ORIGINAL: pe reivers

Since Dick started this;
Although it is a sidestep from the actual noise measurement, how to keep the noise down is equaly important
Dick already mentioned two, but there is a scope of measures that can be taken. One should bear in mind, that if any one sound source exceeds the other one by three dB, it blankets the other source. For example, if prop noise is predominant, there is no use in improving the exhaust system. Tackle that prop noise first, and then maybe there is still a need to tackle exhaust noise or something else.

Sources of noise:

1) Exhaust noise: The loudest one.
Order of power loss by muffling:
1a tuned pipe with aftermuffler(s) - no power loss even at best muffling levels (drawback: very large systems)
1b canisters (2-3-4-chamber designs) slight power loss as chambers increase (size is within reason)
1c side nufflers have serious power loss if any form of damping is present (size is small but losses are huge)

2) Prop noise.
Tip speed should stay below Mach 0.7, or the prop may start showing flow speeds approaching speed of sound. "ripping" like the crack of a whip is the result. Thinner foils with thickest foil section far back rip later and at higher tip speeds. Thin foils can be had with carbon or metal props
Lower tip speeds can be realized by keeping the pitch up. Pitches of half prop diameter or less easily enter the noise danger zone. The sound waves may set the prop in vibration, which endangers the prop and operator as well.

3) engine vibration (caused by prop unbalance, or whatever. This will set the airframe and wings in rapid oscillating motion, and the plane becomes a noise radiator. Especially the light hovering planes are bad offenders here. They just love to vibrate worse than the engine itself. To prevent this vibration, auxiliary mass dampers can be located in the tail structure and wing tips. To prevent engine and prop vibration reaching the frame, the engine can be vibration isolated by rubber mounts etc.

4) Carb noise. Some engines produce quite a lot of carb noise. Worst offenders seem the reed engines, followed by side carb port systems. Rotary valve systems are the most quiet.
Carb noise can be dampened by ducting the air inlet from the fuselage and fitting air filters or damping chambers. (plenum chamber on the carb mouth.

5) Prop buffeting. If the prop is at any point of rotation near an airframe part, that is sure to generate quite some noise, depending on the distance from the prop hub.