A prop can resonate at diffrent rpms. It depends on the natural resonance of the prop.

A 14" prop will start making the speed of sound noise you hear around 12,000 rpm.

Props will make noise, sometimes without getting even near the speed of sound. I have a Tower 46 two stroke. With a 10-6, it'll run around 14,400 rpm which is not near the speed of sound for a 10" prop but you can hear the prop sound over the sound of the engine.

Whatever it is, I know the four strokes begin making a certain resonant sound at the higher rpms

Props will begin to make a substantually louder noise somewhat before they reach the actual speed of sound.

Often the resonance of the airplane itself enters the picture with the covering acting as a drum. You can hear this sound over the sound of the engine or prop. This seems stronger with the four stroke engine operating at lower rpms during 3D exercises.

Enjoy,

Jim