Sound absorbing materials can be used to attenuate sound travelling in ducts. Typically, sound absorbing materials are used to line the internal surfaces of ductwork or are used in splitters in packaged attenuators. A duct system was constructed to test various materials such as rockwool, polyurethane and polyisoprene where the expansion chamber that filled with these materials has different diameters 16, 20 and 24 cm. The acoustical parameter that evaluates the effectiveness of these materials are the sound attenuation, which can easily be measured and give a general idea of the frequency response of the silencer. In this study, the selective acoustic attenuation achieved through the use of different thicknesses of the material means that an innovative technique can be adopted for the design and construction of sound mufflers.