Silencer box design

[gullfo]

you could tap on the aerofoam and test if it's a good conductor. another option is to create a "shell" which is split to round the flex duct in. the split gap is sealed with caulk. this provide a mass barrier from the air space between the units and the flex duct keeps it fully decoupled.
(i'm assuming you mean the connection between the interior unit and exterior units. otherwise if it's between units in the same space, just use whatever you want - aerofoam, flex, or hard duct even).