This paper will demonstrate the signal effects on the normal-incidence absorption coefficient. The test method covers two microphones located in impedance tube to measure this coefficient. It should analyze the measurement procedure and formulae theoretically, furthermore to propose the coherence function as the criteria for measurement results. Experimentally, different signals, generated by a sound source, input to the loudspeaker in impedance tube to produce the plane wave and two microphones can measure the sound pressures. Through different time weightings and average times on the two microphone signals, and after analyzing results of the frequency response, the coherence function and the normal-incidence absorption coefficient, the coherence function is a major factor to check the normal-incidence absorption coefficient reliable, even the test system stable or not.

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