Abstract
The acoustic radiation from a source inside a hard-walled semi-infinite duct or tube, such as for ducted fans and rotors, is investigated. In particular a study of cutoff modes, which have the potential to affect the far-field pressure radiation for sources close to the duct open end, is presented. A large amount of work has been done to mathematically and experimentally investigate propagating cuton acoustic modes. However, there is very little published work on the radiation of cutoff modes. This paper concerns the radiation characteristics of cutoff modes to determine their behaviour, and their likely influence in the far-field directivity for sources close to the duct open end. It is shown that cutoff modes are an essential contributor for the radiation in the rear-arc.
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