>What is the differens between the formula to home sub >and the car sub???
No difference.
For a car audio environment though, you're going to get cabin gain of roughly 12dB/octave below around 80~120 Hz, depending on the size of the car. Best way is to measure the actual gain (transfer function) and add it to the predicted response from the calculations to determine what its actual response might be like in-car.
