ok ill try to explain between rounds here goes take your subwoofer and put it in a sealed box and run test tones one at a time with your amps bass boost all the way down and your headunits bass all the way down and run at one volume at a moderate level to keep voltage steady and dont mess with it at any time as you scan and do readings and then take the box out of the car and test at the exact same volume as before and mark it down on paper in car and out of car then youll have your cars cabin boost and resonant frequency
isnt there a formula.......i seen something like that before.........cant remember where i might be wrong...........O i think that was for just how to tune the ports for the loudest bass.
what's been said. you pretty much find the sympathetic frequency by using an RTA to monitor response, and a tone generator to sweet the range between about 40 to 70 Hertz. See where it peaks or spikes. That's pretty much it.
scott the digital hand held from radio shack is 50 bucks its an ok mic but it only reads up to 126 db plus graph so technically 127 db but the way i approach this its all id need if i wanted it cheap but I also bought a 150 db meter now