Aims: In this study, the amount of the reduction in absorption silencer sound filled with Iranian absorbing materials and influence of thickness and density of adsorbent material in silence was investigated. Materials and Methods: We used galvanized channel 0.6 mm with 30 × 30 cm 2 dimension and axial fan. Length of absorptive silencer was 50 cm, and there used the mineral wool absorbent. Absorptive silencer as channel was designed to cross a section of channel and silencers after embedded the absorber be identical together. This study deals with a number of variables that affect performance of absorptive silencer in the channel and their impact on reducing the sound pressure level be examined. Results: In the frequencies 125 and 250 Hz a reduction of sound pressure level was found. While in frequency 500 Hz minimal reduction is achieved. In this study, an increase in the thickness of the absorbent material of absorption silencer, there was a little change in the volume decreasing but with increasing density from 80 to 100 Kg/m 3 better results were seen to reduce noise levels and it caused to increase NR rate. Conclusion: According to data from the present study we can used silencer with 5 cm thick and 100 Kg/m 3 density in the reduction of sound pressure level at frequencies lower than 250 Hz in the ventilation system as an effective device.