When I built my Stainless Steel pipes I used 1 and3/4 inch perforated pipe for the central core ,this was wrapped in silencer packing for the entire length of the core.
I found it to be still a trifle loud ,so I then made and inserted the reducers that I have told you about.
Before you start drilling I would think about buying or making a fully perforated inner central core the same length as the one in the photo,wrap it in silencer type wool and see how you like the sound.Everyone has a different noise level that they are trying to obtain.
Hope this has been of assistance.
Steve
