How many drivers recommended for IB?

Discussion in 'Archived Threads 2001-2004' started by RichardHOS, Mar 11, 2003.

  1. RichardHOS

    RichardHOS Second Unit

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    Yep, building a new house and HT, and I think I'm taking the IB plunge. HT room is fairly large, coming in around 7000 cu. ft.

    I've been perusing different drivers, doing a bit of research, trying to find what will make a good candidate for IB. Naturally, the AV15 and Tumult look very attractive.

    I can't help but think though that for IB, where enclosure size isn't really a concern, 2 AV15's for the price of 1 Tumult would be a better route to take. More displacement in the linear region it would seem. Any thoughts about the AV15's T/S characteristics and how that would work for IB is appreciated.

    My real question though is number of drivers. Again, 7000 cu. ft. How many AV15's (or other) would it take so that I never regret not having built a larger IB plenum/manifold? My first thought was "4 should be enough for anyone." Then I thought about how large the room really is, and thought "at $200 per driver, 8 would not be terribly expensive, and I can always upgrade amps later for more power." Now I'm thinking "maybe 6 will keep me from feeling really stupid." [​IMG]

    If the AV15's are dual 2ohm voice coils (like the Tumult and other contenders), then 4 would present a 4 ohm mono load (or 1 ohm with alternate DVC configuration). 8 would be an 8 ohm mono load (or 2 ohm). 6 would be a 6 ohm mono load (or 1.5 ohm). A mono 8 ohm is easier for most amps to drive.

    Thoughts?
     
  2. ThomasW

    ThomasW Cinematographer

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    For a world class IB build a 18-20" cube with 4 AV-15's.

    Or to define a new world order for IB subs, build a slightly larger cube and mount 4 AuraSound 18's. They cost only $1130/driver ....... [​IMG]

    [​IMG]
     
  3. RichardHOS

    RichardHOS Second Unit

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    Really... just four AV15's? For some reason that doesn't seem like "enough" overkill... but I do have that personality flaw, so maybe I should take your advice. [​IMG]

    Hmm... 4 AV15's would present either a stereo 2 ohm or mono 4 ohm load. Most amps increase distortion a good bit going from 8 ohm to 4 ohm bridged (if they can handle the 4 ohm bridged). Though, if that distortion isn't audible it really cuts down on amplifier cost (half the drivers - 4 vs. 8 - and half the impedence on the amplifier... only need 1/4 the amplifier power).

    Maybe I'll go for six, and have a mono 6 ohm load. Nice compromise? Looks like mounting arrangement will actually be a consideration in my decision though. Argh.
     
  4. ThomasW

    ThomasW Cinematographer

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    For all intents and purposes amplifier distortion differences with lower impedance loads are in essence inaudible. This is especially true in the passband where sub amps are operating.

    4-AV-15"s will displace ~15.2L. My 12 Shiva IB displaces ~18.4L and has never been operated above 50% of capacity.

    Mounting arrangements for a manifold using more that 4 drivers can get a little complex. Although stacking drivers like my big IB is an option, optimal performance is obtained using the shortest manifold possible
     
  5. RichardHOS

    RichardHOS Second Unit

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    I have a related question that I would normally start a new thread for, but since you seem to be the IB guru and are here I'll just keep it in this one...

    About the mounting issues - and related to optimal sub positioning:

    I've seen an AES paper referenced quite a few times lately that apparently concludes that the best placement for subs is one in the middle of each wall. In my particular room, I have the front wall and both sides wall available for IB (lucky, huh?). With box subs, I would probably try it out, knowing I can move subs around. With IB's... well, that's pretty difficult. [​IMG]

    I'm a bit concerned about generating nulls from multiple subs that I won't be able to correct, but this AES paper suggests that the multi-mid-wall placement is designed to minimize nulls.

    I have one mounting restraint on the front wall that is somewhat of a pain: height. I'm keeping my RPTV even when I add a front projector for everyday TV viewing (the room isn't quite a dedicated HT... more like HT/TV/music room). So I have to stuff a manifold, RPTV (recessed), and large center channel all within the 10' height.

