Dual vented box from Alpine?

back in 2003 i purchased an alpine SBR-122CR, which is SRW-1241 (12" Type-R dual 4 ohm) in alpines custom vented box. This last winter i ended up blowing it apart. When I pulled the sub out of the box I noticed that the two tube ports that lead in the main sloted port. I was just woundering if anyone else has see something like this and if there's any benifit to this "dual" venting or is this a common pratice? this setup sounded great and hit hard for a single 12"(was driven by alpine MRD-300) and i was thinking of tring to make a similar box for the 2 SRW-1242 i'm running currently. Any feed back on this would be great!

werd

Dual ports sometimes are used when 1. the port length is relatively short, and 2. more vent opening area is desired rather than to use one larger diameter port.  Usually the overall box size is on the small side as well, because doubling the number of ports (without decreasing their size) also means doubling the port lengths (which amounts to a much larger volume of airspace dedicated to the ports instead of to the drivers).

Sometimes in the design stage of a vented enclosure, you find that the port length is extremely short.  I have come up with tuned boxes whose ports could have consisted of only a hole cut into the MDF to get the desired tuning frequency.  Since this is not desirable, there are ways to increase the port lengths and still get the same tuning freq.  One way is to decrease the box volume, another is to increase the port opening area, and another is to double up on the number of ports.  Another major factor is the amount of space on the baffle where the port(s) exit...sometimes two smaller ports will fit better than one larger one.   A box designer has to look at every possibility to finally arrive with a good enclosure for the intended purpose.

It is likely that the designer of that particular enclosure finally decided on that design because of all of the above.  If I understand you correctly, there were two tube ports that both opened into a slotted port, which was visible on the outside of the box (the tube ports were hidden within).  If that was the case, the total square inch area of the slotted port would have been equal to the combined square inch area of the tubular ports.  The total length of the port would have been the length of the slotted plus the length of the duals.  For example, if each round port were 4", each of them would have a square inch opening area of 12.56".  Combined, the square inches is 25.12".  The slotted part of the port should equal that, so it could be something like 2.3" X 11", for example.

It may just have been the easiest way to manufacture that box, considering it was made to "fit" a variety of vehicles and wallets.  Using a slotted all the way through might have made the box a bit heavier and bulkier than they wanted it to be (because of the MDF walls used in slotted port construction).