Modelling and Compensation of Nonlinear Loudspeaker

The nonlinear response of the loudspeaker is a undesired phenomenon which produces audible distortion. The construction of the loudspeaker is a trade-off between various factors and therefore makes it difficult to build a linear loudspeaker. This work takes another approach to address this issue. By using digital signal processing, the terminal voltage can be pre-distorted to compensate for the nonlinear behavior and render the system input-output linear. In order to compute a controller which achieves this, a model of the moving coil loudspeaker was implemented. This model was made from the extended ThieleSmall model. The theory of exact input-output linearization was used to compute the control law from this model. Since the control law assumes full-state feedback, a state estimator is used. Two types of state estimators, a feed-forward based and a observer based, were computed and analyzed. The feed-forward one simply uses the terminal voltage to estimate the states of the loudspeaker and can therefore be implemented without sensors. The observer one uses the theory of the unscented Kalman filter to estimate the states using the terminal voltage and the current passing through the loudspeaker. The simulation results showed that the loudspeaker can be rendered fully linear assuming that the hardware does not limit the control signal and that the loudspeaker model is perfect. Various simulation were done to simulate hardware limitation and process noise. This resulted as expected, that the system was able to compensate for the nonlinear behavior but its performance was still affected by the limitation and the noise. The unscented Kalman filter proved to be more capable of estimating the states than the feed-forward estimator when affected by process noise.