Actually, a LOT of noise comes through the floor and through the nose wheel well.  The NWW is only a very thin layer of composite so it transmits noise very easily.  On Dave Morss' plane, we eliminated most of the noise by insulating the floor, nose wheel well and behind the firewall.  Additional absorbers were placed on the glare shield and sidewalls but the floor and firewall were the big noise makers.  I built barrier blankets that fit tightly behind the firewall using a glass/.5 lb /sq. ft. barrier sheet/foam blanket.  For the floor (which is opposite the exhaust remember), I used a 1/4" fire retardant felt/.5 lb barrier and another felt.  This went under the floor mats. the aircraft was very noticably quieter after the floor mats were in.  I suspect there may be more aerodynamic noise coming in via the main spar so if you were to pull out the seats and make a very tightly fitting felt/barrier/felt blanket over that, I suspect it would be quieter still.

 

Dan Newland