Vb vs. V(total)

[gerryc]

I see these values on the Eminence cabinet design pages, I assume the results they show are right out of Eminence Designer. Does anyone know the difference between the two? I first thought that Vb is the theoretical or net volume of the enclosure with the driver volume taken into account and that V(total) is the actual physical enclosure volume (not taking into account of the drivers) since V(total) is larger than Vb. But comparing a number of their designs it became obvious that's not what they mean. For example, for sealed enclosures they are equal. Thanks.

[gerryc]

OK, I installed Eminence Designer demo and looked through the documentation. This is what it says for the two quantities:


Vb The net internal volume of the box or chamber. This is the volume that remains after subtracting the volume inside the box that is displaced by internal objects like the driver, vent and bracing. Units: liters, cu.cm, cu.m, cu.ft or cu.in.



Total The total internal volume of the empty box or chamber. It is calculating by adding to Vb all the volume that was displaced by all internal objects. The total internal volume is the volume used for all box dimension calculations. Units: liters, cu.cm, cu.m, cu.ft or cu.in.

So I apparently guessed correctly, although something doesn't quite make sense on their sample design PDFs. (Vb cannot ever equal Vtotal unless you have a magic driver that displaces no volume ).

[Sydney]

That's why when using design programs/tables/guidelines; Some chose to include a "fudge factor" to compensate for bracing and driver volume, ports, etc.
But IMVHO it can be over-analyzed in the context that since commercial drivers are not high precision +/- 1% devices, the impact of any volume difference of this size is not the primary variable or most significant.

[Mikey]

That's why when using design programs/tables/guidelines; Some chose to include a "fudge factor" to compensate for bracing and driver volume, ports, etc.
But IMVHO it can be over-analyzed in the context that since commercial drivers are not high precision +/- 1% devices, the impact of any volume difference of this size is not the primary variable or most significant.

+1 ... driver specs have some variance from driver to driver, so if you design a cab based on published specs, your precise calculations are meaningless. "As close as practical" will get you close enough without throwing your desired end result too far out of whack. The easiest way to give yourself variance with a cabinet is to build it so the back is primarily adjustable, so that it can be moved slightly in or out to adjust the cab volume. Once you have it where you want it, glue it and screw it.

[Sydney]

If you get a hold of simulation software, and start jamming ( doing what-ifs basically ) you will see that while an error of a few percent in volume has a marginal effect; the jamming done by varying the T/S parameters has a much greater effect on the way it performs. Basically the fine ultra-accuracy of the enclosure volume is not important, compared to other matters.
If you are not following a proven design with components with consistent quality, then you have to expect to experiment.
Without the ability to measure then it becomes assumptions and opinion tainted by builder's bias.
Some enclosure designs are more dependent on accurate tuning then others.

[gerryc]

Agreed. What threw me off was in some cases, seemingly their closed box designs, they showed Vb = V(total) which made no sense to me. Either I misunderstood the meanings of those parameters or whoever ran those simulations fudged around a bit (never ran the program so I don't know) and didn't take the driver volume (or any of the other bits) into account in the simulation. It's obvious now that it's the latter case. I wouldn't build one of those designs verbatim but only use them as a guide for my own simulations and to leave some leeway for final tuning.

[Frankenspeakers]

OK, one thing that they might be doing on the sealed boxes is allowing for the stuffing to make up for the lost volume due to bracing, driver mass...etc. If you stuff with about 6-8 ounces per cubic foot, the box will appear 25% larger to the driver. It's a thought...

[Bill Fitzmaurice]

If you stuff with about 6-8 ounces per cubic foot, the box will appear 25% larger to the driver.

-1, though this is a common misconception. Stuffing a sealed cab lowers the system Q, which can help tame a midbass peak, giving a response curve similar to that of a larger box. But whereas a larger box will result in a lower fb that is not the case with stuffing a smaller box. As in all mechanical systems the addition of a passive element can only consume energy, it cannot create more. In a stuffed box the energy that otherwise would give a response peak is instead burned off vibrating the higher mass of the stuffing plus air compared to air alone.