I was talking to guys who like to do the SPL stuff last night. They build these gigantic boxes with massive slots to pressurize the car. But they tune the boxes to the resonant frequency of the car. They get the resonant frequency by placing a 10 inch sub in the car and running test tones and measuring with a microphone when the highest response level is and then tune to that frequency.
I'm honestly not sure how that will achieve what they want which is higher SPL and they weren't explaining it well so my understanding of it is weak at best. I've always built enclosures to the resonant frequency of the subwoofer in the enclosure, not the resonant frequency of the car.
Needless to say, they want to try it my way and see what makes more noise.
The way I see it, you want that sub working at its peak efficiency in the enclosure. If it is operating at it's peak efficiency, it is using the power you are providing it the best that it can and it is moving air through that vented box as best as it can. In order to do that, you want to build the enclosure and tune it to a frequency that is smack in the middle of the subs response peak. That would be Fs. So if the sub's Fs is 40 Hz, you build a box that will be tuned to a frequency somewhere between say 37 Hz and 43 Hz. That will give you maximum performance from the sub because you efficiency will be high.
Now a concept I don't think they totally grasped was port flow vs. port velocity and what inertia of the cylinder of air in the port has to do with it. They have gigantic slots which move air by sheer volume alone. They are not understanding how you can get similar air flow with a few smaller ports. The big fat slot has low velocity but large volume. The small, round tube has low volume but velocity that can be as much as 10 times as fast. But when you are going for big SPL, you need to make sure that you aren't moving that air in the tube so fast that it cavitates insides the tube and reduces port efficiency and overall velocity.
Is any of this making sense?
Anyway, if some can explain to me that car resonance thing, I'd appreciate it.