Hi....I have a question...how to calculate Enclosure volume when we use lets say 4" fullrange with VAS 2 liter and 6" sub with VAS 8 liter in same enclosure.
Does a VAS of FR change when we put a high pass filter on full range driver?
You'll end up with the smaller radiator becoming a passive radiator to the woofer and vice versa. That is, imagine a low frequency signal. 20 Hz to 40 Hz coming out of the 6" driver. That signal will end up coming out of the 4" driver as well, without much control (using common alignment techniques) of the delay or phase of the output.
You must put each one in it's own enclosure to maintain any sort of predictability or control over the design with commonly accepted practices. I read (briefly) the link GM posted. It seems you can do this, but what a bunch of additional work, for what seems to me a purely academic, and not very useful, result. I mean, cool, but who would want to? Among other things, the additional radiated output will eat into the Xmax of the individual drivers, reducing your maximum output. I could see this reducing the midrange's maximum output by half or more due to the excursions required as frequency decreases. Not a compromise I'd like to make.
Sreten's commens are correct. If you can electronically cut off the 4" high enough, or use a high pass filter to compensate for the rising FR bump the actual volume of the cabinet may be greatly reduced from the ideal.
I use WinISD as a quick way to examine driver behavior in different cabinets. Here is a couple of exmaples with the Eton 5" 5-200/A8/25 driver, the box sealed box WinISD recommended box is 3 Liters. However, if I force the volume to 1 liter, the response isn't much difference until around 800 Hz. If that's going to be my low frequency cut-off, then I can deal with the amplitude rise in the Eton's high pass filter.
If you use XSim, and circuit blocks, you'll use a high pass filter with a Q less than 0.7 to compensate for this, rather nicely.
So really the only way to change the Vas is to alter the driver itself, or get another driver. Your questions were not about altering Vas but calculating the cabinet volume when multiple different drivers are used.
I know this thread was pretty much dead, but I wanted to add another issue. If the amplifier has feedback, having two different drivers share the same volume of air gets really complicated. First, the amplifier will try to correct for any movement of the driver.
If you don't believe me, try this. Turn your amp off. Press on the woofer. Feel how loose it is? Turn the amp on. Try it again. Now the woofer feels stiff. That's feedback in action. The point is, the amplifier will now (maybe, maybe not, depends on phase.... ) be working extra hard to compensate for this extra voltage appearing in the circuit.
And on top of that, you have the movement of the acoustically driven diaphragm putting voltage and current back into the overall crossover.
Gahhh!!! I have no doubt it's possible to figure out a how, but I hope I've covered enough reasons not to for most.