Propellers and the Speed of sound I was reading this website that described a novel wind turbine technology
and it has this quote:

In the mid-20th century, open propeller-driven planes with piston engines reached performance limits as blade tip speeds neared the speed of sound.

In my mechanical engineering thermal-fluids class I vaguely remember there being some discussion about the change in turbine/compressor design needed for supersonic speeds, but I don't remember any of the concepts or equations. Why does it matter that the tip of the turbine reaches the speed of sound? Does it have any relation to the airspeed approaching from the front of the turbine? 
Is whatever sonic phenomena also independent of the size of the turbine? Like a very rotationally-slow but super large radius turbine?
EDIT: to clarify, the company is not claiming anything in particular about the speed of sound in their technology but rather drawing some kind of vague comparison between the limitations of propellers back in the day and how they are supposedly overcoming the same sort of technological barrier.
The question is what does the speed of sound have to do with limiting the ability of a propeller? It is not obvious to me why it's important that a propeller can't break the sound barrier.
 A: If the tips of the propeller blades are moving near the speed of sound a shock wave can form.  Supersonic flow has very different character than subsonic.  A propeller designed to operate at subsonic speeds will be inefficient at supersonic ones due to shock waves.  In general, shock waves cause a loss of efficiency.  You might have noticed that subsonic airplanes often have swept wings.  This is because flow over the top of the wings accelerates and can reach supersonic speeds even when the aircraft is traveling well below the speed of sound.  Sweeping the wings decreases the normal component of the velocity relative to the wing.  Likewise, you will sometimes see helicopter blades with the tips swept back.  To avoid supersonic flow at the inlet of turbine blades you will observe an increase in area from where air enters the engine to when it reaches the blades.  As long as the flow entering the engine is subsonic, the increase in area will slow down the speed of the fluid and avoid supersonic flow over the blades.
A: A prop is actually a wing. If you've seen the shock wave of a supersonic jet you see how it actually comes at a higher degree away from the aircraft wing, like a 45+ degree angle. It seems like it would just flow along side the jet,but it's like trying to push too much water out of a dam, if the opening is too small the pressure of all the force balls up. That same effect happens to a prop.  Now that they finally realized what the rest of the world already knew, that angle of attack against air caused more propulsion than ... faster air over the top of a wing "sucking it up" into the air, you can see that the angle of attack of the leading edge actually leaves too much low pressure behind the blade. It's like trying to gain more speed when there is nothing to push against. imo anyway. A way to kind of think of is as a boat prop cavitating in a way.
A: The statement in the first paragraph "In fact, today’s standard turbines are based on the same physical principles as 18th century windmills." is marketing hooey.  They are hanging their hat on the fact that the windmills were unducted props and most of today's turbines are the same, which is true.  The airfoils used today are not 18th century designs.  They claim that ducting is magic and increases efficiency amazingly.  A couple decades ago there was an effort to design ducted props for aircraft.  I am not aware of any that are in commercial service.  I suspect that the designers of wind turbines are aware of this and decided not to duct their turbines.  There are gains available, but are they worth the cost?
A: The highest wind speed ever measured outside of a tornado was 113m/s, approx. one third the speed of sound. Needless to say, no wind turbine will be operational at that wind speed, let alone operate with its wing tips close to supersonic mode. Smells like the pseudoscience it is. 
