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Discussion Starter #1
The CBT is finished! I don't have it installed in the theatre yet, I'm in San Jose, CA at the moment but if all goes to plan I will be home this weekend and have it installed by early next week.


It performs pretty much exactly as the math predicts. We made measurements on axis to 70 deg off axis in 5 deg increments but I'm posting 10 deg increments to keep the graph less crowded. The lowest curve is 70 deg off axis, just count lines up from there to see the result of 60 deg off, 50... etc. By the time you get to 30 deg off axis, the lines are very nearly aligned with the on axis response. As you can see, the off axis response drops off very uniform within the CBT's range. Starting about 240 Hz, it begins to loose control of directivity and by about 160 Hz, it's no longer acting like a CBT. Because it's open baffle however, it still maintains some control over the low frequency off axis response.


We also made an on axis measurement with and without a rear wall reflection. To do this, we set up a 4 x 8 sheet of MDF behind it at the position where the rear wall will be in the theatre. In this graph the two red curves are with the wall in place, the blue curve is without the wall. The out of control wiggly red line is with the wall bare. The other red line is with the rear wall reflection absorbed by Owens Corning 703 fiber panels. These are 2" thick. We had two layers over the entire MDF board, three layers on the bottom half of the board and then additional 2 x 4 sheets right behind the center of the speaker. We also had some 1" sheets and the thickest part was in total 15" thick. I've just ordered twenty four more 2" sheets to add to this once it's in the theatre.


We wrote all over my driveway to do this test, you can see the marks where we drew out the test points for the 5 deg increments.


We also have a 1" layer of the fiber directly behind the twenty 6" Dayton Reference drivers and the fifty Aura 2" drivers. There is no fiber behind the one hundred thirty two Tang Band tweeters because those are closed back anyway.


Given the direcivity control this speaker exhibits I think the dialog quality should be unparalleled. It's very important to absorb that rear wave however. The nice thing is that you know exactly where to put the fiber... the thing is pretty much like a magnifying glass focusing that rear wave toward the center point of the 120 deg arc of the speaker. This arc is on a 49" radius which will be the distance to the rear wall. I'm going to have a lot more absorption there than we used in the outdoor test.


Once I get it installed I'll post some photos of that. Maybe I'll eventually get around to updating my web site with more photos and details.


All seven channels of the Emotiva UPA-7 amplifier are used. Three channels for the 6" drivers and two each for the 2" and 0.5". The Behringer DCX 2496 sets the - 3 dB increments for the 2nd and 3rd driver banks on the 6" drivers. Resistors set the attenuation on the 4th and 5th banks. There is also a 0.5 dB attenuation between the crossover and the amplifier on the center bank of tweeters. It's too bad the Behringer didn't have seven outputs, that would have eliminated the need for this external 0.5 dB attenuation. It was of course measured using Arta, the mic is the Earthworks M30.


If you are not familiar with CBT (Constant Beam width Transducer) arrays google Don Keele and read his papers on the topic.


mk




 

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Looks cool - I need to make a trip to Austin! I was at the shop yesterday looking at some new drivers
 

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Montekay,


This is great stuff....it seems that if someone has a solid screen, CBT would be one of the best ways to go for center channel sound...I think I would be willing to put up with a slightly skewed center channel image to make the arc smaller than 120deg, though....but that's just me.


You have great response over a 70-80 deg arc! Amazing how deadening the front wall affects things...How did you go about designing the crossover? What did you decide on? Will it remain active, or will you build a passive version?


The vertical dispersion in one of these designs is impressive per Keele's data. Do you plan on mounting sound absorbers at ceiling reflection points?


JSS
 

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Discussion Starter #5

Quote:
Originally Posted by maxmercy /forum/post/17853364


Montekay,


This is great stuff....it seems that if someone has a solid screen, CBT would be one of the best ways to go for center channel sound...I think I would be willing to put up with a slightly skewed center channel image to make the arc smaller than 120deg, though....but that's just me.


You have great response over a 70-80 deg arc! Amazing how deadening the front wall affects things...How did you go about designing the crossover? What did you decide on? Will it remain active, or will you build a passive version?


