6. Positive Position Feedback (PPF) Control

This chapter presents the design of a positive position feedback (PPF) controller based on low-pass filters and band-pass filters. In Sect. 6.1, the principle of the PPF controller is presented based on the single degree of freedom (SDOF) case. In Sect. 6.2, the loudspeaker–duct model is developed, several model interconnection methods in MATLAB are presented, and then the influence of loudspeaker dynamics is discussed. In Sect. 6.3, for the loudspeaker/microphone pair at the same location, the design of a PPF controller with an all-pass filter as phase compensation is presented. The Nyquist diagram, gain and phase margin, and root locus analysis are used to analyze the stability of the PPF controller. In Sect. 6.4, the PPF controller is extended for non-collocated loudspeaker/microphone pair. The calculation results show that the similar sound pressure reduction can be obtained by using a PPF controller with a non-collocated loudspeaker/microphone pair. In Sect. 6.5, the multimode control is discussed by using a single loudspeaker/microphone pair. In Sect. 6.6, a GUI program is given to design and analyze the PPF controller for a loudspeaker–duct model. And then we discuss how to share data between Simulink and GUI programs. In Sect. 6.7, the analog circuit for design of PPF controllers and all-pass filters are presented. Finally, some experimental results are presented to verify the simulation results.