So, personally I like to use the Efficiency Bandwidth Product (EBP) method of dividing Fs/Qts method to determine what's best.
- If Fs/Qts <50, then that sub is really considered a "sealed box" sub.
- If Fs/Qts >100, then that sub is really considered a "vented box" sub.
- If it's between 50 and 100, then it's usually considered usable in either sealed or vented, but then your selection depends on your application, you might prioritize other variables, or want to skew more one way or the other.
So for your subs:
- NVX EBP is 76.3. That's smack in the middle, should work fine in either sealed or vented.
- Thielle EBP is 72.47, which is as close to the same as to barely factor into your choice between this and the NVX.
- Scanspeak is 53.125, so it definitely leans a little closer to being a "sealed box" sub.
For isobaric, those are fun - I used to do those all the time back in the 90's before Kicker disrupted the whole subwoofer market with their "solobaric" subwoofers and ushered in the next wave of "small box subs"... and note all they did originally was double the mass of the subwoofer's moving mass, double the BL, and use a stiffer suspension, to really simulate what happens when you isobarically couple two subwoofers. Now, pretty much everyone does that to begin with, save for some purist SQ subs. They've since evolved the Solobarics (fun fact: the L7 are also of that Solobaric lineage) to increase excursion capability over time, as well as some other things, mostly cosmetic. Funny thing is L7's really skew "sealed" by EBP standards, though obviously they work awesomely well in vented boxes, so there's always exceptions - use this as a guide only, although I wouldn't shop for a subwoofer that violated this EBP rule unless I explicitly knew it would work.
But your question on isobaric - the only spec that changes when you isobarically couple two subs is Vas. You cut it in half. So if your modeling software doesn't specifically offer an "isobaric" option, all you have to do is enter a second version of that sub with the Vas cut in half, and bam - you are simulating an isobaric pair of those subs, rather than a single sub. Easy.
Basically, all you are doing is creating a turbocharged single subwoofer. I say that because even though you are technically using two subwoofers, you really only have one cone acting on your listening space and on your box With the air trapped in between coupling them, you now you have two motors driving it, and of course you have double the mass compared to an actual single sub.... that might sound bad but it washes out since you have double the BL/Re motor strength, and you have double the suspension which washes out since you have the rest doubled as well.
One nice sound quality benefit is that suspension non-linearities (the surround moves slightly differently, makes a different shape, when the cone moves in, vs when it moves out) are ironed out, since you always have one cone moving in and the other moving out, at least if you clamshell them together.
...and don't forget to wire the outside (listening space) woofer out-of-phase, so the subs work together rather than fight each other. You'll know if you forgot... "where's the bass?" :lol:
For bandpass, here's where things start to get a little muddy (and I don't necessarily mean "sub response is muddy" as a pun)-
Depends on what kind of bandpass:
A 4th order bandpass is one where the rear chamber is sealed, and the front chamber is vented.
A 6th order bandpass is one where the rear chamber is vented, and the front chamber is vented.
The role of the rear chamber in a bandpass box is to define and control the shape and gain of the lower frequency portion of your designed frequency response.
The role of the front chamber in a bandpass box is to define and control the shape and bandwidth and gain of the upper portion of your frequency response.
So - for a 4th order bandpass box, you can get away with a sub that is a "sealed box" sub, because the low frequency chamber is sealed. However, I'd personally want a little more motor strength, because there still is a port in the equation, and that will still have SOME impact on cone motion... you obviously don't want it so overdamped that it is lifeless and won't play low (like if you used a ported-only sub in a sealed box), but you also don't want it being too easily upset by a vent that it ends up muddy.
I'd feel good being right in the middle of that EBP > 50 but EBP < 100, like your first two subs are.
For a 6th order bandpass box, you definitely want a sub that's in "vented box" territory, although you can get away with one that's in the "either" range. I'd want to get a sub that's EBP >75, if not EBP > 100. The NVX and Thielle would probably work just fine, the Scan Speak not so much.
Modelling - for
either bandpass box, if you are modeling them up - again, you might not find software that allows you to do both "bandpass" and "isobaric" at the same time, that's OK. Again, all you have to do is manually manipulate that Vas parameter. Just cut your Vas in half... and now you are simulating an isobaric PAIR. It's that easy.
Build that bandpass box (be VERY careful with your precision when building, as a millimeter off here and there can have big consequences on your ACTUAL frequency response), and then mount the drivers clamshelled together on that center wall (make sure you account for one sub's displacement in each chamber!) and bam - you've made an isobaric bandpass box. :thumbup:
If that's your goal.
So for that purpose, I'd think either your NVX or that Thielle would be fine.
And if Hexibass has used those Scan speak subs in a 4th order bandpass box - then I'd trust that for consideration as well... it does make sense from the standpoint that 4th order bandpass is at least fundamentally like a sealed box, and those subs are slightly leaning that way.
But the other two are seeming just as good, 4th OR 6th order ...or sealed, or vented, if you ever change your mind down the road. :wink:
