Originally Posted by Jazzi View Post
Large enough that it won't influence the behavior of the woofers in a significant way. The best way to do this would be on a very large baffle with no enclosure on the rear. The larger the baffle the better (six foot+ diameter circle would be a goal to aim for). If you can build a big one like that, then you can make quasi-anechoic measurements to get the on and off axis frequency response measurements. Then you need to splice those together with measurement from less than an inch from the woofer's dust cap to get a good measurement of the bass.

This entire process of making quality measurements is not very straight forward or easy to do. But if you're trying to make the best use of the equipment you have and the chance to review the gear you will will have access to, then it's worth trying to do your best, right? You can learn all of these things from Testing Loudspeakers by Joseph D’Appolito. It's very affordable and written in plain English for the average enthusiast to learn from.
I just purchased that book and Loudspeaker Design Cookbook to further expand my testing methods. Reason I wondered about the air space is because some vehicles have extremely well sealed door panels other than the window gasket at the top. Would that still act like an Infinite Baffle instead of a sealed enclosure? I am guessing, 1.5 cubic feet air space on average in a fairly well sealed door?