Noise breakout from flat-oval ducts

Abstract

Two theoretical treatments concerning the transmission of internally propagated sound at low frequencies through the walls of “flat-oval” ducts are described. One of these is based on a numerical solution to the equations of motion for the duct walls, and the other on a forced wave duct wall response to the internal sound field. Only the plane internal acoustic mode is considered. Both theories are in reasonable agreement with experimental data at low frequencies, though discrepancies are noted in the region where higher order acoustic modes can propagate within the duct. The chief feature of interest in the wall transmission loss mechanism of flat-oval ducts is that the flat walls radiate most sound energy at low frequencies, and a mass law behaviour predominates, but at high frequencies a ring resonance effect, connected with the curved walls, is noted; this lowers the transmission loss near the ring frequency, where the curved walls radiate the greater part of the acoustic power.