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The farfield radiation of turbofan noise from the inlet of an engine has recently received considerable study. Relatively simple approximate expressions have been developed for single mode radiation as a function of cutoff ratio and for multimodal radiation. Convective effects upon radiation which are essential for interpreting static engine tests have been developed. This information has been essential for the design of inlet suppressors. This paper represents an initial effort to provide the same type of radiation expressions for the noise radiation from the aft duct. The results of approximate aft radiation equations are compared to more exact Wiener-Hopf radiation results. Refraction as well as convective effects in the multiple flow streams must be considered. The possibility that the acoustic power distribution is altered during transmission is also discussed. This has an impact on the design and testing of aft duct suppressors.

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Recently, approximate equations [E.J. Rice, AIAA Paper No. 777–1281 (1977)] for the farfield acoustic radiation pattern in the forward quadrant from a flanged circular duct were presented for both single and multimodal excitation. The single mode radiation equation showed that modal identification could be suppressed and that the resulting approximate equation could be expressed as a function of mode cutoff ratio only. In this paper, several exact radiation patterns for modes with similar cutoff ratios will be compared to the approximate equation to check its range of validity. The multimodal approximate equation consisted only of a simple function of directivity angle and an acoustic power biasing function, and was checked in the above reference for the equal acoustic power per mode case. Several cases of exact calculations other than equal acoustic power per mode will be presented here to test the validity of the multimodal approximate equation. A discussion of significant modal participation in a multimodal radiation pattern will be included.