Modeling is extremely important to make sure you get the response you want so that you have good bass instead of terrible bloated bass or anemic bass. This video demonstrates what kind of response you can end up with if you don't model your box. Ever hear a ported "one note wonder" subwoofer? This video will explain why and how to avoid ending up with one.
The program being used is Jeff Bagby's Woofer Box and Circuit Designer Excel spreadsheet and can be found here: The subwoofer being modeled is the same subwoofer used in my first video, the Dayton Audio RSS256HF-4. Again, it is a great subwoofer with low distortion and a very clean sound. I've used it in the bottom of a 3-way speaker and the 12" version as a subwoofer before and I love it! They are very hard to beat for the price.