applied by three different methods: the leakage algorithm based on the transformation method using a penalty function; the re-scaling algorithm based on the active set method; and the simple practical (clipping) algorithm which just clips the output if a constraint is encountered. A comparison of the three algorithms shows that the re-scaling algorithm can usually work successfully under the constraint, while the leakage algorithm usually needs a large leakage coefficient to satisfy the constraint with a resulting performance loss. The clipping algorithm has potential problems both with the stability and convergence speed.

A study of time-domain FXLMS algorithms with control output constraint

The acoustic environment in an automobile cabin has a significant effect on the perceived quality of the vehicle. There are two components of the acoustic environment in the automobile cabin: the noise due to automotive processes and the sound produced by the car audio system. In both cases active methods can be employed to improve the acoustic environment and this thesis presents an investigation of both active control of automotive noise and active sound reproduction systems in cars. In the context of active noise control, cost-effective systems are investigated for the control of both engine noise and road noise. A model of structural-acoustic coupling is first derived and the effects of coupling on the performance of feedforward and feedback controllers is investigated. Feedforward control has been highlighted as a cost-effective method of controlling engine noise. In order to achieve low-cost active control of road noise, however, a number of multi-source, multi-sensor feedback active noise control systems are investigated. A modal feedback control system is investigated and shown to offer significant levels of control of a single dominant resonance. In general, however, a fully multi-input multi-output (MIMO) feedback controller is necessary to achieve control of the multiple resonances present in a road noise disturbance. Measurements have been conducted in a small city car to validate the proposed MIMO feedback controller and a real-time feedforward engine order control system has been implemented. Active control of sound reproduction has also been investigated with the aim of producing independent listening zones in the front and rear seats of the automobile cabin. Simulations have first been used to investigate the effect of implementing a personal audio system in a car cabin and to compare the performance of acoustic contrast and least squares optimisation strategies. In order to achieve control of the sound field over the full audio bandwidth two arrays are implemented, one employing the four standard car audio loudspeakers and one employing phase-shift loudspeakers mounted to the headrests. The performance of the system is validated using a real-time implementation in a people carrier.

Active control of the acoustic environment in an automobile cabin

This paper studies the application of the linear least-mean-square (LMS) algorithm with leakage to a system having a memoryless saturation-type nonlinearity. This approach represents an interesting alternative to the nonlinear LMS (NLLMS) algorithm. The major drawback to implement the latter is that the model parameters of the system nonlinearity must be known. In contrast, the linear LMS algorithm with leakage does not require such knowledge. It permits to approximate the performance of the NLLMS by properly selecting a constant leakage factor value. To cope with an eventual change of the environment, a strategy for a variable leakage is also proposed. Several numerical simulations are presented with the aim of ratifying the feasibility of the proposed approach

On the LMS algorithm with constant and variable leakage factor in a nonlinear environment

Adaptive algorithms are presented for constrained multichannel feedforward control. They minimize the sum of squared error signals while limiting the value of either the sum of squared control signals (total effort) or the mean square value of each individual control signal (individual efforts). It is shown that the exact least squares solutions for these problems have a simple form, which can be approached with an adaptive algorithm. The properties of the resulting algorithm are illustrated by simulation.

K.H. Back

Papers

Effort constraints in adaptive feedforward control