| SPL != loudness
The idea with SPL is to create pressure, putting your subs somewhere so they fire forward unrestricted is not very effective at creating pressure. It will probably be louder, but won't have better SPL. Putting the box firing backwards as close to the back of the car as possible will probably give you better SPL, because bouncing the waves off the back of the car will help create pressure, and give you better numbers.
If you want an example, I know of a guy who recently bought a new car for SPL. He had a very large box from his previous vehicle (a van) that has 4 10s or 12s in it, I can't remember. Previously in his van, he was hitting high 130s with the subs all firing up, presumably the roof was helping create the pressure. In his new car, he simply turned the box upright with the subs firing forward and had the box behind his back seat. It was kind of like a wall, but not really since it was just a box sitting on the back seat and not a real sealed off wall. I sat in the car and it was really really loud, but when he went in the lanes to do his run, he hit like 122dB. Why? Because his subs were freely firing forward with no means to pressurize the wave. Had he had a real wall, the wave woould have pressurized between the windshield and wall and he would have got a high number, but with the box simply on his back seat, NOTHING. But it was loud.
So yeah, SPL does not equal loud. Loud does not equal SPL.
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