I'd struggling to find what you're referring to.
There's a Celestion 15"er for that much, but no 18"s in sight.
I'd be tempted, but ensure they have plenty of Xmax before going for them. Going this low is all about moving lots of air, so be sure to compare Vd (cone area x excursion) with other drivers before going for it.
Power handling is also important, as designs like these tend to be very inefficient, so need a rocket up them to get serious SPLs.
For the majority of the time, however, subwoofer drivers have enough power handling. Its only when you want lots of output from a tiny sealed box that power becomes an issue.
Celestion mostly make PA and guitar speakers, the latter of which won't even be discussed for subwoofer use: a bendy cone and <1mm of Xmax? No thanks.
So, PA drivers.
Generally speaking, the primary purpose of PA speakers is to survive putting out loud SPLs for a long time.
So, first up they make them as efficient as possible. That means you get more sound out for a given power input, so you don't need to hit them with a couple of kilowatts to hear them. To get a high-efficiency speaker, all the moving parts must be nice and light, and the motor assembly must hit a compromise between efficiency and linear excursion.
Efficiency is key, so they concentrate the magnetic field over a fairly small area, and fit a voice coil that's only a little bigger. That way, little of either magnetic field (from the coil or the speaker magnet) is wasted. So far we have a driver with relatively small Xmax, but high efficiency because of it.
But we want more.
Lets take a look at the cone and suspension. If we make the cone nice and light, there's more efficiency to be gained - its easier to wave a light thing around than a heavy thing.
The suspension must also be fairly stiff, particularly so toward the ends of its travel: PA speakers must be designed to survive abuse, mechanical or thermal. If some really low frequencies come along that hit the speaker, a HiFi speaker will often bottom out. Coil hits backplate, nasty clacking sound, and you know you've done something wrong. Chances are the voice coil is also ruined and needs replacing.
A big bunch of 18" drivers suddenly subjected to infrasonic turntable feedback must be designed to survive, despite the lack of cone support from the cabinet (we're using resonant enclosures that "let go" of the cone below tuning). You can't run around re-coning them all every time someone does something stupid.
So, the suspension is designed to gradually lock up when driven past the driver's relatively small Xmax.
Check out the excursion specs for some Eminence drivers - Xmax is tiny (that's the linear bit), Xlim is usually quite big. Home drivers often have Xlim and Xmax within a few mm of each other, as the suspension tries to lock up all at once, (with some success). You get clean bass while you stay in Xmax, and it suddenly goes horrible if you push too far.
PA speakers distort gradually with low-order distortion, because of the gradual change in suspension compliance. Some find it pleasing to the ear, and it certainly doesn't sound as bad as a home driver driven past its Xmax.
So, now we have a PA driver with a fairly stiff suspension, a light cone and a fairly small Xmax. The first two raise the resonant frequency of the driver, which is a big knock against bass performance. The latter means the low-bass SPL is limited, but it'll go much louder higher up in the frequency range (say, 60Hz and up, where it really hits you in the chest).
Based on that lot, PA drivers aren't fantastic for use as subwoofers compared to a similar-sized home driver, but they will go effing loud if you need them to.
Note that WinISD simulates for ground plane SPLs. As soon as you back the subwoofer up against a solid wall, you get +6dB for free, and another 6dB if its in a corner.
You also get gain when the half-wavelength exceeds the longest room dimension (IIRC), but that's for perfectly sealed rooms, so don't rely on that.
Also remember that concrete turns to paper at frequencies this low, so take the groundplane SPLs as roughly what you'll get, and anything over that is bonus.
Chris
Low End!!! can you guys recommend me some drivers? thank you! *edit* topic got moved i think, just so everyone knows i'm looking to make a set of speakers for that $500, not just a sub.
