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27th September 2010, 04:37 PM  #1 
diyAudio Member
Join Date: Jan 2004
Location: Sydney

DIY CBT Design: (i) The Story So Far & (ii) A Question Unresolved...
Hi all,
I'm looking at building a straight delaycurved CBT (Constant Beamwidth Theory) line array (a la Keele), using 24 3.5” fullrange drivers (Vifa TC9FD1808 (apparently aka: Peerless FR35/8); sensitivity's okish at 86dB, but good power handling for a small driver (50W/100W) with good widerange performance 110Hz15K. Qts = 0.72). CBT arrays look VERY promising; just Google CBT & Keele for more info, but to sum up, much better dispersion than std linearray; less vertical lobing, more consistent & homogeneous sound field near & far field. However, I have a question re acoustic outputs & drivergroup sensitivities for mixed parallel/series combinations of drivers that I’m hoping someone could please help me with, but see Keele's .pdf presentation on CBT's & then read on for the context first: (Incidentally, I'll stress; all my efforts are for a PERSONAL system; in no way am I attempting to build a commercial unit. I'd also encourage all who follow to observe & respect the patents that JBL & Keele have on CBT technology). Right: now that the legal caveat is out of the way... So far, I’ve established the appropriate Legendre shading amounts for each of the drivers (like I said, see Keele's preso'!), which I’ve then grouped around 3dB increments (ie, 0dB, 3dB, 6dB etc) to simplify the amplification logistics. My shaded groups end up looking like this: Qty . . . . . Shading (dB) 1 . . . . . 18dB 1 . . . . . 12dB 1 . . . . . 9dB 2 . . . . . 6dB 3 . . . . . 3dB 4 . . . . . 0dB (ie, no attenuation) 4 . . . . . 0dB 3 . . . . . 3dB 2 . . . . . 6dB 1 . . . . . 9dB 1 . . . . . 12dB 1 . . . . . 18dB Those of you who understand the maths may notice that I’m actually one driver short; there should be a 9th driver at 0dB in the middle, but I’ve decided to omit it for each of amplification (8 x 8ohm drivers: yeah! But 9 x 8ohms drivers… hmmm…). I’m hoping the fact that I’m roughly approximating the curve using the 3dB increments will allow a degree of fudging. Re the amplification: Rather than try & driver the entire lot of one amp (which I think would involve having to come up with some very strange combinations of drivers to ultimately establish a workable end impedance (ie, 16 >= Z >= 4ohms)), I’ve decided it’s probably easier to use a small number of multiple amps to drive select combinations of the groups above that naturally occur 6dB apart in relation to each other (although see the caveat below before you jump on any errors in my assumptions), ie: Group . . . . . Qty . . . . . Shading (dB) C2 . . . . . 1 . . . . . 18dB C1 . . . . . 1 . . . . . 12dB B2 . . . . . 1 . . . . . 9dB A2 . . . . . 2 . . . . . 6dB B1 . . . . . 3 . . . . . 3dB A1 . . . . . 8 . . . . . 0dB (I’ve added the two 0dB groups together) B1 . . . . . 3 . . . . . 3dB A2 . . . . . 2 . . . . . 6dB B2 . . . . . 1 . . . . . 9dB C1 . . . . . 1 . . . . . 12dB C2 . . . . . 1 . . . . . 18dB Ignoring the symmetrical layout & combining group numbers for simplicity, this should result in the following: Group . . . . . Qty . . . . . Shading (dB) A1 . . . . . 8 . . . . . 0dB B1 . . . . . 6 . . . . . 3dB A2 . . . . . 4 . . . . . 6dB B2 . . . . . 2 . . . . . 9dB C1 . . . . . 2 . . . . . 12dB C2 . . . . . 2 . . . . . 18dB Factoring in series/parallel (S/P) combinations, I hope to achieve the following (once again, see the caveat below before you jump on any errors in my assumptions): Group . . . . . Qty . . . . . Shading (dB) . . . . . Combo impedance (ohms) A1 . . . . . 