Yep I agree. Because a car is a small space the waves hit the front of the car and travel backwards before they have a chance to play out fully. With the windows down they get to travel out and dont cancel themselves out! As a side note, does your box match the subs air space requirements?

 

what it is there is more pressure outside the car than there is inside so when you open windows the pressure is increased its cancelation .my car is the same way i hit 153.2 db on a termlab mic with door open and 151.1db with doors shut

 

oh well, i've just learned to live with it...and yes the box matches the sub. when i want that extra rumble i just crack the windows a bit

 

Same problem in my truck its an 02 ram with a JL w6 15"

 

Yeah, like everyone else said. With your windows up it causes standing waves, due to the sound waves bouncing off the windows instead of going through the windows. If it happens to create a perfect standing wave, it will just be quiet. As if you were just sitting in your car with no subwoofer at all. Which is why they say don't make a perfectly square box

 

This is explained by reverb cancalization. What is happing is when the soundwaves "echo" back off the window it cancels out the inital carrier causing a lower soundwave to enter your ear. If you where to sit in a diffrent area of the car the effect would be diffrent.

To fix this you will need to move the sub to a diffrent area or adjust porting if you have it.

 

Here's a simple explanation.... a vented box makes your subs louder than a fully sealed box right? ... well a "vented" car makes your bass louder than a "sealed car". Get it? Got it. Good.

 

you know, that was my next theory because i just have to crack the window like an inch and boom instant bass, so i kinda had doubts about the cancelltion theory because te window practically still up so sound waves can still reflect off it.
if i stand outside and put my hand by the open part of the window i can feel the air moving....sort of like a port. so i'm guessing the car has such a good seal that it was sort of dampening the bass, I don't know [&:]

 

This is not true. In using this example your assuming your inside the sub box. A ported box gives you more spl outside the box, but if you where inside the sub box ant it was sealed you would have much higher spl