This paper presents an analysis of perfect noise cancelling controllers for acoustic ducts. The analysis is applicable to many acoustic noise control applications such as active noise suppression in air-conditioning ducts. Implementation issues for an 'ideal' noise cancelling controller are discussed. Practical difficulties posed in otherwise ideally suited infinite-dimensional controllers are outlined. A mathematical analysis is given to demonstrate that the control system designed based on the perfect cancellation technique is highly sensitive to imperfections in any practical implementation of the theoretical infinite-dimensional controllers.