If you look at the rotor propulsor system; basically you had four, narrow diameter, jet effluxes.
To provide enough energy to lift the beast bodily you required a very high jet efflux velocity to provide the overall mass flow out of the burners enough to give sufficient rotational thrust into the rotor system. These four,narrow diameter,extremely high velocity gas effluxes then had to dissipate their energy in to the ambient air. This velocity shear was what made the noise and,incidentally is why today's large diameter high by-pass jet engines are quieter, by flattening this gradient over a series of stages of progressively lessening efflux velocities.
In autorotational flight the Rotodyne was in the same order of noise magnitude as any other twin turboprop,however the translation to and from hovering flight (and its duration) was another matter entirely . The claimed "end in sight" for the tip jet noise problem was wishful thinking ,a few db possibly by exhaust noise attenuators, but the efflux velocity shear noise situation could not be realistically overcome.
Excuses made that this problem only affected a relatively small area close to the touchdown/lift off point would not have placated those powerful entities working in the City of London for example,I would suggest.
Now,perhaps,had you blown that efflux out along most of the outer trailing edge of the rotor instead ,taking notice of the Hunting H. 126 "Jet flap" concept ........
Last edited by Haraka; 3rd Nov 2017 at 09:55.