Abstract: The characteristics of porous sound absorbing material have great effects on the behavior of a pneumatic silencer. Measurements of the centerline mean velocity, total pressure, and distribution of turbulent intensity were made to investigate the flowfield generated by compressible air passing through the porous material (polyvinyl formal) with different thickness. The details of the flowfield are different from those of the free jet of a convergent nozzle. The airflow from the porous material includes common micro-jet flow, combining and spreading flow. The effects of the porous sound absorbing material on the noise attenuation include two aspects one is the total pressure is decreased, and the other is the outer flowfield such as its axial velocity profile and distribution of turbulent intensity is changed due to a great deal of micro-jet flow interaction.