In reality, there are certain cabinet styles that will work reasonably well with a free air driver. I don't believe that ported is one of them. Mount the subwoofer to a solid wood baffle and try to prevent the back wave from passing through the seats and the opening into the passenger compartment. Pioneer subs hammer quite well when in the right environment. Incidentally, if you have a vehicle without a trunk, you're in trouble. Choose the cabinet version of the Pioneer sub instead, or try that old form of learning called "experimentation". You can learn a lot!
Impedance is the resistance that a component exhibits while in operation. The amplifier is the component that reacts to the speaker's impedance and causes a change in sound. Overall, when designing a cabinet, the three most important characteristics are Vas (volume compliance), Qts (the resonance character) and Fs (the natural resonant frequency), followed by thermal power handling. Impedance is a fair ways down the list of concerns.
I recently talked a friend into buying a Focal 31B 12" Polyglass subwoofer from an online store. Before his purchase I searched up and down for information/ reviews on this subwoofer to no avail. The recommended use for the subwoofer was in a sealed enclosure, according to the Web site. These woofers were older models and were going at good prices, so based on the information that was given to us he (my friend) bought the subwoofer.
A few weeks later the woofer came in the mail with no specs whatsoever, so I had no indication as to what size/volume to make the box at. So I contacted Focal about the woofer, they sent me literature about the product through snail mail and also indicated to me through e-mail that the woofer was meant to be used in Free-Air. I have never used a Free-Air woofer in my life, nor did my friend or I desire such a woofer, aside from the fact that he owns a Ford Explorer and a free-air woofer wouldn't be all too easy to set up in a vehicle like that. So I contacted Focal one more time and told them my situation and they said the woofer COULD be used in a box that was 15 cubic feet. At that point I was about ready to cut my losses and buy the woofer from my friend and throw it away since it has caused nothing but grief.
As a last attempt before doing so I contacted the company I bought the woofer from, and they told me that they advertised the woofer as a closed box woofer because their supplier gave them a Car Audio magazine article that featured the Focal 31B in a 2 cubic foot sealed enclosure. I searched your Web site for archives and/or e-mail addresses that I could request such an article from. Please pardon my way of contacting you but this seems to be the only source I can use to request such information. Could you guide me in the right direction in order for me to acquire information on the specific article(s)?ThanksDarren Mills
Darren, I finished this column and the next day received your question. I felt compelled to include it in this column since it was right on the topic. In many ways, the previous question ties into your situation. And, no, I'm not psychic!
It's not unusual for a speaker manufacturer to introduce a refined version of a speaker and still keep exactly the same model number. It tends to get confusing for everyone, but is often essential for marketing. Since the supplier claims that the version you bought is able to function in a 2-cube box, I would throw together a test box, set it in the passenger compartment and see if it works. Either the speaker will sound good, or it won't. You should know right away if it will do the job for you.