That noise will be one of the focal points of the study.
Most fans that I have ever seen are straight. Which is very noisy due to the supersonic turbulence at the tips of the blades. Hence, why they are ensconced in a shroud or cover. The article says that these covers are also there for safety. Should a fan blade fail. A concept that made me chuckle a bit. Nothing in an airworthy airframe is going to stop a failed fan blade.
I believe that I have read some of the supersonic blade tip noise can also be reduced by aerodynamic shaping of the blades. Which is illustrated. There’s a price to be paid in efficiency by doing so. Also, as you say contra rotation or even stationary blades behind the fan could possibly help with the turbulence and reduce the noise.
Interesting plot you posted. 
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