Arrangements of the secondary source on the performance of active noise control systems

Three active noise control (ANC) systems, which incorporates different arrangements of the secondary loudspeaker, have been examined experimentally, based on the analysis of the characteristics of sound wave propagation in a duct. Measured transfer functions of the plant and the error paths show good agreement with the theoretically derived transfer functions in the acoustic modal frequencies of the systems. Real-time experiments showed a more effective noise attenuation with a tree-shaped configuration than with the traditional T-shaped ANC system. The symmetric arrangement of two secondary speakers in the tree-shaped system helps in compensating the node observed at 200 Hz in the error path. This compensation can also be achieved by using an equalizer