After Dave's thread about passives it struck me that there might be a need to cover some of the basics on passives.
I have seen some talk about passives being merely a rip-off as it fools consumers into believing that they're getting more woofers than they really are. That's far from the truth, if you used two subs in the same size enclosure that the driver and passive are in you'd get a huge decrease in the lower register. The passive serves the same purpose as a port but offers more flexibility than most conventional ports.
First: Enclosure size. Sure you can go with a T-line and have the sub mounted at one end of a tube so that you only have the displacement of the port in the footprint. Unfortunately they're very tricky to calculate and aren't forgiving of different drivers in the same enclosure. It could work or not. A PR
box only needs to be the size that accommodates the sub and the passive. That enclosure will work fine for any sub you can fit in the hole provided that the passive meets it's needs. If you're using a tunable passive it will increase what it'll work well with.
Second: Port noise. If your port's too small and it makes noise you're pretty much up a stump. If the passive isn't up to the task you can replace it with one that suits your needs. If the passive is making noise it's because it's moving too far. One with longer excursion will solve that dilemma.
Third: Tunability. You can easily lower the FS of the passive by adding mass to it. This in turn lowers the tuning of the enclosure. This can be accomplished by gluing weights to it or going with a step larger passive than the sub or adding another passive. While larger and more won't work well in an eastablished enclosure, added mass is fairly easy. Many passive radiators even offer a mount for more mass.
Passive radiators are fun to play with. You can tune and listen, tune and listen and keep at it with tiny change until you get it where you want. If you decide later that it needs a change it's not a big deal to go back a few steps if you use a tunable passive. Glued weights are a different story though.