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Constant Beamwidth Transducer (CBT) Speakers

yourmando

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I thought I’d cross post my CBT question here to see if any you CBT people have any ideas.

In short, I’m wondering how the JBL CBT extension modules work in conjunction with the main module.

Either they are an almost magical technical feat where you can extend vertical pattern control down more octaves by willy nilly stacking modules that just contain woofers (plus some unknown delay + shading scheme)—or I’m missing some simple line array principle that would make this concept work! Details below:

I've been reading Keele's papers, JBL tech docs, and watching lectures on CBT technology. Fascinating stuff!

I'm stumped on one thing--how do the bass extension modules technically work (phase & shading) in conjunction with the JBL CBT 70J and CBT 1000??

I'm wondering if any line array gurus such as @Bjorn , @Selah Audio , etc can shed some light or ideas?

Here's what I know, and what the JBL docs spec:

The main purpose of adding the 70JE or 1000E extension module to their base CBT module is to extend the +/- 20 degree vertical pattern control by doubling the length of the line array (it also doubles the power handling, etc.)
  • The CBT 70J has vertical pattern control down to 800hz. Adding the 70JE extension module extends the vertical beamwidth pattern control down to 400hz (with still good control below that w/ the vertical beamwidth gently widening and providing good attenuation off axis). Similarly, the CBT 1000 has pattern control down to 600hz and combining with the 1000E extension module extends the pattern control down to 300hz.
  • The FAQ states you can even add more modules, such as adding 2 more extension modules to double the length again and bringthe vertical width control down another octave (you'd have to add EQ though because the out of box EQ assumes it's alone, so w/ more woofers you need to EQ down some of the overlap).
  • The FAQ states the extension modules would even technically work on the other CBT lines, such as adding a 70JE to a CBT100LA-1, which uses completely different drivers and is a different width! (see FAQ snippets attached).
At first I assumed the CBT extension modules simply extended the phase delay and shading pattern of the base module CBT array. But this doesn't make sense upon further thought.

The JBT CBTs are "free standing" CBT arrays (as opposed to the ground plane version expects to be mirrored on a reflective surface).

This means that the center of the CBT "arc" is in the middle of the 70J or 1000 module, and as you move toward the ends of the array there is more phase delay and attenuation.

The attenuation at the ends of a CBT array is pretty substantial. Keele worked out a version that roughly approximates the Legandre shading, is less attenuated at the ends, and is quantized into 5 banks instead of continuous shading. Even in this less attenuated version, the drivers at the end of the arrays are -12db.

View attachment 121335

So it seems unlikely that the added woofers are simply extending the 'arc' starting from -12db and attenuating even more from there to continue the shaded arc.

In fact, there can't be much shading or phase delay going on at all with the woofers. The 70J only has 4 woofers, so there would only be room for 2 banks of delay & shading! (For example the 2 middle woofers would be at 0db and 0 phase delay, leaving 1 more more step of attenuation and delay for the 2 end woofers). Keele's CBT arrays use much smaller drivers so both the woofers and tweeters can be bucketed into 5 banks with several drivers in each bank (each bank has the same shading).

So it also seems like most of the CBT behavior must come from the array of 16 or 24 tweeters (70J or 1000, respectively), which have enough drivers to start to simulate a shaded arc. So perhaps we have something like a frequency dependent shaded and delayed CBT array network.

This made me wonder how the extension modules, and the woofers in the base unit work.

Could the woofers + extension module simply be a straight (no delay) and unshaded line array?

I thought about that a bit, but it seems like a simple straight array wouldn't work because a straight (non-delayed) array does not have constant vertical pattern control. The phase delay/curve is necessary for this. A straight array would instead have vertical beamwidth halved with each doubling of frequency. (The curve angle controls the beamwidth, while the shading and center to center driver spacing control lobing/fingering issues. The vertical dispersion is controlled by arc degrees--vertical dispersion is about 64% of the arc degrees so a 20 degree vertical pattern needs an arc of about 31 degrees.)

