Abstract: Four small rooms have been modeled by finite element method, and the proper position for the absorptive material is found through the investigation on the sound field below 100 Hz. It is shown that the modes' maximum pressure occur in all the corners in the rectangular room, but in the non-rectangular rooms, the pressure distribution is complex in some corners where some modes' minimum pressure may even occur. By comparing the average SPL curves with different absorptive material positions, it is seen that the proper absorptive material position contributes greatly to the flatness of the curves, which is also proved by experiments in a reverberation room. So the absorptive material should be placed not in the corners where the mode's minimum pressure gathered but where the modes' maximum pressure gathered. In order to place absorptive material effectively in non-rectangular rooms, the conclusion drawn by modal superposition method in rectangular rooms shouldn't be applied directly, and it is favorable to analyze the non-rectangular rooms' sound field by the finite element method.