improving my subwoofer enclosure

jmalbright, very well written and supported topic;  I commend you.

I believe the intent of DYohn's post "in-car group delay under 20 msec  at 20Hz and under 10 msec at 40 Hz" was referring to one gradual slope, that is, the slope on the upside of the peak.  You are looking for a sloping change from better to worse as you look at the response down through the frequency range.  In this case, the peak has already ocurred at 24 Hz before the group delay becomes "acceptable" at 20 Hz.  The reason for acceptable response below 24 Hz, in this chart, is because of lack of output;  there is greatly diminished sound below 24 Hz.

You may not be able to improve this response and still build an enclosure that will fit the car and give you everything else you're looking for.  It's one of the tradeoffs you'll be balancing as you complete the design.  And keep real life in your vision as you do these models;  how much of the material you will listen to will fall into the below-30Hz realm?

DYohn will correct me if I erred in this response.  And DYohn, thanks for clarifying for me.  Standing waves have always made my head hurt.  I'm trying now to find out how I would determine the standing wave frequency of a given room, per your instruction "Determine the standing wave frequency of the space".

Believe me, people:  there are a lot of things that come easy to me, but physics is not one of them!

http://www.glenbrook.k12.il.us/GBSSCI/PHYS/mmedia/waves/swf.html

  Thank you.  And I assume if we're dealing with an automobile that it is trial and error, and experience.

As Mr. James Dyson, aka Dr. Bose Jr., says in his TV ad about the vacuum cleaner (use a British accent as you lip-read this):  "I made 5000 prototypes - that's 5000 mistakes - before I got it perfected."

Nice.

1.  In general, no, although subwoofers tend to be more predictable with the port opening on the same baffle as the driver.

2.  PVC pipe is what I usually use.