Acoustic Impedance of Outlet of a Hard-Walled Unbaffled Cylindrical Duct for Multimode Incident Wave

In the paper, the specific acoustic impedance of outlet of an unbaffled duct for incident wave composed ofoneormanycircumferentialcut-onductmodeshasbeenderivedandpresentedongraphs. Whenconsideringthemultimodeexcitationtheequalenergypermodeprincipleandrandomphaseshavebeenassumed. Experimentallyverified assumption on equal energy per mode allows for quantitative analysis of the sound field and thusprovides more physical insight into the in-duct and out-duct phenomena. The assumption of random phase wasimplemented by means of the Monte Carlo method and so the expected value, the standard deviation and thepercentilecurvesoftheimpedancewerepresented. Numericaldataobtainedaccordingtothederivedtheoreticalformulaeforsingleandmultimodeincidentwavemanifeststrongmodalcharacteranddependenceonthemodesphases. The assumption of multimode incident wave has brought the theoretical model closer to what is beingobserved in practice, especially when ducts of large radius, such as heating and air conditioning systems orturbojetengines,areconsidered.PACS:43.20.Mv,43.20.Rz