THE EFFECTS OF FLANKING TRANSMISSION ON SOUND-ATTENUATION IN LINED DUCTS

It is well known that flanking sound transmission can affect the acous tic attenuation in ducts lined with dissipative material. The exact na ture of the flanking paths, ''air-borne'' (sometimes termed ''radiatio n by-pass'') through ''breakout'' and subsequent ''breakin'' of sound through the duct walls, or ''structure-borne'' through axial transmiss ion of weakly coupled (and lightly attenuated) structural/acoustic wav es, is less well known. In this paper, an approximate analysis is pres ented for the case of a lined sheet metal duct located in a reverberan t sound field. A Rayleigh-Ritz procedure is used to model the structur al/acoustic modes propagating in the duct. The principle of reciprocit y is then used to model the effects of breakin from the reverberant en closure. Theoretical predictions of sound pressure on the duct axis ar e compared to experimental data for a typical air-moving duct, and the relative importance of the two flanking mechanisms is made clear.