The Mirror Filter-A New Basis for Reducing Nonlinear Distortion and Equalizing Response in Woofer Systems
A new filter structure, derived from the applicable nonlinear differential equation and inserted in the signal path, reduced loudspeaker nonlinear distortion caused by displacement-sensitive parameters (force factor, stiffness, and inductance) and by the Doppler effect. This filter can also be used for optimizing the linear frequency response (resonance frequency and Q factor) and for protecting against mechanical damage. There is no need for a permanent sensor, a requirement in feedback systems. To adjust the filter parameters automatically to a particular loudspeaker, an iterative method is presented, based on the electrical or acoustical measurement of the overall transfer response. Both the filter and the auxiliary systems for protection and adjustment are implemented in a DSP 56001 and result in a self-learning distortion-reduction system. The system was tested on different loudspeakers, and the measurement results are compared with listening impressions. The possible consequences in loudspeaker design are discussed.
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