I'm building a pair of subs each with 2x Dayton Audio SD215A-88 8" drivers and a single 4" flared port.
With a target volume of 3ft^3; I'm wanting to make it roughly the same footprint as my Infinity Towers, which is 12" deep (outside). If I assume 1/2" MDF/plywood and a double baffle, that gives me 10.5" deep (inside), but with my port flush mounted, that gives me 11.5" from the front baffle to the inside of the rear wall. My 4" port (actually measured 3 15/16") needs to be 8.5" long to produce the tuning I want (31Hz) plus the flairs. The flairs are 1.25" each end or 2.5" total, so my total port length, including flanges, is 11". That only leaves me 1/2" of space between my rear flange and the rear wall, which is obviously not enough space. If I bump my depth to 14", I have 2.5" of space to work with, which still seems a bit tight to me, but I'm not really sure why.
My question is, how much space do I need to leave between the rear of my flange and the back of my speaker box to avoid unwanted effects? Is there a general rule of thumb with or without flairs? Is it based more on air speed than anything? If it matters, I'll only be pushing 100w total through both towers (50w per tower), so at 50w each, that a port velocity of 16.5m/s. If I were to hook it up to a bigger amp and run a full 160w through each tower, that would be 30m/s.
With a target volume of 3ft^3; I'm wanting to make it roughly the same footprint as my Infinity Towers, which is 12" deep (outside). If I assume 1/2" MDF/plywood and a double baffle, that gives me 10.5" deep (inside), but with my port flush mounted, that gives me 11.5" from the front baffle to the inside of the rear wall. My 4" port (actually measured 3 15/16") needs to be 8.5" long to produce the tuning I want (31Hz) plus the flairs. The flairs are 1.25" each end or 2.5" total, so my total port length, including flanges, is 11". That only leaves me 1/2" of space between my rear flange and the rear wall, which is obviously not enough space. If I bump my depth to 14", I have 2.5" of space to work with, which still seems a bit tight to me, but I'm not really sure why.
My question is, how much space do I need to leave between the rear of my flange and the back of my speaker box to avoid unwanted effects? Is there a general rule of thumb with or without flairs? Is it based more on air speed than anything? If it matters, I'll only be pushing 100w total through both towers (50w per tower), so at 50w each, that a port velocity of 16.5m/s. If I were to hook it up to a bigger amp and run a full 160w through each tower, that would be 30m/s.
Update:
I've decided to go with 3/4" Ply. After modeling up in Sketchup, I think I'll go with a (outside including double baffle) 39"H x 12"W x 16.25"D box. It looks decently proportioned in my model and it gives me 4.5" between the rear of my port flange and the back of my speaker wall, which seems more reasonable. I'm still interested in everyone's thoughts on this topic though, for sure!
I've decided to go with 3/4" Ply. After modeling up in Sketchup, I think I'll go with a (outside including double baffle) 39"H x 12"W x 16.25"D box. It looks decently proportioned in my model and it gives me 4.5" between the rear of my port flange and the back of my speaker wall, which seems more reasonable. I'm still interested in everyone's thoughts on this topic though, for sure!
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I do not know. It should be at least such a distance that the restriction does not have a smaller cross sectional area than the port itself, which would be achieved at a distance of abuot 2 inches to the opposing wall.
Having the port exit near any wall will increase its effective length. Measure the tuning frequency after the speakers are built and shorten the port if required.
Having the port exit near any wall will increase its effective length. Measure the tuning frequency after the speakers are built and shorten the port if required.
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