There is a rectangular plate in between the rear speakers and I was thinking about porting a sub band pass directly at the rear window. I talked to a custom stereo installer that told me that bouncing the bass off the glass is a12 db boost?

 

That is such a generalized statement that I would question their credibility and true knowledge.

A bandpass system is very finicky when it comes to enclosure volume and tuning. You just can't guess at it. If you are planning on modifying an existing box to function as one be prepared to be disappointed. If you wish to pursue the bandpass route, you will need to build a custom box specifically for the application. If you tell me what woofer you have, send me the specs (thiele/small parameters), and tell me what you listen to most of the time, I can design one for you. A bandpass is going to be more boom than quality though.

Just as an example of window effects, I once built a system in an Accord that has 2-15's firing toward the back seat. It was ported and the port was firing straight up through the rear deck at the rear window. That thing sounded like crap. There was severe cancellation from the port location near the window. on other occasions in different cars it worked just fine. Strange isn't it? I've done bandpass boxes firing in different places too. You may get a gain or you may not. The bandpass won't be as picky in regards to port location as the ported box but it will have an effect. No one can definitely tell you how much it will gain unless they have already built the exact same system in the same car. A 12db gain is HUGE. If this were really true then everyone would be doing it. It is unpredictalbe and may or may not work very well. Just don't try it the way you are proposing.

 

Thanks. Sounded to good to be that easy. Thought I could conserve space with a band pass. I'm looking to add some bass to my system and want quality, not car rattling bass just a good bump to compliment my upgraded speakers, Polk 6/9 three way in the rear deck Polk 5/7 two way in front and an inch and a half Cerrwin Vega set replacing the stock tweeters. Any suggestions?