I'm not the worlds greatest expert on small turbines, I'm mostly quoting experience. On the SAAB Gripen for example, the perceived noise level from the APU is around 4 times that of the main engine during ground checks, and the small turbine (made from a truck turbocharger unit I seem to recall) we had in the underground lab when I was at University needed 3 layers of glass AND ear defenders to become bearable. I could be wrong about RPM being the main player, but certainly all my experience is that small turbines tend to be significantly louder than large ones.
G