The vertical dispersion in one of these designs is impressive per Keele's data. Do you plan on mounting sound absorbers at ceiling reflection points?


JSS

I'm using the Behringer crossover and yes it will remain fully active. I've found the best way to go about dialing in a crossover is to first run each frequency band full bandwidth with the possible exception of some low frequency filtering on the tweeters. For example, I turn the crossovers completely off on the 6" drivers and run them flat. I then use the Behringer to EQ the response as flat a possible over as much bandwidth as possible. I notch out any peaks but never use any high Q boost. I will use shelf boost to extend the bandwidth and low Q boost to bring up any wide band low areas but never any high Q boost, if there is a narrow band notch in the response, I leave it. After that I apply the crossovers. Once this is done for all bands I level match and do any time alignment required.


I have proven to myself, forgotten or disbelieved myself and then proven to myself again that driver resonance peaks must be removed even if they are well beyond the frequency band where the driver is to be used. If you don't do this, the speaker will sound harsh. The strange thing is that this is true even if there is no visible effect to the total system frequency response measurement. That's why I always disbelieve myself and end up having to prove it to myself again.


I'm going to have a bunch of absorption behind the speaker but little if any on the side walls or ceiling. The CBT will be mounted horizontally below the screen so the CBT will control the off axis response horizontally and the dipole effect will reduce the floor/ceiling reflections. I may eventually put some absorption on the walls however. I do have one single panel on the back wall behind the back row of seats. The plan was always to put a few more but I just haven't gotten around to it. It might pay to also up a couple on the side walls down near the front just in front of the speakers.


mk
 

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Discussion Starter #7
The CBT is up and playing in the theatre. I have to say it sounds pretty amazing! As expected dialog clarity is just outstanding no matter where you sit.


Overall, this speaker makes for an interesting listening experience. Especially when it was setup outdoors you could easily tell the directivity. You could stand in front of it and then begin walking around the arc and hear virtually no change for the first 30 deg or more each way. Then it began to drop off just slightly and once you reached the end is was almost like someone turned it off.


Another interesting is the apparent source location. It always seems the sound is coming from the center point of the arc. As you walk around it the sound follows you, it reminds me of one of those paintings where the eyes follow you as you move around the room. With it on the floor in the dining room it was facing a wall about 10 ~ 12 feet away. If you stood just off the end of the speaker you really couldn't tell anything was coming from the speaker, it sounded as if it were all coming from the wall in front of it. Because you were out of the speakers coverage area, all you could hear was the reflection off the wall. It was almost spooky how well this worked.


Setup in the theatre was pretty easy. We had dialed in the Behringer crossover to a flat response with the speaker outdoors lying flat on the ground tweeters down. Once it was mounted on the stands in the theatre it sounded thin as expected due to having been calibrated with the ground plane effect. The only correction required was to bring up the low end to where it had been with the speaker on the ground.


As usual I had to rush out of town shortly after getting it going but we did watch some scenes from the new Star Trek and later the movie, Wall E. We also moved around the room trying different seating locations and it just sounded amazing in every seat. I couldn't be happier with the performance and also with the fact that all the measurement data completely agrees with the predictions of the theory behind this speaker. I really appreciate all the support and assistance Don Keele provided on this project. At one point I can concluded it wouldn't work but Don brought me back on course and now that it's completed, I'm sold, CBT is simply amazing!


On a side note, my UPA-7 Emotiva amplifier provides the power to this thing very well. This was the final test for the Emotiva and it passed with easy. I figure this speaker has the resolution to reveal any flaw the electronics might have and I believe the Emotiva performs just as well as my Anthem. I'm selling the Anthem PVA-5 btw but I don't recommend it, you're better off saving the money and getting the Emotiva.


mk

 

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How have you liked your CBT as of now?


I still haven't made a center channel yet and having a CBT for a center channel seems doable but I would have a much smaller one for my room. I have BFM SLA's for now for the L/R. Plus I would have to see what it would cost to build one.
 

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Discussion Starter #9
Quote:
Originally Posted by chrapladm
How have you liked your CBT as of now?


