There are three effects of grazing mean flow on acoustical characteristics of the micro-perforated panel absorber （1） on radiation impedance of the orifice, （2） on acoustical impedance of the construction at sound wave angle incidence, （3） on sound propagation property in a duct lined by absorber. Based on the acoustical fundamental principle, these effects were analyzed respectively, and relative formulas were derived. Some qualitative tendencies were shown that radiation impedance of an orifice, value of ξ in function coth（ξ） which estimates cavity impedance, and transmission loss in a lined duct all will decrease with flow speed increases as well as the resonant frequency will move to lower frequency caused by Doppler Effect, when sound wave propagation direction is the same with flow direction.The discussion was also supported by a relative experimental study.
According to the thickness and the configuration of sound-absorbing material,the cavity＇s acoustical impedance and the aperture＇s radiation acoustical impedance is analyzed in detail.Thus,to predict and optimize the sound absorption performance of the composite structure of sound-absorbing material and microperforated panel,a theoretical model is established. And the acoustical properties of several composite structures used commonly are comparatively analyzed both in theory and experiment.Good agreement between the theoretical calculations and experimental results shows the availability of model.Taking into account the performance and economy,the composite structure of sound-absorption material in the middle of cavity is suggested.
