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You may or may not choose to include a dampening material, such as polyfill, which can result in your enclosure giving a response of an enclosure up 1.4 times the size (depending on the amount of fill per cubic foot).

I am trying to work out my design in reverse and am not exactly sure how to do it.

Say my design calls for a cabinet that is 24"x12"x12" (interior volume). That is two cubic feet. Then I have to subtract the displacement of the drivers and bracing and the volume of the port. I am unclear as to whether or not I should include the effective increase in enclosure volume from the polyfill.

The problem is that the volume of the port is dependent on the volume of the enclosure. So if I start off with 2 cubic feet, subtract .2 cubic feet for drivers and bracing, that is 1.8. If I plan on using a polyfill amount that increases effective volume size by a factor of 1.2, I now have 2.16 cubic feet. If I figure out a port for this size, I get a port with a volume of .3.

So now what? Do I subtract the .3 from the 1.8, giving 1.5, x1.2 for polyfill, which results in 1.8 again. However, if I model this enclosure size, I get a different port size, which then sends me back to the beginning.

In other words, everything is interdependent.

Am I missing an easy way to figure this out?

Am I correct that the port volume is entirely subtracted from the enclosure volume?

Am I correct that polyfill's influence on apparent enclosure size will in turn alter the tuning of the port and therefor should be included in the model?