The Socata material is very flimsy and disintegrates within a year or two.
Has anybody used a better material?
For better sound insulation one would inevitably need something heavier (in weight) and that could be another problem.
It’s obviously uncertified, but a low-cost solution ?ould be a spray foam sealant, which does exist in fire-retardant versions (e.g. Dow GREAT STUFF Fireblock).
There is some really interesting stuff. That cool-it stuff especially.
The thickness needs to be about 10-15mm; any more than that and it would get in the way. It also needs to withstand temperatures up to maybe +100C which is what one gets in there if one parks up in still air, straight after a flight, in the summer.
I am going to get some samples. Many thanks all!
This stuff has been mentioned in the Socata site.
OTOH if the 35kg/m2 is not a misprint, this is damn heavy! The adhesive would need to be very good, and hi-temp capable.
Amazon.de probably won’t ship to the UK and also the stuff looks very similar to the Socata stuff, even down to the colour of the backing waxed paper. I would be interested whether anybody from Germany has bought this.
it is 35kg/m3..
I think you read the description incorrectly. It is 35kg per cubic meter, not per square meter 
I bought some of the stuff and just got it delivered this morning. It “feels” better than the original Socata-stuff, i.e. less prone to abrasion and the covering on the non-sticky side has an almost rubber-like quality to it, not with the pores open like the original. No experience yet with it in the plane though.
35kg/m2 at 10mm thickness would be 35g/cm3…. that would be denser than any normal matter, I believe 22g/cm3 is the densest you get.
OTOH, how would, say, a tungsten firewall improve sound characteristics?
35kg/m2 at 10mm thickness would be 35g/cm3
3.5g/cm3 
It’s a slow day, evidently!
