When putting two identical woofers in a single ported box.
-Double the single-driver box volume.
-If your port has "sharp" ends on both sides, then you can just double the port tube length.
-If your port has rounded ends on one of both sides, then you should run a few calculations to get accurate construction details.
-The port resonance will increase, and this might become audible. You may want to change port diameter.
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With high-output woofers, a single volume with two woofers "might" generate angular rocking of the cones which "might" generate voice coil rubbing on the pole steel. Pro-woofer cabinets often use multiple ports in balanced-pressure baffle locations.
You pretty much double everything with two drivers.
Two boxes, two ports, two crossovers.
Here's the thought experiment.
Ok that's easy enough, so in that case you'd keep Vas the same and just double the enclosure volume as well as number of ports?
Double the cone area, double the cabinet volume, double the port area.
Imagine two identical single driver ported boxes - your port length won't change, but the area will have doubled.
Chris
The correct information is that you would double the volume of the single driver case and, if the port diameter remains constant you would reduce the port length by around 1/2 or more to keep the tuning frequency constant.
Unibox
WinISD
Unibox suggests Vb=21.5L, Fb=43Hz and a port= 5cmx12.5cm per single driver.
For 2 drivers in the same box, Vb=43L, Fb=43Hz and the port=7.5cmx12cm (or 2 x (5cmx12.5cm).
But here's a thought - put each driver in a separate chamber with its own port. This has the added flexibility of allowing you to fine tune your speaker's LF response to your room by stuffing a single port if you find that the bass ends up on the boomy side.