Conclusion

Aero-acoustics in aeronautics... A fascinating field, thanks to the diversity of subjects covered, the knowledge required and the improvements achieved. It's also a difficult field, because of the multiplicity of sound sources involved simultaneously, and the logarithmic sensation in response to excitation (Weber-Fechner law). In other words, halving acoustic power only provides a gain of 3 dB, which is hardly noticeable; it has to be divided by ten, i.e. gain 10 dB, to give the impression that there is half as much noise. What's more, manufacturers want their aircraft to be as quiet as possible to make them competitive, but they also want (or even demand) that aerodynamic performance is not degraded.

Finally, there are other aspects to consider, which go beyond acoustics stricto sensu