View attachment 121337

I'm not sure if it's even possible for the extension modules themselves can possibly be a CBT, because:
  • You can stack them in any order, and on either end (The CBT 1000E even has brackets available mount either on top or on bottom).
  • You don't have to flip the extension module depending on which end you stack it--so shading or phase can't go in one direction. The CBT1000E bracket manual shows the module being stacked on either end without having to flip it over to maintain a direction.
  • You can stack multiple of them, without having to keep track of which part of the 'arc' it belongs to.
  • When you add them, there's no switch or electronics activated to make the whole array reconfigure itself or change any settings. They are simply wired in parallel.
View attachment 121338

So however the extension modules work, it seems pretty clever, and almost certainly has to be a simplified line array that somehow can build on the main module's CBT pattern control to extend vertical directivity, but without having to extend the line of tweeters, cutting costs (the extension module is 1/2 the cost of the base module).

In addition, even though these are not ground plane CBTs, putting these CBT + extension modules on a reflective ground plane would still double the length of the woofer arrays, just like doubling by adding bass extension module(s).

So it seems like you can get another octave lower of pattern control if you put this on the ground plane!?!

For example, if I have a the CBT 1000 + 1000E system on the ground, I have a slim tower that's nearly 7' tall, putting both seated and standing positions on axis. The main module controls vertical directivity +/- 20 degrees (-6db) down to 600hz, the extension module...extends the vertical pattern control down to 300hz, and the ground plane reflection doubles that whole thing bringing pattern control down to 150hz! Apparently I can add bass modules willy nilly, so this must work, right? (Again, this is not a ground plane CBT where the center of the 'arc' is at the floor, but if adding any number of bass modules extends pattern control, then I'm also doing that with the reflection.)

View attachment 121342View attachment 121343

I just can't picture how the extension modules can seem to implement this behavior.

What phase delay and shading scheme could it be using, if any? Thoughts?

View attachment 121335View attachment 121337View attachment 121338View attachment 121342View attachment 121343
 

brombo

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I enjoy my cbt36k's and Dayton audio subwoofers (18 inch kits) with the home theater. I am currently using a minidsp crossover (coax spdif input) but would like to use a high quality (Asus Xonar) soundcard in my htpc as the dsp crossover. I am running Ubuntu 20.04, any software suggestions?
 

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AdamG

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I enjoy my cbt36k's and Dayton audio subwoofers (18 inch kits) with the home theater. I am currently using a minidsp crossover (coax spdif input) but would like to use a high quality (Asus Xonar) soundcard in my htpc as the dsp crossover. I am running Ubuntu 20.04, any software suggestions?
Welcome Aboard @brombo.
 
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Rick Sykora

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I enjoy my cbt36k's and Dayton audio subwoofers (18 inch kits) with the home theater. I am currently using a minidsp crossover (coax spdif input) but would like to use a high quality (Asus Xonar) soundcard in my htpc as the dsp crossover. I am running Ubuntu 20.04, any software suggestions?

Suggest you try your question in the DIY forum and take a look at the Open streamer project thread.
 

Bjorn

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I enjoy my cbt36k's and Dayton audio subwoofers (18 inch kits) with the home theater. I am currently using a minidsp crossover (coax spdif input) but would like to use a high quality (Asus Xonar) soundcard in my htpc as the dsp crossover. I am running Ubuntu 20.04, any software suggestions?
You could use Jriver Media Center which has the possibility of crossover, delay, shelving, PEQs and can also do home theater processing. JRiver can also use a plugin from someone that offers FIR filters or something more advanced.

jriver2.jpg
jriver.jpg


It's not the easiest to set up and even more challenging to measure the result, but the forum can help you.
 

