Wonder how fast that propeller tip is traveling? Check out this chart. A lot of the noise generated by an RC plane comes from the propeller. When the tip reaches about 65% of the speed of sound you get a propeller induced pressure that will start to get loud. At 70% it will be quite loud (approaching 98-100dB) and at 75% it will be ear splitting &emdash; guaranteed to put you over the limit on the sound meter. So if you're turning a 12 in. prop at 15,000 rpm you're going to raise some eyebrows. That's about as scientific as we're willing to get on this subject.