Chris, the firewall from memory is about 6.0mm phenolic material and the
drumming
qualities are quite different to the rest of the fuselage; higher pitched around
2000hz. If you have a dB app on your phone you can work these out by going
around tapping sharply with finger tip and noting all the areas identified.
On 181's firewall this is the forward facing footwell surfaces and the upper
vertical firewall/cabin surface. Engine vibration travels through the entire
airframe.
These surfaces can often act more like a speaker cone vibrating when excited by
another "modal" frequency impacting anywhere upon the body.
Technicals aside the DCI produced butyl mat only needs to be applied to 10% of
the entire drumming surface area to still be most effective. Other brands will
require 20% coverage due to density. It can be applied on the interior side of
the firewall.
DCI also produce a 3M adhesive polyester backed AL sheet which I have used
successfully
lining automotive roof panels; it's very light weight, fire resistant,
sound and heat insulating, applied across entire panel, and looks great when
the edges are folded over to form an edge bead.
The 3M adhesive is a "once on never comes off" solution. You get one chance only
at putting on. On composites it will tear the base material layer away from
the body before it comes away from the adhered surface. So finger tip size
tacking
points to align followed by soft paint roller application is required.
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