It just so happens that I have had a transfer function measured in the same style escort. Deon measured several cars when at Klipsch about 6-7 yrs ago now. Here is the transfer function. Red is with windows all down and yellow with windows all up.
With windows up, by 70hz you have 3dB of gain. At 50hz there is about 18.5dB of gain. By 20hz there is 32dB of gain. This clearly is the reason for lack of upper frequency output in relation to the lower frequencies. In addition playing the front stage and overlapping the subwoofer gives issues with cancellation and you often get a null at the xover point until you either time align, adjust phase, or move position of the box.
I had gone through this a few times but I figure might as well post it once more. The following is a model of the AV15H adding in the actual transfer function. Blue is sealed model. Orange is vented. The green curve is the sealed box with the measured transfer function applied. The yellow is the vented box with transfer function applied.
Even with the sealed box there is still a large drop in response above 50hz due to the cabin gain. There is a bump centered just below 50hz, then a dip and another peak below 20hz. The vented box fills in the dip between the peaks. Response is +/-3dB from 15hz up to 50hz which is very flat. Also look at the scale there. With only 10W input the levels are up in the 120dB+ range.
Then there are two basic options if you are looking for an SQ system with flat response. One is to bring in the front stage at the point where the rolloff of the sub happens and adjust levels appropriately. You still need to keep the phase coherent between them or there will be a null at the xover point. If you need the sub to go higher, you can use a bandpass filter with about a 24dB slope on both ends to bring that level down. You'd then have the sub flat from 10hz to as high as you need it with huge amounts of headroom in reserve.
John
