The work described in this thesis was directed towards the development of a compact hybrid Passive/Active noise absorption system that would be effective over a wide frequency range. The Passive/Active system efficiently integrates both passive and active noise control methods. Both theoretical and experimental investigations were carried out in this work. A simple numerical model was developed to simulate the Passive/Active system. The passive sound absorbing material used in the system was the partially-reticulated polyurethane foam. This material was characterized using its sound propagation constant and its characteristic impedance which were empirically determined. Three different control strategies were investigated for the Passive! Active system. These control strategies were: (i) directly minimizing the reflected wave, (ii) inducing a pressure-release boundary condition on the back surface of the absorbing layer and (iii) a new approach consisting of minimizing the reflected wave in the airspace. The latter results in a match between the impedance in the air cavity and the impedance of a plane wave in air. This impedance-matching control approach was selected for the Passive/Active system because it meets the goal of optimum absorption over a wide frequency range and offers practical advantages. In this system, the error sensing process takes place inside of the airspace which results in a compact design with all the necessary sensors and actuators built into the system. Parametric analysis were performed on the impedance-matching control approach to investigate the sensitivity of the performance of the system to variation in absorbing layer thickness and airspace depth. The performance of the system was determined to be independent of the airspace depth and marginally sensitive to absorbing layer thickness. In addition, in order to study the feasibility of using the Passive/Active system for other sound absorbing materials, an experimental investigation was performed with the polyurethane foam replaced by a porous metal sheet, commercial name FELTMETAL®. The result of this investigation showed an improvement in the absorption property of the FELTMETAL, opening a number of potential applications for the Passive/Active system by using different absorbing materials. / Master of Science