DIY CBT Design: (i) The Story So Far & (ii) A Question Unresolved... - diyAudio
Go Back   Home > Forums > Loudspeakers > Planars & Exotics

Planars & Exotics ESL's, planars, and alternative technologies

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 27th September 2010, 03:37 PM   #1
diyAudio Member
 
Join Date: Jan 2004
Location: Sydney
Default DIY CBT Design: (i) The Story So Far & (ii) A Question Unresolved...

Hi all,

I'm looking at building a straight delay-curved CBT (Constant Beamwidth Theory) line array (a la Keele), using 24 3.5Ē full-range drivers (Vifa TC9FD18-08 (apparently aka: Peerless FR35/8); sensitivity's ok-ish at 86dB, but good power handling for a small driver (50W/100W) with good wide-range performance 110Hz-15K. Qts = 0.72). CBT arrays look VERY promising; just Google CBT & Keele for more info, but to sum up, much better dispersion than std line-array; less vertical lobing, more consistent & homogeneous sound field near & far field.

However, I have a question re acoustic outputs & driver-group sensitivities for mixed parallel/series combinations of drivers that Iím 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, Iíve established the appropriate Legendre shading amounts for each of the drivers (like I said, see Keele's preso'!), which Iíve 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 Iím actually one driver short; there should be a 9th driver at 0dB in the middle, but Iíve decided to omit it for each of amplification (8 x 8ohm drivers: yeah! But 9 x 8ohms driversÖ hmmmÖ). Iím hoping the fact that Iím 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)), Iíve decided itís 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 (Iíve 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 1-2-4-8 parallel/series combo)
B1 . . . . . 6 . . . . . -3dB . . . . . 5.33ohms (3 paralleled sets of series-pairs)*
A2 . . . . . 4 . . . . . -6dB . . . . . 8 ohms (straight 1-2-4 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 1-2-4-8 parallel/series combo)
A2 . . . . . 4 . . . . . -6dB . . . . . 8 ohms (straight 1-2-4 parallel/series combo)

B1 . . . . . 6 . . . . . -3dB . . . . . 5.33ohms (3 paralleled sets of series-pairs)*
B2 . . . . . 2 . . . . . -9dB . . . . . 4ohms (parallel pair)**

C1 . . . . . 2 . . . . . -12dB . . . . . 16ohms (series pair)
C2 . . . . . 2 . . . . . -18dB . . . . . 16ohms (series pair)***

* No, Iíve no idea if the B1 combo will naturally yield a group sensitivity thatís -3dB with respect to A1; thatís yet to be established & may need a little cut/boost if required. But Iím estimating that itíll 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

Iíll 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) Ė this may require an overall C-group attenuation if the parallel-configí raises the sensitivity, but I didnít want to series them due to the 32ohm impedance that would result.


So, in essence, Iím 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, hereís the main caveat (and the point of my postís question):

Am I correct in my assumptions about the combination in Group A achieving a -6dB difference between the two sub-group pairs? Iíve 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 1-2-4-8 parallel/series combo)
Connected in series to:
A2 . . . . . 4 . . . . . -6dB . . . . . 8 ohms (straight 1-2-4 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 sub-group 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
__________________
___________________
There's a fine line between 'hobby' and 'mental illness'...
  Reply With Quote
Old 10th January 2013, 07:39 PM   #2
blueznl is offline blueznl  Netherlands
diyAudio Member
 
blueznl's Avatar
 
Join Date: Mar 2011
And then what happened next?
  Reply With Quote
Old 11th August 2013, 01:43 AM   #3
Renron is offline Renron  United States
diyAudio Member
 
Renron's Avatar
 
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
__________________
"If it doesn't work properly, hope it catches on fire"- Nelson Pass @ BA3
"I fired up the prototype. Literally." The Prophet Pass.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
My First DIY Loudspeakers [Story & Pics] bmwman91 Multi-Way 22 10th April 2013 08:42 PM
Tube Amp Design Question... Langenbauer 1 Tubes / Valves 9 21st July 2009 01:29 AM
The story of my life: diy speakers (as told by an EE) JLC7 Multi-Way 40 26th August 2008 06:02 PM
Help needed with DIY chill amp II Spasticteapot Chip Amps 1 14th March 2006 12:14 AM


New To Site? Need Help?

All times are GMT. The time now is 07:11 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2