| Description |
This dissertation investigates techniques to combat perceptible nonlinear distortions introduced by audio systems during playback. These techniques are based on physicsbased models of the distortions and their generating processes. Two nonlinear audio devices investigated are direct-radiator woofers and waveguides coupled with compression drivers. The first contribution of this dissertation is a method to compensate for driver-induced distortions. A novel equalizer is introduced to compensate for nonlinear distortions in direct-radiator loudspeakers in a closed cabinet by means of an exact inverse of an electromechanical model of the loudspeaker. This system performs equalization of excursiondependent and current-dependent nonlinear distortions as well as distortions due to eddy currents. The states are estimated directly from a desired output signal and, using preequalization, generate the required driver stimulus which is fed to the driver. In the ideal case of exact parameter availability, our approach demonstrates upwards of 20 dB reduction in both total harmonic distortion and intermodulation distortion. Under the assumption of perturbed model parameters, these distortions are reduced by approximately 10 dB. Next, this dissertation describes a method to perform identification of the nonlinear time-varying speakers. Equation-error identification is used to characterize a 12-inch subwoofer in a block-wise manner. Assuming a parametric lumped-element nonlinear model and using Hartley modulating functions, the governing equations are transformed into a ‘linear-in-the-parameters' system of equations that can be solved using least-squares. In our preliminary evaluations, normalized root-mean-squared prediction errors from the estimated models are comparable to the standard in loudspeaker measurement and identification for low and high stimulus levels. This enables identification and adaptive tracking of the loudspeaker nonlinear parameters which further enables the construction of distortion mitigating equalizers and reliable speaker protection systems. The nonlinear parameter estimation method is described with a normalized root-mean-squared current and displacement error under 8.5 and 7.37 %, respectively, at high-stimulus-signal levels. Finally, a model-based precompensator for propagation-induced distortion of acoustic waveforms in an air-filled waveguide at high sound pressure levels is developed. The nonlinear distortions introduced by propagation are modeled using Burgers' propagation model. This equalizer is stimulus-independent and mitigates distortion at some predefined distance. The preequalizer digitally propagates the waveform backwards using a sign-inverted propagation model. The output of the preequalizer is fed to the waveguide. When the forward model completely characterizes the waveguide, the waveforms arriving at the chosen destination will be identical to the input signal fed to the preequalizer. Experimental evaluations on sinusoidal and multitone testing stimuli demonstrated approximately a two-fold reduction in the intermodulation distortion and and a four-fold reduction in total harmonic distortion at lower frequencies when compared with a modelbased method available in the literature. |
