The effects of flanking transmission on sound attenuation in lined ducts

It is well known that flanking sound transmission can affect the acoustic attenuation in ducts lined with dissipative material. The exact nature of the flanking paths, “air-borne” (sometimes termed “radiation by-pass”) through “breakout” and subsequent “breakin” of sound through the duct walls, or “structure-borne” through axial transmission of weakly coupled (and lightly attenuated) structural/acoustic waves, is less well known. In this paper, an approximate analysis is presented for the case of a lined sheet metal duct located in a reverberant sound field. A Rayleigh-Ritz procedure is used to model the structural/acoustic modes propagating in the duct. The principle of reciprocity is then used to model the effects of breakin from the reverberant enclosure. Theoretical predictions of sound pressure on the duct axis are compared to experimental data for a typical air-moving duct, and the relative importance of the two flanking mechanisms is made clear.