Originally Posted by timmyb
My buddy just picked up an '07 X45 and we are getting ready to build a box for his JL 12W7. We were thinking of putting it behind the drivers seat and removing the factory 10" JL and just leaving the grille on to vent the sound out. Does anyone have pics of their X45 sub installs? I found a couple of TeamTalk but not any W7's specifically. Is that factory 10" opening going to be enough of an opening or are we going to need to cut in another vent?
In a perfect acoustic world the subwoofer/enclosure baffle would be sealed airtight to the existing opening via a VERY shallow manifold. That way the woofer output would be direct-radiating and not into the seat console. Thus, you would avoid any losses. Also, if the existing opening was the same suface area of the woofer's Sd, then you wouldn't lose anything. The deeper the manifold the greater the loss and the greater the unwanted impact on the woofer's response.
Now, if the sub/enclosure rests in the seating console and plays freely into the air space within, then the dynamics change quite a bit. That air space, including the adjoining gunnel cavities, is huge. Therefore, that air space is very compliant. As a result, the vent as a ratio looks small in comparison, represents resistance, and there is little impetus for the bass radiation to flow through the vent. More of the radiation is lost into the console and gunnel than is funneled through the exterior vent. The expansive interior air space stores and releases energy which is delayed in time. Think of an echo....not long enough to be heard as an echo but long enough so that the direct radiation of the woofer and reflected radiation from the interior space is seriously out of phase. This acts to filter certain frequencies more than others, creates output losses making the sub and sub amp work harder to create the same output, and generally smears the bass tonal construction. You end up with a lot of boat shaking and tactile bass but less in the way of bass pitch accuracy and much less upper/transient/attacking bass. This bumpy response also makes it difficult to coherently and seamlessly splice the sub with the satellites.
Having explained the reality of this type of set up I also have to say that some boats give you little to no other option. So you are forced to take what the boat gives you. In that case you should expand the vent as large as possible. Usually you can elongate the vent horizontally and make a custom vent. It looks better and the boat doesn't look like swiss cheese.
Do not take polarity for granted. Experiement.