IB manifold opening...

[nelsondog]

I'm quite sure the questions I have have been answered many times over. I tried searching through the countless threads without success. Your patience is very much appreciated.
I have a pair of Mach 5 Audio MJ-18 woofers on the way to and would like to get a little head start on building the baffle.
It's obvious to me that the sides be large enough to accommodate the drivers and not much more. What about the distance between the opposing drivers? I have read to build the enclosure large enough to allow for the addition of more drivers in the future, but if that time ever comes I will worry about that then. What would I be looking at for the internal dimension of the throat opening when completed?
Thank you in advance!
Brian

[Otto]

Hi Brian,

Form my reading, the "best" opening to have would be the size of the sum of all the drivers being used. This essentially becomes an array. However, people have built them of all shapes and sizes. Sometimes the drivers are facing each other "pretty closely". I've seen pictures where it would appear that the drivers have maybe six inches between them. This would be one of the more extreme cases. You've probably seen them over at "The Cult" as well.

Since IB is frequently situated in an attic, basement or adjoining room, there are obvious dimensions that work out nicely. For example, most homes are built with 16" on center framing. Like you said, it's easy to make the IB manifold big enough to accommodate the width of the driver. The 16" OC framing makes it pretty easy to have the OD of the short side of your box be 14.5", which fits nicely inside such framing. That means that the ID is going to be about 11.5" if you are using double thick 3/4" MDF in your design.

I've not heard a lot of actual first-hand reports on the performance of IBs with "smaller" openings. I'm sure that you could certainly get it very functional in various sets of circumstances. That said, I would have to agree with the information that you've read elsewhere. Make it big enough to accommodate two more drivers in the future. That gives you a bigger throat in addition to the upgrade path. Here's a link to an IB that I made with the same drivers that you will be using. My manifold simply hangs from the joists. And my opening is probably about the size of one of the drivers I'm using. Unfortunately, I don't have any first hand experience with a line array, so I'm not sure how mine would compare to that. There are a few around here that do have line arrays, and may have heard manifold designs as well.

Good luck and have fun!