A few questions from a noob

[waynep]

I’m contemplating an IB project and have been rummaging around this site getting lots of good information.

I have a few questions.

From what I’ve seen here, the size of the room is an important factor, but the seating distance from the IB is, too. My room is about 9200 cu. ft. because it opens to the dining room and kitchen, and also has a high cathedral ceiling. However, the IB manifold would be about 15 ft. from the main listing position, just to the right and above the right speaker.

• Any idea of how many drivers I’m looking at? Say, 15”, which is the size I’m contemplating.
  
  
• Any considerations in driver selection for a room this big that would minimize having to EQ boost the lowest frequencies to get good extension down to 20 Hz or lower? Or that even a concern (boosting the lowest frequencies, that is)?
  
  
• I’m trying to sort out the recommend drivers list. I assume it’s all about “moving air,” so I assume that if you use long-excursion drivers you can get away with fewer of them?
  
  
• I assume that’s the same reason you’d use 18” drivers instead of 15”? I.e., you can use fewer of them to move the same amount of air? Or are there other reasons to use 18-inchers?
  
  
• If Driver A has an x-max that’s twice that of Driver B, which is better to use – one of the former, or two of the latter? I’m asking from a performance perspective, not cost.
  
  
• I’ve seen that the size of a manifold opening is important – i.e., the bigger the better. If the drivers are long-excursion, I assume a larger manifold would be needed vs. the same-sized drivers with less excursion?

Thanks for any enlightenment.

[ThomasW]

If the drivers are the same quality it's all about moving air, so the goal is to have the highest Vd possible.

From a performance perspective it's better to use any driver in the middle of it's excursion range. So using many drivers and having each only run where there distortion is lowest results in the best sound quality

Linkwitz has a spreadsheet one can use for calculating the output of excursion limited drivers
www.linkwitzlab.com/spl_max1.xls
In addition programs like Unibox can be used to examine the output of multiple drivers in a virtual IB, when the size of the box is the largest enclosure the program will allow.

[ryans]

• Any idea of how many drivers I’m looking at? Say, 15”, which is the size I’m contemplating.
  
  An IB is very much like a sealed subwoofer. So if you have any experience with a known sealed sub it would be very instructive. If, for instance, you think a sealed Tempest would be plenty then 4 Tempests in an IB would more than get the job done.
  
  
• Any considerations in driver selection for a room this big that would minimize having to EQ boost the lowest frequencies to get good extension down to 20 Hz or lower? Or that even a concern (boosting the lowest frequencies, that is)?
  
  Generally not a concern. If you need to boost ... then boost. So long as you have sufficient woofage and ampage, boosting's not a bad thing. Its much more important to check you IB locations for room nulls that can't be fixed by boosting.
  
  
• I’m trying to sort out the recommend drivers list. I assume it’s all about “moving air,” so I assume that if you use long-excursion drivers you can get away with fewer of them?
  
  Exactly.
  
  
• I assume that’s the same reason you’d use 18” drivers instead of 15”? I.e., you can use fewer of them to move the same amount of air? Or are there other reasons to use 18-inchers?
  
  Most of your money pays for the woofer motor which is often identical for a 15" and 18" woofer in the same line. So for the small extra cost to move up to an 18 you can move a lot more air.
  
  
• If Driver A has an x-max that’s twice that of Driver B, which is better to use – one of the former, or two of the latter? I’m asking from a performance perspective, not cost.
  
  Generally Driver B will also tend to be higher quality than driver A. And its probably going to be cheaper to ship 2 of Driver B than 4 of Driver A. So if B costs 2x the price of A, then I'd tend toward Driver B.
  
  
• I’ve seen that the size of a manifold opening is important – i.e., the bigger the better. If the drivers are long-excursion, I assume a larger manifold would be needed vs. the same-sized drivers with less excursion?
  
  If the concern was air chuffing then yes, this would be true. But that shouldn't be a problem with the dimensions we're dealing with. Instead the concern lies with whether the manifold dimensions are severe enough to start changing the tuning of the enclosure. And even if this took place I would think you could still EQ it flat. I can't recall reading any actual complaints about a small manifold distorting the sound.