Here's a useful formula that came from an article by Steve Boser, at Sensenich Propellor Co.

It was in his reducing sound levels article, of the Nov/Dec 2009 issue of Airboating Magazine. (page 41)

Prop tip speed (in miles per hour, or mph) = prop RPM x prop diameter (inches)/336.

It was a pretty decent article. If you care about sound level, you should read it.

Thanks,

Duane