Aims: The purpose of the present work was to consider the effect of different samples thicknesses on the acoustic absorption coefficient. Materials and Methods: An impedance tube was built with two microphones accordance to ISO-10534 and the American Society for Testing Materials-E1050 standards. For the measurement of absorption, the study was carried for 25 and 30 mm thicknesses of closed cell polyurethane foam, polystyrene, polyvinyl chloride (PVC), rubber, mineral wool, carpet, and glass samples. Measurements were performed by impedance tube and VA-lab4 software. Results: In carpet and mineral wool with more thickness, the absorption was increased but, the carpet with less thickness showed more sound absorption in the frequency range of 1500-3600 Hz. The peak of the absorption coefficient of 25 mm glass was 0.36 that the amount was reduced to 0.2 in the 30 mm thickness. Furthermore, the difference between the peak absorption of two thicknesses in polystyrene sample was equal to 0.29. In fact, polystyrene with less thickness had better sound absorption. The same situation was happened for glass in frequencies of below 4500 Hz with less thickness. Conclusion: Incident sound energy, which is not absorbed, must be reflected, transmitted, or dissipated. The porous materials had a higher absorption coefficient. Carpet and mineral wool samples had the highest absorption coefficient, but the materials such as polyurethane foam, PVC, and rubber had lower sound absorption.