WOW, lot's of response, great!:D
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Originally Posted by lfh
Yep, a main lobe in the horizontal plane is the expected behaviour, and the pattern should be symmetrical above and below the "x-axis". No unpleasant surprises. :) The graph basically shows the variation in readings you would get when moving a mic along a half circle (at large distance from the array) in the vertical plane around the array center (and on axis in the other dimension) in an anechoic environment. The frequency response at any desired angle (e.g. 30 deg above/below horizontal plane) can be derived from a set of such polar plots calculated for a sequence of frequencies.
It wasn't until now, that I realized this. I thought a nice even curve looking like a half circle was the target at all times.:o
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Note that such a FR is due solely to the array geometry (and possibly source phase and level diffs), and the corresponding interference patterns. A more realistic simulation would (at least) take the driver directivity patterns into account (possibly modeled with the well known directivity patterns of simple propotypes such as a piston in a baffle), but the model used in the program still gives a good demonstration of the fundamental properties of a certain array.
Isn't it possible that the Behringer UltraDrive will take care of this, or isn't it the same thing as time aligning the drivers?
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EDIT: In this particular case - using a 10" and a 15" in an MTM setup - I'd want to use the piston model (or better yet; measured magnitude and phase data) for the simulation. Such a hack is on my to-do list, but ETA is like Q2... Perhaps you could convince SG to add this (piston model), along with a few text book filters, and means to enter driver acoustical centers?
I'm not sure I understand these abbreviations and also, wouldn't these additions to the program complicate it quite a bit?
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EDIT 2: Then (after reading Earls post again), what would you do with the more accurate plots? New conclusion: Point sources are just fine. Time better spent would most likely be to build test boxes and experiment! "Probieren geht ueber studieren." :)
"Aber natuerlich". Though being able to predict or knowing even, makes it possible to avoid spending a lot of time and money on building boxes, when the odds are stacked heavily against any solution.:D
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Well, it depends. Sealed boxes behave like 2:nd order high pass filters whereas ported boxes behave like 4:th order ones. Thus, a sealed box has the theoretical possibility to perform better in this respect than a ported one. However, performance depends on the "filter" tuning/alignment. A closed box with a high Q-value will "ring"/"boom", and a ported box with a shallow roll off will have a "tight" character. That said, a 2:nd order HP characteristic with a Q around 0.6 (or even lower, when factoring in room gain at lower frequencies) is pretty much as good as it gets, but a ported box tuned with transient performance in mind yields very good results as well.
I have already been using JBL's "SpeakerShop Enclosure Module 1.0" and stopped at a net volume of 7L sealed box (10L minus 3L driver displacement) and a Q of 0,591, giving a Fc=166,9 and F3=205,7 and no filling.:D
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Often the latter is the better overall compromise, when weighing in other aspects such as the desired cutoff frequency, the acoustical output capability etc. Yet another thing to keep in mind is that the "best" box type to a large extent depends on the driver at hand. Typical JBL drivers often work best in ported enclosures (if you still want some LF, that is).
I don't think I've ever seen the 2123 in a vented box and I was hoping I'll be able to EQ the 2123 to some extent, giving it a wider FR and at the same time lowering the output somewhat.
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You're not doing a middle-of-the-road design here... Do you have some references of similar "hybrid MTMs" (IIRC Earl runs such stuff - I must read the entire K 145 thread) and have you looked into D'Appolitos designs - noteably the crossover networks? How will you tackle the low end where the 2123s runs out of steam but the 2215s keep pumping? I think some careful thinking is needed here when designing the two boxes. (Sure, you're "cheating" with your trick EQ and all, but some master plan on how to design the disparate bass boxes for the desired combined LF response (10"+15"+EQ+room) is called for IMO.) Finally; why is the tweeter not above the mid horn?
Mainly I'm acting on a very strong hunch and then I wanted to use the things at hand! I've been reading some D'Appolito papers and I've seen a few asymmetric solutions. So when Earl K suggested that I should build just that, I knew I had to.:D
About the overlapping, I'm hoping it will help creating a sonic "glue", like Earl K is suggesting. To me it seems very reasonable, given the need to look at the combined sound level and a lot of fiddling with x-over points, slopes and EQ'ing.
Hmmmm, "master plan", I don't think it ever came to that...:o
The tweeter has been moved around a few times. at one time there wasn't enough room between the 2441 and the 2123. Then again Earl suggested a placement, which resembled the JBL 43XX design and therefore should be quite a safe position.
I take it you would like to place it above? If so, could you please elaborate on why? Hopefully it would help me understand the principle a little better.
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Bullock on boxes is a great read on this subject.
He says a properly tuned low Q woofer will sound every bit as good as a seal design. He then goes on to discuss the finer points of reflex tuning, accounting for Rx, QL and actually T/L measurements.
I know myself you can alter the quality of the bass in sublte but audible degrees by optimising the woofer for a given enclosure Vb, tuning frequency Fb and Rx the total Dcr behind the speaker.
Ian
Thanks Ian, where can I find this read? Is it to be found on this forum?
BR
Roland