Thank you for looking it over.
I completely new to this, but for now I have chosen 24Hz box tune in an effort to juggle port size, port length and cabinet size. Any deeper and the port becomes unmanageable in length if I want a decent port size. The SVS has a shorter 4” port, but if I try that dimension port velocity increases somewhat and comes outside the recommended values in Unibox. Also, as the port length increases the port resonance creeps down. I want to X-over at 80Hz and chose to keep the port resonance at roughly 3 x 80Hz. I am not sure if the port resonance is very important but 240Hz or so feels safe as it would a long way down when x over Is applied.
I have done some more calculations and with the proposed 20” cube I’m close to the anticipated internal volume with 2 internal bracing frames, the driver and the port. I have increased the port size a bit to keep port velocity down and it stays at 24m/s with 250 watts applied. I don’t understand how SVS can get away with a 4” port. In my box that would give me 34m/s at 250 watts applied. Am I missing something?
Going forward building the actual box I’d like to ask if there I any restrictions in placing the internal port? I’m building a square/slotted port (16 x 7cm) and the natural way to do it would put the internal port right up against the back of the cabinet. Is that OK or would it somehow impair the airflow not having the port “in midair”? I could build an internal "chimney" if that is better?
As for internal dampening I think I’ll go with lining the cabinet walls. I’m thinking 1” soft insulation is a good place to start. It’s not a lot but it certainly is a lot more than my old XTZ 12" sub. Maybe 2” insulation is better?
I’d like to round over the port ends, hoping to minimize port noise. Unibox predicts a 1” increase in port length with “2 flared ends”, however I does not say anything about the size of the flares. I’m building the whole box, including the internal port with 1” MDF so I have sufficient material to make a decent round over. I guesstimate that the predicted 1” increase in port length can be applied if I round both ends with a ½" bit. Is that in the ballpark?
Thank you for your time :-)