somebodyelse

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I enjoy my cbt36k's and Dayton audio subwoofers (18 inch kits) with the home theater. I am currently using a minidsp crossover (coax spdif input) but would like to use a high quality (Asus Xonar) soundcard in my htpc as the dsp crossover. I am running Ubuntu 20.04, any software suggestions?
It depends how you're using Ubuntu. If you're using a GUI the easiest is probably Pulseaudio Crossover Rack. Also have a look at BruteFIR and CamillaDSP which will take more effort to configure, but better suited to headless use. Some other options are listed in the EQ software thread, although most are building blocks rather than an all in one solution.
 

brombo

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I looked at JRiver. The problem is that on linux (Ubuntu) you cannot have a JRiver virtual sound device (on windows you can) thus I could not use it with Kodi. I am looking at Pulseaudio Crossover Rack and on this website I also found Camilladsp which I had not know about. I wish to continue to use Kodi since on linux you can use Kodi to play commercial bluray disks if you have Makemkv installed.
 

laetoli37

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I bought my CBT24 right after reading the Sound & Vision review. After almost 4 years, I am still floored by how fantastic they sound to my amateur ears.

I am quite disappointed that these are no longer available from Parts Express!

Have anyone used the JBL CBT 100LA-1 in a home theatre setting? Greatly appreciate sharing.

Thanks
IMG_5479.JPG
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laetoli37

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By sheer luck I found an AV Pro with a couple of JBL CBT50LA-1 for sale from a recently decommissioned installation.
I bought 3 pairs from him at very reasonable price. Installed them as my side surrounds, overhead Atmos, and Center to complement my existing CBT24 in a 5.1.2 configuration.
It’s very hard to explain to someone who has not heard of the CBT sound.
It’s a bliss.

01 Surround + Atmos.jpeg
02 Center Behind Screen.jpeg
03 LCR.jpeg
 

gavincurtis

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Just got my CBT36 finished; years after receiving the kits as of this posting. These are ones I have upgraded to pro-audio output levels to fill a large area with sound. The shading resistor network's power handling capacity has been increased and liquid cooled for pro audio output levels.

I have 2 of the Cerwin Vega TS-42 21" folded horn subwoofer to use with these.

Now setting up the DSP aspects of the sound processor for real sound tests aside from simple tones etc next couple of weeks as my schedule allows. :)
 

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Newman

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units that can be stacked and wall-mounted
Big problem with that idea is that the "CBT effect" is destroyed by being close to walls, either side or rear. If you want to use these things in a home, they need to be well away from side and front-of-room walls.

Did you notice how DBK did his demos in large rooms or arenas, with the speakers way out from any walls..... for good reasons.
 

youngho

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Big problem with that idea is that the "CBT effect" is destroyed by being close to walls, either side or rear. If you want to use these things in a home, they need to be well away from side and front-of-room walls.

Did you notice how DBK did his demos in large rooms or arenas, with the speakers way out from any walls..... for good reasons.

Can you please explain why that is the case? Why do the Jbl CBT models seem to work mounted to Walls? My Epique CBT24 speakers seem to work quite well close to the wall behind them. Why is that the case?
 

youngho

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By sheer luck I found an AV Pro with a couple of JBL CBT50LA-1 for sale from a recently decommissioned installation.
I bought 3 pairs from him at very reasonable price. Installed them as my side surrounds, overhead Atmos, and Center to complement my existing CBT24 in a 5.1.2 configuration.
It’s very hard to explain to someone who has not heard of the CBT sound.
It’s a bliss.

View attachment 148345View attachment 148346View attachment 148347
I'm sorry, but @Newman says "the "CBT effect" is destroyed by being close to walls."
 

Newman

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Not sure how many members have CBT speakers, but I know a few have shown interest.
I had enormous interest in the early days of the tech. I even bought a full set of drivers in 2009 for a DIY design I had in mind. Yes, I was excited.

But something was bugging me. I eventually realized what it was (duh, slow). The constant beamwidth is only in the vertical plane. In the horizontal plane, the beamwidth behaves just like a small 2-way turned on its side. NOT GOOD. There are good reasons why small 2-ways are designed to be used in the vertical position.

