I tried an isobaric alignment last week and came to the conclusion that... it's not worth it.
An iso pair is -3db from a single sub because of the efficiency loss. Now, the VAS is halved also though meaning only half the volume is needed. But let's do the math.
These are just sensitivity numbers and theoretical, but bear with me...
Say a single has a sense of 90db. An iso pair will be 87db. (2) iso pairs will get you back to 90db. What's that mean? That means you need (4) subs to equal a single in a standard alignment. Yikes.
What about space? Well, let's say your single needs 1cf/2cf for a 4th. Iso pair needs half so .5/1. But you need (2) iso pairs to get back to the output of a single so double that and you're back to 1cf/2cf but now with (4) subs instead of just one and the exact same theoretical output on the same power.
BUT... your power handling also quadruples with the (2) iso pairs. So, even though you need all (4) subs to meet the sensitivity of a single, you can surpass it's output by sheer power. Somewhere in the theoretical neighborhood of +6db. Small box, big power twice as loud. If you have the power, that is.
Is it worth it? If you are strapped for space and have planty of power on tap... maybe. But, there is also another benefit of the iso alignment. Since you are saving space, you can increase the ratio and as such, increase the gain in the bandpass to make up for the loss of efficiency. Even if you can design in a conservative +3db gain with a larger ratio, you now start leveling the playing field while still staying smaller than all (4) in a standard fashion.
With all of that being said, if you have the room to do it normally, go that route. If not, iso may benefit you if you have the power or if you just want to do something different.
