Moble Enclosurs 10+ year member
Box Designer/Builder
This is true. The vehicle will determine more of the LF response than the enclosure will. This can be calculated for, optimally, if desired and I would not recommend a design without that information. This is why we all have said 29Hz seems low. But again, this is always your decision, not ours.if you do not have time alignment on your sub channel passive radiators will not work. also with these low xmax subs you can get away with much less port area. Lastly are you sure 29hz will play flat in car? that sounds way low with out a separate higher tuned sub woofer
As far as passive radiators, this is true, the delay will be a factor, but unlike ported designs, the resonances within the enclosure are less of an issue for output, due to compliance. So, you will not have major phasing issues like you do with a conventional design, but not to say it is not an issue, by all means. But other factors of a PR make resonances less of an issue in such a small space. This would be more of a concern in a larger acoustical area than a vehicle. Either way, it is definitely something to look at, as all variables are. SO, that being said, great point.