The resonance frequency of your car isn’t 65Hz. This is a common belief and one that is completely incorrect. In fact, without a diaphragm, or some other vibrating object, there is no resonance. The term “resonance” describes vibration or movement in a mechanical system, which must include something that vibrates. The air inside the vehicle is part of the system, but without a vibrating medium, the system is incomplete. The resonance of the vehicle, for purposes of SPL competition, must include the speaker system. Because the speaker or speakers will be the vibrating medium, their compliance and moving mass combined with the compliance of the air within the smaller of the two enclosures will determine the system resonance. If you don’t believe me, try this experiment: Place a microphone inside a car. Connect the microphone to an RTA and configure it to measure frequency response. Tap on the car’s window and record the frequency of resonance. Then, place a big woofer in an enclosure in the car and tap on its cone. The resonance frequencies are not the same. The resonance of the system using the speaker as a vibrating medium will be lower than that of the system using the window as the vibrating medium (unless the speaker is very small, and the window is very big). The speaker is more compliant than the glass, and provides a lower system resonance.