This got me thinking about my old Cessna 185. It was on floats, and was equipped with an 86" propellor that turned at 2850 rpm for takeoff. That yields a tip speed of 729 mph, or ~Mach 0.96 at sea level standard air pressure. Between the engine and the prop, it was an incredibly loud machine

Planes like the C-185 floatplanes were the ones I was thinking about when I wrote, "Even for some light planes whose props exceed about Mach 0.9, the noise levels get very high, making them unpopular in populated areas." A T-6/SNJ trainer seems to fall into this category as well. As long as we have people building houses next to airports and then complaining about the noise, we have to be aware of just how loud some planes actually are.