Sources of the Sounds We Hear:
For our powered airplanes, the sound heard comes from several sources. The four major sources:

• Exhaust
• Propeller
• Intake
• Airframe

Measuring Sound Levels:
The standard we currently use for measuring is:

• airplane on the ground (grass) - pointing crosswind, exhaust on the upwind side.
• sound meter is 10 feet away from the crankshaft on upwind side of the model.
• plane is mechanically held - or by a person on opposite side of the meter.
• Readings are taken at FULL throttle using the "A" and "SLOW" settings on the meter

Alternate methods are being considered for future including:

• raising model from ground to minimize echo or muffling from ground.
• changing distance / dBa level to base on 25 feet to better allow for larger planes. The dB meter is quite close to the wingtip of 40% plus airframes at 10 feet.

Sound Measurements:
Some measurements we have made in the past using the method above:
Sound Measurements

Experience has shown us many planes can make our dBa limits with only minor changes, ranging from a different prop, to new mufflers.  We have had planes ranging from Giant Scale Gas Powered planes to a 40% TOC style aerobatic plane meet our noise limits.  The costs were not significant (in proportion to the cost of the model!) to achieve this.

Resources:
While Skymasters is one of the quietest clubs in Metro Detroit area, other parts of the country - and the world - have done more. Here are some links to sites in UK where noise has been a large problem. Note for reference, the UK standard is measuring at 7 meters.

A very thorough explanation of noise and ways to reduce it, and other reference links:
(I was going to do a lot more typing till I saw this site!)
Model Aircraft Noise Reduction

The experiences of the Felbridge Flyers (UK)
Their Noise Reduction Chart

Thanks to Skymaster Sherman Dickson for providing the above links.
If you have other resources of interest, please email us, and we will include them!

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