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).

[ACSlater]

Thanks once again for your valuable advice Glenn. Your assumption was right regarding the connection between interior/exterior units. I have used the Aerofoam duct to connect the units as I have enough faith in the material that it doesn't transfer vibrations from the steel duct. It doesn't contain the sound passing through the duct itself as it's a very light closed cell foam, but I figured the sound leak doesn't matter at that point as it's in the spring ( between the mass) and coming from inside the baffle box.
Here's how it is now:

I had to use two conduit reducers on both sides as the steel tube has a 125mm diameter and the Aerfoam is 150mm.