I would just put in a test tone CD with individual frequency steps and measure the loudest response.
You should find cabin gain increases greatly at 43 Hz but the peak is around 63 Hz.
Anthony B. Davis
2Deep2 Silver SVT
2002 IASCA SQ Ult Ama 601+ World Champion runner up
2002 SLAP dBQ Champion

2000 USAC SQ World Champion
2002 USACi SQ World Champion