Here are few dilemmas I had not too long ago when modifying existing well-made enclosures in order to load them with a replacement woofer of a different type, re-working the vents, plus new x-overs.

Working with a previously-made good box (thick plywood, glued, screwed, siliconed inside, panels braced heavily-some with hardwood 2x3" (!) and already painted ) can be a great time saving opportunity. In choosing the replacement woofer, the process naturally involves running T/S parameters of the proposed drivers in the speaker design software, to establish suitability and try to find the best combination of response curve/box tuning frequency(Fb), knowing the cabinet's volume is already determined and there's not much I can do about that (other than over-lining (i.e. much thicker) damping material, creating more absorption-sometimes called "virtual volume"- which "tricks" the woofer into "seeing" a larger box volume). However, with over-lining you may be begging for trouble with regards to required vent dimensions, unless you have a large box. Finally, the extent of woofer choices also depends on the extent of the wallet...

I decided against over-lining in my small box (about 1.5 cu. ft.) considering the heavy bracing plus damping material aldeady in place, in addition to a brace I wanted to add from front panel to back panel, and ducted port required dimensions/space to "breathe"... Another dilemma !

Fitting something like a 2x2" front to back panel, will most probably, in my case, be a nuisance to vent performance by creating turbulence. This need not be rocket science: doing a simulation with a piece of wood of similar dimensions placed in the right location and looking through the ducted port from a few inches will reveal if there's something in the way (i.e. problem with brace too close to vent). If you can't see the brace in the ducted port (looking up, down, right and left from a few inches) you're most probably in business. Otherwise you're in trouble. In some boxes, like this one, bracing strong front to back with vent location in the middle of the front panel a little above the woofer don't go hand in hand. One will impair the other. I have to resolve this issue.

I want ALL the goodies the vent can give me, plus in this case I intend to drive this box hard...beat the crap out of it ! Simply stated the usefullness of the vent is two-fold: First, the vent permits optimization of woofer/box frequency response (e.g. flat, bump, etc.) to suit purpose/need by tuning a "pipe" to a certain frequency (Fb). That frequency depends on air volume in the box and the tube's dimensions (diameter and length); Second, the vent also acts as a "helping hand" for the woofer, by sharing a good part of the burden (moving air) at low frequencies, consequently reducing driver strain/distortion, which is even more evident as sound level goes up. Why would you go without those almost free benefits ? There's one condition though: things have to be done correctly, and vent space to "breathe" is part of that.

Now that I can't have it both ways (best front- to- back bracing and best vent action) I have to choose, make a compromise between the two, based on the above. I decided to sacrifice a bit on the bracing, since I'd rather have 100% vent performance with say 80% front- to- back bracing efficiency, than the other way around. The cabinet is already well braced as mentioned earlier, even the front panel has an horizontal brace just above the woofer. The idea of a larger front-to-back brace has been tossed away and replaced by two smaller ones (about 1x1")made of hardwood and repositioned a bit lower and to the sides, away from the center vent. Vent clearance as seen from looking through the ducted port is pretty good now. That means I will be able to drive the woofer harder (though not unlimited) by feeding it higher power at and around Fb with less fear of reaching Xmax too early or woofer damage.

Right from the start, the purpose of building a vented box is to benefit from its advantages, otherwise go for a closed box which is simpler (no vent). However, remember that, all other things being equal, in a closed box woofer distortion rises more rapidly (no help from vent), and damping material is not lined but stuffed.

Richard