    That creates a problem for optimum manifold. If I did the suggested cube with four subs, the depth of the speaker+cube+speaker would be roughly four feet. Add in clearance (don't want those huge magnets near center channel or TV), the five foot TV, and center channel, and it's likely not going to fit.

    Mounting drivers only on the 'sides' of a manifold cuts down some 30" on space requirements. But that means either a pair of drivers in a cube, or an extended manifold going away from the wall with two or more drivers per 'side' (none top and bottom). I know timing issues come into play with a longer manifold. A possible solution would be multiple cubes along the width of the front wall with a pair in each, but I'm more afraid of spreading out the sound along the front wall than following the suggestion of "midpoint of each wall."

    Or... following the suggestion of the AES paper, I could do a three cubes with a single pair in each, one mounted in the front and side walls each.

    Any thoughts on my dilemma?
     
  6. Jack Gilvey

    Jack Gilvey Producer

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    Just a complaint for Thomas: I shorted-out my keyboard when I saw that Aurasound 1803 pic...please use better judgement next time.


    [​IMG]
     
  7. ThomasW

    ThomasW Cinematographer

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    Oops, sorry Jack. At least no milk squirted out your nose......[​IMG]


    Richard,

    Everything in life is compromise....... [​IMG]

    Obviously I choose to avoid multiple sub locations. My feeling is that the potential for nulls and other room induced weirdness is multiplied by having several locations. And I'm too lazy to build lot's of IB's, even though my situation would allow placement of them.

    Regarding multiple sub locations the only way to know for sure is to whip up a few sealed boxes. Place them in the room, then 'map' the LFE with measurement equipment. This is a ton of time/labor; but in the end, it's the only definitive way to answer your question
     
  8. RichardHOS

    RichardHOS Second Unit

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    Yeah, that would be the preferrable method. However, it seems very likely that I will install the IB well before final room accostic treatment is in place (not to mention furniture), so test results would likely be very limited in usefulness.

    I don't think I want to postpone the IB construction until after the walls are treated, carpet is down, etc. So, I guess I'll have to make the choice in advance of any test data.

    At least with multiple subs, you have the option of altering gain among them to modify nulls/peaks, but like you I'm a bit queasy about the interaction of three manifolds with unknown characteristics. [​IMG]

    Thanks for the help though.
     
  9. Bryan Michael

    Bryan Michael Supporting Actor

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    i have 2 av15 in a 3200cf room and that poens to the rest of a 8000 foot level. i hafe hit 125 db with it at 50hz it is tuned to 17 hz and prety flat to there. i am driveing these with 500 wats a piece.
     
  10. ThomasW

    ThomasW Cinematographer

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    Richard

    Standard room treatments have almost no effect on bass unless the absorbers themselves are huge/thick and are of the membrane type. The best design I've seen for true bass traps are at this WEBSITE. I think a DIY version of these wouldn't be all that difficult

    If you want to get an idea of the basics of the room download one of the freeware room mode calculators. There are several available. The Harman one is HERE The SGHT worksheet is available HERE

    Either of these will give you some ballpark info as to what's going to happen in your room.

    BTW can you do a 'stacked' manifold, 2-15"s on each side (one above the other) firing towards each other? Something like this only rotate 90 degrees.

    [​IMG]
     
  11. RichardHOS

    RichardHOS Second Unit

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    Yeah, I can do a rotated 90 degree 'stacked' manifold like that. I'm torn between doing just that on the front wall, or doing the three cubes with a pair in each on the front, left side, and right side walls.

    Not many people have a room with the ability to do an IB on five of the six walls... and I would almost feel guilty if I didn't use more than one of them. [​IMG]
     
  12. ThomasW

    ThomasW Cinematographer

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    I'd suggest a 4-15" manifold in the front. But start with only 2-15"s in it. Take the other 2-15"s and put one in each of 2 sealed boxes (with the proper design these could be used as manifolds later on). Place them around the room for an audition. If there aren't any problems then do single or dual 15" IBs in the other positions.
     
  13. RichardHOS

    RichardHOS Second Unit

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    Sounds like a great idea!
     

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