My former club invested in a sound level meter. Loudest plane we ever measured was a Byron P-51 with their quiet mount and a Quadra (42?). Over 120 dBA slow scale at roughly 100 feet distance and airborne at roughly 50 feet altitude. Second loudest was an electric powered Weight Lifter for the College competition. 118 dBA slow scale at 9 meters on the ground. The Astro 120 (I think I remember it being a 120) was turning a 16-6/8 reinforced fiberglass prop fast enough for the tips to be supersonic. The class actually had several props delaminate under the stress. It hurt to stand close. Quietest high power we measured were a couple turbines. 72 dBA Slow scale at 100 feet and 60 feet altitude, full throttle. Many small electrics have been measured at higher than that.
Interestingly, we had been at the club's current field for over 6 months before any of the neighbors, 1800 and more feet away, even came by to see what was up. That was when the gas engine people started showing up. Before that, with just glow and electric, the noise level was not high enough to attract attention.
There have been a number of articles on engine noise in the model press. A large part of the noise does come from the prop, especially small diameter high rpm props. The noise level drops off significantly when running larger diameter props at lower speeds. Then there is the sound of an internal combustion engine exhaust. Can be reduced with efficient mufflers. Then another component of sound from the internal combustion engine, glow or gas, is actually the noise generated by the airflow into the intake. This actually is measurable, and with a great muffler and slow prop speed, can even be heard. And sometimes,on high wing area planes, resonance can set in on the wing covering and structure, creating an even higher sound than even an unmuffled engine might make. And I would imagine I'm not the only person who has heard flutter from a plane, often just before an aileron, or even a tail, decides to go it's own way.
Engine noise is quite a complex topic of discussion.
Last edited by 50+AirYears; 04-30-2014 at 10:16 PM.