I used this when I modified mty first subwoofer box in my 1971 chevelle malibu and was astonished at the the results it had on the box out. At that time my fun with car audio had just begun and my understanding of how this alternate port worked was beyond me. What is this alternate port method a "passive radiator". This method has an advantage that it can be tuned just like a port without any port length and has no port noise to contend with. My first experience with this method was achieved using a blown 12 inch subwoofer and using just the suspesion part of the speaker and cutting the rest off and this was larger than the 8 inch pyramid phase 3's I was using in my once simply sealed enclosure the output and sound was very noticable and to my liking. Just another food for thought when building your enclosures guys and gals.
I was dead set on using a 15" PR with 4 RE8's as this gave me loads of output, a small box and a nice low tune. But after asking around to checkout my idea I got this advice.....
"Problem: Passive radiators are too heavy to be used in cars. The forces of driving cause the mass to shift around. This can even damage the active driver if the passive radiator is suddenly forced into the box and it has a displacement greater than the active driver(s). Also, when you go over bumps, you'll weaken the foam surround, and its life will be shorter. As you slow down or start up or turn, the passive radiator's mass will try to keep going straight, so it will pull the diaphragm out and push it into the box, so you have much less available excursion. Passive radiators are just plain unsuitable for cars."
This put me off investing a wad in a PR at the time. I feel the advice has some sense.