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Post by mike1234 on Mar 23, 2008 9:29:11 GMT -7
Hi All. I'm new here but not completely new to DIY audio. Firstly, I did a search for several terms all of which returned negative results.
PROBLEM: I really like well-implemented IB systems but there is no attic above the front wall of my HT room (tall ceiling) and the rear wall has a loft above it with a lower ceiling.
SUGGESTED IDEA: I'm considering the use of four separate quasi-IB systems each with an 18" driver. Huge sealed boxes disguised as rather large/fat columns will run from floor-to-ceiling (13 feet). The boxes (columns) for the rear two 18's must run through the floor of the loft to achieve the same box volume as the front drivers... or I could use fatter "columns" in the rear. However, larger columns will look stupid in that area of the room due to the lower ceiling under the loft. BTW... no wife to cut my subs smaller.
QUESTION: How much will the tall box shape (tuned pipes) affect resonance and system FR? I'm sure much of this can be tamed with acoustic stuffing, driver placement height, and EQ. Also, since the end of the "pipes" are sealed, resonances are confined insided the boxes (pipes) but affect SQ via coloration attacking the rear of the cones.
Please educate away my ignorance. I need HELP!!!
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Post by ThomasW on Mar 23, 2008 9:49:20 GMT -7
Hi,
First there's no such thing as a quasi-IB. One either has enough box volume to make an IB or they don't.
Since you haven't provided specific dimensions it's a little difficult to comment, other than to say with regard to bass there are few 'pipe' resonances if the enclosure is properly built and damped.
Beyond that using WinISD Pro or Unibox will answer most of your questions..
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Post by mike1234 on Mar 23, 2008 11:23:02 GMT -7
And again, please forgive my ignorance.
Boxes (columns) Inner Dimensions: Approximately 13' tall by 1.6' wide by 1.3' deep or a total of 27 ft3, which is enough volume for IB alignment.
Driver Parameters: 18" Diameter, Znom= 4 Ohms, Fs= 16Hz, Qts= .42, Vas=570L, Xmax=25mm each way.
To be more specific... how would I "properly" damp a long box like this to achieve true IB results? And does it really make no differenece with regards to driver placement along what is essentially a pipe? It seems that proper placement could make a substantial difference in taming resonance peaks. Or am I just being stupid? Frankly, I dunno.
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Post by ThomasW on Mar 23, 2008 14:05:07 GMT -7
To be more specific... how would I "properly" damp a long box like this to achieve true IB results? You can't imitate an IB, what you can do is make a sealed box... The bracing needed for your boxes and the damping material will deal with any resonances.. |
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Post by chrisbee on Mar 23, 2008 15:01:21 GMT -7
Mike The idea of using columns fascinates me because I looked at similar ideas to get an IB into my attic AV room. I didn't take that route in the end but still wonder how I can use my above-ceiling volume instead. Your first problem is that you are looking at drivers with a huge Vas. Let's look at the Fi Q18 instead. Vas 268 liters = ~10 cu ft. Minimum IB enclosure volume is 4 x Vas per driver. So you are looking at 40 cu ft minimum per driver. You'd really like to see more volume but at least it gives you a ballpark minimum figure. A square column 2ft x 2ft x 10 feet high just about gets you to 4 x Vas. Extend to the ceiling and you gain a few more cubic feet. Open at the top into the attic and you reduce back pressure. 4' x 13' = 52 cu ft. You are up to a Vas ratio of 5.2 already. The 13 feet column open at the top will have a resonant frequency of about F = V/2L. V =1130 ft/second. 1130/ 26ft = 43Hz. The larger you make the cross sectional area of the pipe the better. A narrow column will have a sharp tune like an organ pipe. I have no idea whether you were considering free standing columns. If you place you columns against a wall they become part of the wall as far as the onlooker is concerned. Nobody looks at a wall anyway. Extend your column width to 4-5 feet x 2' and you have enough volume for 2 x Q18s. That's a fair bit of displacement. Extend the width and your cross sectional area soon becomes very large. 2 feet deep x 8 feet wide is just enough for the 4 x Vas of Q18 drivers. Perhaps enough to call it a true IB enclosure if you leave it open at the top and line it out with rockwool or fiberglass.  |
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Post by mike1234 on Mar 29, 2008 11:48:54 GMT -7
Hmm... that's a lot of "column". Maybe I should just build fake walls :-)
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Post by mike1234 on Mar 29, 2008 11:53:36 GMT -7
ASSUME: A 4x IB system each with two AE IB15's. These will be mounted in the four corners of a 15K ft3 room.
QUESTION: Will it help cancel driver motor inequities to mount them front to back? By this I mean each manifold will have one driver facing into the manifold and the other facing the attic.
