Propeller tests have shown, that noise produced sharply increases when the prop tip speed goes beyond 0.6 mach. Extra sonic waves are produced even before the flow reaches mach 1.
If mach 1 equals 18500 rpm, then the best speed for low noise will be below 0.6x18500=11100 rpm. This is lower in rpm, than is customary in the US. In Europe, a 14"prop would be kept below 10500 in locations with noise restrictions.

Explanation for above stated:
1) The airflow around a foil varies quite a lot, and in small foil sections near the leading edge it may approach sonic speed way before the foil speed itself does. This seems to set in at 0.6 mach as indicated by prop noise measurements. Much depends on the local foil thickness and nose radius.
2) The prop rotational speed is not constant, but is maximum at the end of the combustion stroke. In a 4-stroke engine, which is worse than a two stroke, the prop has to do a lot of work before the next combustion stroke, so it slows down a lot. At a mean speed of 18500, the max speed (if maintained) may wel be equal to 20.000 rpm, which is way above sonic.

Summing up 1) and 2), any speed above 11.000 rpm using a 14"prop is prone to produce extra noise, to the extend, that "supersonic slap" can be heard in flight when rpm rise.