For my LNC2 I soundproofed the area underneath the rudder pedal subfloors
only.  This area is basically inaccessible once the floor is installed.  For
all other areas of the plane, I plan to evaluate the noise level with an ANR
headset.  If I'm not happy then I'll start adding more soundproofing,
although I don't expect to need any.  For this reason I will do my initial
flights without the interior installed.  (Plus, I expect to be broke at that
time....)

I used a single layer of ADC-006, which is a thin foam with aluminum backing,
stuck to the inner skin with the peel-and-stick adhesive it comes with.  Over
that I put a custom shaped pillow of fiberglass batting with a vapor-proof
heat-sealed cover (the vapor barrier is kinda like plastic tissue paper that
you seal up with a household iron).  The subfloor goes over that.  

If anyone wants more details I can go look at the boxes to get part numbers.  
The minimum order was enough for at least two Lancairs, but it's still a
small amount of stuff.  My recollection is that I only added about a pound of
stuff (both sides combined).

- Rob Wolf