Excellent point, the more blades there are the more noise the prop/fan produces. By using the appropriately shaped scimitar blades the noise level goes drastically down yet it is still considerable when compared to those hubs with a smaller number of blades. A typical prop has two, three or four blades, at most 5 or 6. A typical fan has from 9 to several times more blades, the more blades the more noise. So if the fan blades are straight the noise really becomes a problem at these rotational speeds. Yet when the shape of the blades are scimitar, curved feather or other shapes the noise level can be reduced quite a lot.Scimitar blades are efficient, in part, because they are quiet. A new C-130 is almost silent from a surprisingly short distance if it's taxiing directly toward or away from you. When I was in the service, there was a designated smoking area barely fifty feet off the wing tip. The old paddle blades would make you abandon your cigarette and go inside if you didn't have over-ear protection. After the switch to the J model, we could smoke and carry on a conversation just by raising our voices.
This is the reason the un-ducted fans being studied by industry have scimitar shape. Noise abatement is serious business in the commercial fleet.
The tip of the blade travels at a higher speed than the middle or the root. By designing the tip accordingly to the speed and the angles of attack most appropriate for that use the noise level and efficiency can be adjusted precisely. It makes sense, a typical efficient airfoil for low speeds can be thick and swept forward for good efficiency, so is the root of the scimitar blade. A typical airfoil for medium subsonic speeds is straight and slightly laminar, so is the middle of the scimitar blades. A typical high subsonic speed airfoil is thin and swept back, so is the tip of the scimitar blade. So if designed properly the blades can contribute to both better efficiency and less noise production.
So you are right, it is not just noise reduction, it is also efficiency improvement that is a bit better. I've seen a blade manufacturer at a fair advertise their shape on a smaller prop model, a good straight prop blade was producing around 2kg of thrust per HP while a scimitar shaped one was making more than 2.5kg. Which is quite a significant difference and it would surely affect range and fuel consumption. Using such an unducted fan for propulsion of a small fighter style experimental homebuilt 'jet' aircraft could assure better range and fuel economy at the sacrifice of speed. If anyone has more ideas or thoughts on this subject it would be nice to read them here...