On the effects of flight on jet engine exhaust noise

Recent flight data on jet engine exhaust noise do not agree with projections based on classical jet noise theories. This paper demonstrates that these flight effects can be explained on the basis of the combination of jet mixing noise and internally generated engine exhaust noise. The source strength of the internally generated noise is assumed to be unaffected by flight, as has been shown in small‐scale free‐jet experiments. The directivity of the internall generated noise is assumed to be the same statically as that given in the NASA interim prediction method for core engine noise. However, it is assumed that in flight the internally generated noise is subject to the convective amplification effect of a simple source. The absolute levels of the internally generated noise are obtained from an empirical fit of some typical engine data. The static and flight jet noise are predicted using the NASA interim prediction method for jet noise. It is shown that in many cases, although jet mixing may be the dominan...