Nowadays, there are mainly three kinds methods used in absorption coefficient measurement of sound absorbing materials. They are standing wave tube, reverberant room and normal loudspeaker reflection method. The standing wave tube and the reverberant room belong to the laboratory methods, which are not suitable for measurement in situ. The normal loudspeaker reflection method can measure the absorption coefficients in situ. However, it has limitations on the size of the material and the measuring room.

How to measure absorption coefficient of sound absorbing materials exactly is a critical point in acoustic engineering. In order to solve this problem, researchers of the Key Laboratory of Noise and Vibration Research, Institute of Acoustics conducted a series of experiments and solve the problem innovatively. 

Based on the high directional property and the plan wave property of the self-demodulated sound from parametric array, under the excitation of sweep frequency sound wave, the researchers could measure the absorption coefficients of the materials in free field and normal rooms. The experimental results are compared to the standing wave tube method, which shows the precision of the measurement result got by the new method. The new method not only settles the cumulative effects problem, but also deduces the influence of the environment to the measurement results.