Once I realized this, I went a bit cold on the idea. I also reviewed all Keele's talks and papers to see how I missed the obvious, and you know what? It isn't very clearly stated at all, that the 'width' in the name is vertical. I think DBK is really talking to more educated professionals than typical self-taught hobbyists like me, and it probably never occurred to him how easily I (we?) could mis-read the word 'width'.

Of course with a single full-range driver for the line, instead of a separate tweeter, the horizontal interference issue is avoided, but then the CBT has horizontal dispersion the same as the single driver, ie narrowing (beaming) in the treble. That is still an issue for me.

JBL seem to have handled this conundrum best with their CBT 70J speaker, having the tweeter and woofer lines coaxial. But the 70J appears to have a lot of engineering development behind it, and I would rather just buy it than try to go a similar path with a DIY effort.

cheers
 

j_j

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I'm sorry, but @Newman says "the "CBT effect" is destroyed by being close to walls."

Well, if it is, something has gone wrong. You may get a specular reflection from the wall, but the beam is still the same, you've just acquired more specular reflections.
 

Spocko

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Not sure how many members have CBT speakers, but I know a few have shown interest. Since there have been scattered ongoing discussions, am creating this thread to give it a home. If you are wondering what CBTs are, they are the work of former JBL and Audio Artistry engineer Don Keele. They offer some major benefits (more uniform sound distribution and better room integration) that are (IMO) way beyond traditional monopole speakers.

Here is an example pic and Keele’s website: http://www.xlrtechs.com/dbkeele.com/CBT.php

View attachment 54390
I'm totally into this but I'd rather not DIY and am just waiting for a company to make one that will fit inside typical 8 feet ceilings
 

j_j

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Not sure how many members have CBT speakers, but I know a few have shown interest. Since there have been scattered ongoing discussions, am creating this thread to give it a home. If you are wondering what CBTs are, they are the work of former JBL and Audio Artistry engineer Don Keele. They offer some major benefits (more uniform sound distribution and better room integration) that are (IMO) way beyond traditional monopole speakers.

Here is an example pic and Keele’s website: http://www.xlrtechs.com/dbkeele.com/CBT.php

View attachment 54390

Um, proper CBT design is not exactly novel for either microphone arrays or speakers (same math). Don did some good work, but there are a few other contributors like Jim Flanagan, Jim West, Dave Berkeley, just for starters.

But as usual, Bell Labs designs were not commercialized.
 

j_j

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Many users of controlled directivity loudspeakers find time-intensity trading to widen the sweet spot when set up so axes cross in front of the listening position and sometimes obviate the need for a discrete center channel speaker (compare with designs from Earl Geddes, Brian Waslo, Duke Lejeune, etc).

Perhaps you should just go back to 1933 and read Stienburg and Snow's paper? Or ask people who produce in stereo why the put emphasis on center-panned signals (voice or otherwise). The far-ear interference is well known and established, and happens regardless of the directional pattern of the loudspeaker. Some level shading can help, to some extent, yes, but you can't eliminate either the issue of conflicting HRTF's or conflicting ITD's vs ILD's.

I'm not sure why you're being so hostile here to other posters, they are, after all, referring to documented acoustic issues.

Of course, improper mixing for 3 front channels can create a problem, as well, but that problem is in production, not in 3-channel playback.
 
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Rick Sykora

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Not an "assumption" proven in 1933 and remains true to this day. It should be the same as L and R. It is THE "main" speaker in a 3-front setting, period.

Suppose just depends on how well matched an OMD-5 is to a CBT24?

If it does not match, then by definition, can not be considered proven (at least based on my experience).

Edit: To be clear, not denying that matching is good practice. IME, has not worked well and wanted to help those who may have had comparable results. As a CBT center is a challenge, just sharing what worked well for me.
 
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