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Old 14th July 2013, 01:51 PM   #1
robywan is offline robywan  Australia
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Default CBT Center Speaker - Intial Design/Wiring

Hi Everyone,

I have been reading up on CBT Loudspeaker technology and done searches here but to no conclusion on how to Design them.

My Idea is to build a proto type that runs off just one Amp Channel (the center channel output on a Av receiver for instance)

I have 10 x HiVi B3S drivers (at Normal 8 ohms) that I got on special and I am thinking of building a Center channel for a Home Cinema. (I can get more drivers if needed for the array design)

How do I start, the Maths really confuses me regarding the angles and resulting angle after Shading.

I know I will need tweeters at a later stage to avoid lobing because of driver spacing in the woofers)

Also once the maths for the angle and arc etc I have no idea how to do the shading/wiring for the 10 drivers, I know the center driver will be at 0db (full signal) and the left and right outermost drivers lower in Db - another thing I don't full understand is the shading depending on angle, do I do to -12 regardless of angle or it has to match the Curve Legendre Shading down to the point at which the drivers at 90 degrees past center each side are at 0 signal coming out (if it was indeed a full half sphere)

I might be confusing my self but I just need a proper step by step description of how to plan it depending on the needs instead of trying to strip out the crucial parts of patents and papers, this I think is why many Diy people are not trying it is because it is confusing.

Cheers

Robywan

Last edited by robywan; 14th July 2013 at 01:53 PM.
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Old 18th July 2013, 09:20 PM   #2
boris81 is offline boris81  United States
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http://www.parts-express.com/cbt36/
Have you seen this page? It describes the Shading and Curve of the Part-Express CBT but it doesn't show the math. It would be easy to build a clone but I have no idea how one would go about scaling it for a different size array.

I am confused as well, I can picture this working for about an octave but how does it hold directivity over such vast frequency range?
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Old 19th July 2013, 03:46 AM   #3
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One of the things that make the cbt36 work is that it uses an active digital crossover, which lets you do things that are difficult at best with a passive crossover.
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Old 19th July 2013, 09:00 AM   #4
AllenB is offline AllenB  Australia
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From what I've read, 12dB appears to be far enough. You should be able to set the levels by using various series parallel combinations with mild padding.
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Old 19th July 2013, 06:32 PM   #5
jeno is offline jeno  Norway
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Have you read Don Keeles 4th cbt-paper?
http://www.xlrtechs.com/dbkeele.com/...T%20Paper4.pdf
It has a practical explanation on how to implement the design. The math goes over my head, but the in practice it is as follows:

I think it is easier to think in half arrays. If you are going for a full array, just think of two half arrays laying feet by feet. The -6dB coverage angle is 64% of the full angle of the half array. If the CBT36 is 36 degrees, then the -6dB point will be at 23 degrees. That means if you are going to use two CBT36 as a center channel you will have a 46 degrees coverage angle. Just target the coverage angle you like and do the math from there.

The drivers are divided in 5 banks. Bank 1 is 0 dB, bank 2 is -3dB, bank 3 is -6dB, bank 4 is -9dB and bank 5 is -12dB. Bank 1 should be approximatly 40.4% of the drivers, bank 2 is 26.2%, bank 3 is 15.5%, bank 4 is 10.5% and bank 5 is 7.4%. You will now see that this is difficult to accomplish with just 10 drivers. For a half array the five drivers will be 20% each.

A practical example would be to use 16 drivers to approximate these numbers for a floor standing half array. Bank 1 would be 6 drivers, bank 2 4 drivers, bank 3 3 drivers, bank 4 2 drivers and bank 5 1 driver. The less drivers you use, the worse the approximation the per cent numbers would be. I have no idea how to do the shading with passive components though. I would have gone active like Monte Kay: Center Speaker

Hope this helps.
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Old 20th July 2013, 12:22 AM   #6
AllenB is offline AllenB  Australia
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Using two drivers in series will set them down 6dB each. Or two in parallel then one more in series with them makes the two around 9.5dB down and the single around 3.5dB down. These numbers might be close enough but a few small resistors could be used. It should be possible to do this from a single amp channel.

First you might want to settle on angle, and whether it will be symmetrical about the listening position and how many drivers then the rest can be worked out.
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Old 20th July 2013, 06:42 AM   #7
robywan is offline robywan  Australia
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Hi jeno,

Thanks for this I remember stumbling upon this paper then forgetting where to find it again.

I will see if I can figure out some of the maths and design and post my results here with what you told me.

There was a guy over on another forum that is friends with Monte Kay that works in the same factory I think, he had 12 woofers on a really crazy wedge angle if I remember correctly.

Thanks for all the comments

Robywan



Quote:
Originally Posted by jeno View Post
Have you read Don Keeles 4th cbt-paper?
http://www.xlrtechs.com/dbkeele.com/...T%20Paper4.pdf
It has a practical explanation on how to implement the design. The math goes over my head, but the in practice it is as follows:

I think it is easier to think in half arrays. If you are going for a full array, just think of two half arrays laying feet by feet. The -6dB coverage angle is 64% of the full angle of the half array. If the CBT36 is 36 degrees, then the -6dB point will be at 23 degrees. That means if you are going to use two CBT36 as a center channel you will have a 46 degrees coverage angle. Just target the coverage angle you like and do the math from there.

The drivers are divided in 5 banks. Bank 1 is 0 dB, bank 2 is -3dB, bank 3 is -6dB, bank 4 is -9dB and bank 5 is -12dB. Bank 1 should be approximatly 40.4% of the drivers, bank 2 is 26.2%, bank 3 is 15.5%, bank 4 is 10.5% and bank 5 is 7.4%. You will now see that this is difficult to accomplish with just 10 drivers. For a half array the five drivers will be 20% each.

A practical example would be to use 16 drivers to approximate these numbers for a floor standing half array. Bank 1 would be 6 drivers, bank 2 4 drivers, bank 3 3 drivers, bank 4 2 drivers and bank 5 1 driver. The less drivers you use, the worse the approximation the per cent numbers would be. I have no idea how to do the shading with passive components though. I would have gone active like Monte Kay: Center Speaker

Hope this helps.
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Old 16th July 2014, 12:05 AM   #8
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Quote:
Originally Posted by robywan View Post
I will see if I can figure out some of the maths and design and post my results here with what you told me.
Did you ever give this a go? I'm curious how terrible the result would be trying to scale down with fewer drivers
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Old 16th July 2014, 09:46 AM   #9
Juhazi is offline Juhazi  Finland
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JBL has a program for CBTsetup (PA)

https://www.jblpro.com/www/product-support/downloads
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Old 16th July 2014, 01:50 PM   #10
flg is offline flg  United States
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The patents I've seen include some wiring examples of parallel series and additional R's for a few differnet designs. Shouldn't be to hard to find.
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