I still haven't made a center channel yet and having a CBT for a center channel seems doable but I would have a much smaller one for my room. I have BFM SLA's for now for the L/R. Plus I would have to see what it would cost to build one.
I've been very happy with it. Comparing it to my straight line arrays lead me to modifying the straight lines to include Hann shading. Eventually I would like to replace them with curved true CBT's. The straight lines have a "hardness" to them the CBT doesn't have. I'm not sure if simply adding the Tang Band tweeters would resolve this or not. The shading seemed to help some but it's still there. It might be due to the fact that in the straight lines the 2" Aura is used all the way to the top. I still might consider adding the tweeters leaving it straight but ultimately I think the right thing to do is to rebuild with the curve. Since these would be standing vertical, the dispersion requirement would be much less thus the arc angle would be much less. Perhaps about 30 deg as opposed to the 120 deg of the center ch. I will probably stay with a free standing CBT as opposed to ground plane just because I'm not sure about the image placement with the ground plane CBT. Inverting them and using the ceiling for the ground plane might work but I have not had the opportunity to listen to a ground plane CBT yet so I'm just not sure.


mk
 

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Wow MK your Center channel looks amazing
,I bet it sounds as good as it looks..


Sorry if I missed it but How long did it take to build it

how many Tweeters & mids are used & what brand..



Cheers...
 

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Discussion Starter #11

Quote:
Originally Posted by cinema mad /forum/post/19580581


Wow MK your Center channel looks amazing
,I bet it sounds as good as it looks..


Sorry if I missed it but How long did it take to build it

how many Tweeters & mids are used & what brand..



Cheers...

Marshall cut the parts on the CNC in North Carolina then flew out to Austin for just over a week. By the time he left it was built but not fully wired. It takes a long time to wire 132 tweeters http://www.parts-express.com/pe/show...number=264-841 The 50 2" drivers are the Aura https://www.madisound.com/store/prod...oducts_id=8465 And the 20 6" drivers are the Dayton Reference http://www.parts-express.com/pe/show...number=295-342 These truncated frame Reference drivers were originally a custom driver for us but PE ordered a few extra for testing the market then made it a standard product.


mk
 

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I was thinking more along the lines of a 4-5" driver and tweeter combo that can be crossed at roughly 100hz. I am mainly wondering about what size arc would I need for a seating area 9-11ft away with walls only 13.5ft wide?
 

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Monte,


Your love of audio, and devotion to rolling your own is a boon to us all. I’ve been an avid DIY audio geek (now professional – but I still roll my own and geek out at home regularly) for ~~ 3 decades now. Having heard the dynamics and low distortion available from Line Array’s – I’ve always wanted to try them in a home theater environment.


My issue had always been two things. 1 – I’m looking for constant directivity over as large a frequency spectrum as I can attain (my holy grail). 2 - the inability to also use a line array for the center channel on its side.


I require my front three speakers to be as close to identical as possible with the same voicing, a straight non shaded line array just doesn’t have the lateral coverage necessary, lobes and combe filters too much – and won’t center the image on the screen’s center..


Your CBT Center is my answer!! I am so glad Don convinced you to give it a try! Thank you for the incredible level of work you’ve gone through in documenting it for us (well that and the actual build
. I am extremely excited to start heading down the CBT path myself… Eventually I would like to build a pair of G.P. CBT’s for L/R duty, several smaller variants for surrounds, and a full CBT for the center..


I am especially happy to hear the legendre shading keeps vocals centered roughly in the middle of the screen as you move off axis. This has been my biggest sticking point – the desire to use a solid screen and maintain a fixed center image directly in the center of the screen.


On a side note – regarding your comment about your reticence to try G.P. CBT’s verses full height CBT’s above for L/R duty – you may have the answer to your fears sitting in your HT already:


After careful review of the measurements Don posted for CBT’s and ground plane CBT’s – I suspect that a half height ground plane CBT would be Ideal as replacements to your current straight line arrays..


Since you’ve already got the straight line array enclosures built repleat with HANN shading– It might be useful to digitally delay the signals and see how they sound. IOW, use digital delay and legandre shading to generate the equivalent of a full CBT, as described in don’s papers.


This way you will have a strong idea of the potential improvement in SQ going from straight to CBT array for your L/R’s.. You could also test whether you would prefer full height CBT’s or ground plane CBT’s all on the same straight L/R line arrays you currently run before ever cutting wood..