8 . . . . . 0dB . . . . . 4ohms (straight 1248 parallel/series combo) B1 . . . . . 6 . . . . . 3dB . . . . . 5.33ohms (3 paralleled sets of seriespairs)* A2 . . . . . 4 . . . . . 6dB . . . . . 8 ohms (straight 124 parallel/series combo) B2 . . . . . 2 . . . . . 9dB . . . . . 4ohms (parallel pair)** C1 . . . . . 2 . . . . . 12dB . . . . . 16ohms (series pair) C2 . . . . . 2 . . . . . 18dB . . . . . 16ohms (series pair)** Or, when viewed in their groups: Group . . . . . Qty . . . . . Shading (dB) . . . . . Combo impedance (ohms) A1 . . . . . 8 . . . . . 0dB . . . . . 4ohms (straight 1248 parallel/series combo) A2 . . . . . 4 . . . . . 6dB . . . . . 8 ohms (straight 124 parallel/series combo) B1 . . . . . 6 . . . . . 3dB . . . . . 5.33ohms (3 paralleled sets of seriespairs)* B2 . . . . . 2 . . . . . 9dB . . . . . 4ohms (parallel pair)** C1 . . . . . 2 . . . . . 12dB . . . . . 16ohms (series pair) C2 . . . . . 2 . . . . . 18dB . . . . . 16ohms (series pair)*** * No, I’ve no idea if the B1 combo will naturally yield a group sensitivity that’s 3dB with respect to A1; that’s yet to be established & may need a little cut/boost if required. But I’m estimating that it’ll be close. ** B2 will require an extra 3dB attenuation to bring them to the 9dB target *** C2 will require 6dB of extra attenuation to bring them to the 18dB target I’ll add the group pairs as follows: A1 + A1 = series (Z = 12 ohms) B1 + B1 = series (Z = 9.33 ohms) C1 + C1 = parallel (Z = 8 ohms) – this may require an overall Cgroup attenuation if the parallelconfig’ raises the sensitivity, but I didn’t want to series them due to the 32ohm impedance that would result. So, in essence, I’m hoping to achieve the shading for all six groups by running them in relative +6dB pairs off three amplified channels, with a little extra attenuation where required. HOWEVER, here’s the main caveat (and the point of my post’s question): Am I correct in my assumptions about the combination in Group A achieving a 6dB difference between the two subgroup pairs? I’ve taken into account gain due to both current draw as well as driver numbers, rather than just driver numbers, so, looking at Group A: Group . . . . . Qty . . . . . Shading (dB) . . . . . Combo impedance (ohms) A1 . . . . . 8 . . . . . 0dB . . . . . 4ohms (straight 1248 parallel/series combo) Connected in series to: A2 . . . . . 4 . . . . . 6dB . . . . . 8 ohms (straight 124 parallel/series combo) Given that A1 has twice the drivers as A2, and that A1 will draw twice as much current than A1 (due to half the impedance), is it correct to say that the overall difference in subgroup sensitivities will be 6dB? Or will it only be 3dB? And does connecting them in series help/hinder this? I hope that all makes sense; can anyone please assist me with this? Thanks heaps! Paul
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___________________ There's a fine line between 'hobby' and 'mental illness'... 
10th January 2013, 08:39 PM  #2 
diyAudio Member
Join Date: Mar 2011

And then what happened next?

11th August 2013, 02:43 AM  #3 
diyAudio Member
Join Date: Jan 2007
Location: Sacramento

Doofus, (up arrow)
I have had the same thoughts on wiring a CBT array, and your ideas / theories are logical. I wish I could help, however, I'm still in the haven't cut any wood yet stage. How did yours turn out. Sorry, I'm not much help, but I'm not a Doofus either. Ron
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"If it doesn't work properly, hope it catches on fire" Nelson Pass @ BA3 "I fired up the prototype. Literally." The Prophet Pass. 
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