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Post by mike1234 on Mar 29, 2008 11:54:56 GMT -7
There are two schools of thought regarding system Q and SQ. Some say that Q does not matter regarding SQ if the system is EQed flat. Others argue that detail/definition suffers with low Q drivers. I just don't know.
Let's assume nothing matters except SQ and EVERYTHING is IDENTICAL except driver Qts. There are four drivers to choose from, all of which have matching T-S parameters except Q. The drivers have a Qts of .20, .35, .50, .70, and 2.0. In all cases driver Fs= 16Hz, Xmax= 25mm each way, etc. Digital EQ is used to flatten overall system response and we have unlimited power with which to drive them.
Again, only SQ matters. Please help me decide.
Mike
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Post by ThomasW on Mar 29, 2008 11:57:52 GMT -7
Why don't you post a floorplan and then we can give you feedback about what's practical and what isn't?
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Post by mike1234 on Mar 29, 2008 11:59:37 GMT -7
Good idea. Will do!!
BTW, You'll find that some of my questions don't necessarily pertain to my particular application. Some are more generalized.
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Post by ThomasW on Mar 29, 2008 12:19:46 GMT -7
QUESTION: Will it help cancel driver motor inequities to mount them front to back? By this I mean each manifold will have one driver facing into the manifold and the other facing the attic. Compound loading will slightly decrease the 2nd order harmonic distortion. If this is audible you have really good hearing... I not going to debate by proxy what people on the PE board have said. My recommendations regarding driver "Q" were posted in the link below quite a while ago... ibsubwoofers.proboards51.com/index.cgi?board=nottalk&action=display&thread=1152113684At this point you're fixating on minutia. Don't sweat the small stuff... FYI, the "Cult" forum has been running almost a decade. There are literally hundreds of successful IB installations constructed using our recommendations. Most of the people posting on other forums have never heard an IB sub, this means their posts are based on 'theory', not hands on experience... |
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Post by mike1234 on Mar 30, 2008 12:05:58 GMT -7
If by "compound loading" you mean the drivers mounted "face-to-face" or "back-to-back" and coupled together... this is not what I intended to refer to. I consider that a waste of drivers. I meant drivers NOT coupled but mounted in the manifiold inversely. This should provide correction of non-linearities without wasting cone area.
Regarding driver Qts... if I understand correctly, a driver with a very heavy cone driven by a miniscule motor should sound as good as a driver with the opposite design if both designs are EQed flat. So a driver with a cone weighing 30 pounds driven with a 1 ounce magnet will sound exactly the same as a driver with a cone weiging one ounce driven by a 30 pound magnet, right? That is if both are EQed falt?
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Post by chrisbee on Mar 30, 2008 12:49:20 GMT -7
I think this is what Thomas is referring to by "compound loading".  The drivers still remain opposed to cancel mechanical forces. |
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Post by ThomasW on Mar 30, 2008 12:55:10 GMT -7
Below is an example of compound loading, also known as push/pull.  That assumption is incorrect. |
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Nordo
Full Member
Posts: 118
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Post by Nordo on Apr 4, 2008 18:23:05 GMT -7
mike1234, There are many posts (and FAQs) on this forum that advise against having IBs (or any sort of sub) at the rear of the room. Also having multiple subs makes it very difficult to EQ and align the system. The beauty, to me at least, is that with IBs, you can put all your drivers in one spot. ie you put them all in a manifold with just one outlet into the room (unless you want stereo subs). I believe that outlet should be no bigger than the combined surface area (Sd) of all the drivers in the manifold. (Thomas has posted elsewhere that, within reason, you can go smaller than your combined Sd). This outlet will mean your low frequencies will be emitting from one spot in the room. All you have to do then is EQ that single source, then align it with the rest of your system. This is easy to do using REW and the "sub distance" adjustment in your receiver / prepro. REW is a fantastic freeware program. You can get it from: www.hometheatershack.com/forums/rew-forum/9865-rew-v4-10-v4-11-release.htmlIf you want to stick with your front subs, your idea of constructing a false wall right across the front of your room isn't as silly as it sounds, so long as you have the room. Make the area behind the wall as big as you can. 4 x Vas minimum, but your not really getting an IB until you are at least 10 x Vas. Also, if you can afford it, clad the face of the false wall with two layers of drywall, and line between the studs behind with rockwool or similar, to try to isolate the rear sound from the room (although low frequencies pass through nearly everything :  . If you can put some vents through the original front wall to the outside, so much the better. This will allow you to reduce the "box" volume your trying to create. BTW, make sure the drywall is well glued & screwed to the studs. Nothing worse than vibrating wall panels  . |
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