Another cause of noise is the airframe itself. Vibrations cause the covering to act like a drum, exhaust stacks cut to just exit the cowl can cause reverb within the cowl, loose fitting cowls, canopy hatches and a host of other things.

I had once had a Yak that (I will not mention the brand) the canopy hatch was very loose fitting. After the engine was running it was very loud and I was running a canister. I thought this bites, I spent all this money on a custom exhaust and it made no difference. Well needless to say I placed my hand on top of the canopy for balance and the plane got really quiet. I was never able to really get the hatch to seal tight enough without breaking it (very cheap balsa).

My current Yak is extremely quiet. On the ground at idle you cannot hear it 10 feet away, run it up to full throttle with the PT prop on it and it's at 90Db, replace the prop with an NX and now at full throttle it's close to 96db and all with a header and KS tuned pipe.

In the air, the saitos are louder than mine. We took a Saito 100 running an APC prop vertical with me going vertical and we could not hear my plane. I'm told the Vess props are even more quiet than the PT models. I'll know later this week how true that is