Today's massive frames and motors add challenges to designing enclosures these days. What is the general "rule of thumb" for the minimum acceptable distance from the back of the woofer's motor/pole piece vent hole and the back wall of the enclosure, or a port wall? I'm playing around with different designs for an older Rockford Fosgate T210D4. I can handle the designs myself, I just need some input in this area.

Thanks in advance!

Whatever the surface area of the pole vent is what you need between the sub and box.

Gotcha... Kinda what I was thinking. Thanks!.. Way back in MY day, we used to just figure half of the cone size.. Example: 6" for a 12 inch sub. I doubt there was any real thinking behind the idea.