Considerable progress has been made in active noise control (ANC) in the recent decade. Compared with the conventional “passive” noise control, ANC reduces noise by introducing a number of controlled “anti-sound” sources (secondary sources) so that the sound field generated by these sources interferes destructively with the sound field caused by the original sources (primary sources). Compared with the conventional “passive ” noise control, ANC reduces noise by introducing a number of controlled “anti-sound ” sources (secondary sources) so that the sound field generated by these sources interferes destructively with the sound field caused by the original sources (primary sources).