After calculating all the potential listening heights angles (which I suspect you already know  ) – choose a ground plane CBT curvature and height that properly covers the entire intended listening area, and truncate the line electronically. Only use those speakers needed. You could use delay and shading to approximate different curvatures even.. You could then invert this effect and evaluate a ceiling verses floor mounted G.P. CBT
.


If you do entertain the idea of testing this – once you build what you’ve decided on, to address the harshness issue of your current L/R’s in the top octave, I recommend incorporating the tweeters used in your center for your new L/R CBT’s or G.P. CBT’s.... The majority of the answer lies in your post #5 under this thread:

Quote:
Originally Posted by Montekay /forum/post/17856535


I have proven to myself, forgotten or disbelieved myself and then proven to myself again that driver resonance peaks must be removed even if they are well beyond the frequency band where the driver is to be used. If you don't do this, the speaker will sound harsh. The strange thing is that this is true even if there is no visible effect to the total system frequency response measurement. That's why I always disbelieve myself and end up having to prove it to myself again.

Those Aura’s have a breakup mode above 10K that can be heard, and I suspect this is the majority of the harshness you currently experience with your L/R’s relative to your center even after shading..


I gotta say – at some point I would love to head down to Austin and give your system a listen!!
 

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dB,


When you build yours, we need a thread....


I promise to do the same when I build some as well...


A ceiling mounted groundplane CBT is near-ideal as a surround, with absorption behind it to avoid 1/4 wave cancellations.


Only thing a center channel CBT needs is absorption behind it and at first reflection points floor and ceiling.


I have said many times that CBT is a great option for those without acoustically transparent screens....


JSS
 

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dB, why must you use a horizontal orientation for a center channel line array? Not familiar with your system... no option for an acoustically transparent screen I suppose?


Looks like the CBT would be ideal if you have to use a horizontal center.
 

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Quote:
Originally Posted by maxmercy /forum/post/21518233


dB,


When you build yours, we need a thread....


I promise to do the same when I build some as well...


A ceiling mounted groundplane CBT is near-ideal as a surround, with absorption behind it to avoid 1/4 wave cancellations.


Only thing a center channel CBT needs is absorption behind it and at first reflection points floor and ceiling.


I have said many times that CBT is a great option for those without acoustically transparent screens....


JSS

Absolutely! Just brainstorming at the moment, but I'm thinking about using the same horns and mids my system currently uses. A Left and Right cutaway view of the current take on a L/R GP CBT using them is attached along with the current settup.

Quote:
Originally Posted by Bigus /forum/post/21518322


dB, why must you use a horizontal orientation for a center channel line array? Not familiar with your system... no option for an acoustically transparent screen I suppose?


Looks like the CBT would be ideal if you have to use a horizontal center.


My HT also doubles as the familly room. To help with some of the lighting issue's - I use a hard screen with high gain. (Vutech Silver Star)

 

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Fascinating - routing / sanding / wiring those tweeters must have been FUUUUUNNNNN.
I just had to do it for a pair of baffles with 16 tweeters each, and that was already tedious.
 

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WOW.


I feel the need to point out that with just the 20x 6" drivers, you have in the range of 2000 sqcm of surface area, more than 2 15's.
 

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"Marshall cut the parts on the CNC in North Carolina then flew out to Austin for just over a week."


sounds pricey...;-)


though i like what you guys have there...
 

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Quote:
Originally Posted by dB-Kicker /forum/post/0



Absolutely! Just brainstorming at the moment, but I'm thinking about using the same horns and mids my system currently uses. A Left and Right cutaway view of the current take on a L/R GP CBT using them is attached along with the current settup.


My HT also doubles as the familly room. To help with some of the lighting issue's - I use a hard screen with high gain. (Vutech Silver Star)

I also do not have full light control and use a gray screen, nor the space for a proper baffle wall and AT screen....


The limitation I see will be w/ the horns. C-C spacing will not be ideal, but I don't know if anyone has measured a CBT w/ horns for high end. Their directivity may be a good thing, not studied in a CBT orientation, AFAIK.....


JSS
 
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