On the characteristics of modal radiation from ducts above and below cut-off

This work investigates the acoustic radiation from a submerged semi-infinite pipe, such as for marine ducted propellers, which are generally close to the duct open end compared to the acoustic wavelength. Radiation from the duct therefore comprises both propagating and cut-off acoustic modes. Whilst the radiation from propagating modes is well understood and can be predicted from classical theory, relatively little work has been undertaken aimed at understating radiation due to cut-off modes. This paper is a fundamental study into the characteristics of modal radiation from rigid flanged circular ducts, with particular emphasis on the radiation from cut-off modes.This paper presents a Fourier analysis of the radiation from the duct to allow separation of the component of velocity that radiates to the far-field and that which decays and is responsible for the duct near-field. We show that cut-off modes can potentially radiate effectively to the far-field and are shown to be particularly important for the radiation towards the sideline directions. Cut- off modes must therefore be included in any prediction of duct propeller noise when the source plane is relatively close to the duct open end, such as in many marine applications.