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On selecting sensor and actuator locations for ANC in ducts

We investigate the effects of microphone and speaker locations on the performance of active noise control in ducts. We study a so-called "symmetric" configuration in which the collocated performance microphone/control speaker and the collocated measurement microphone/disturbance source pairs are located equidistance from the duct center. By applying an "alignment angle" analysis to a duct model, we show this configuration to be prone to plant-disturbance misalignment. As a result, amplification at the measurement microphone and reduced stability margins occur. Stability robustness to modeling uncertainty is improved if the measurement microphone/disturbance source are de-collocated.