Based on the analysis of the characteristics of sound wave propagation in a duct, three Active Noise Control (ANC) systems, which incorporate different arrangements of the secondary loudspeaker have been examined experimentally. Calculated and measured transfer functions of the plant and the error paths show good agreement in the acoustic modal frequencies of the systems. Real-time tests show a more effective noise attenuation with 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 be also achieved by using an equalizer.