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Constant Beamwidth Transducer (CBT) Speakers

Rick Sykora

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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. Edit: As noted later in the thread, many others contributed to CBT development too. Don has been their biggest promoter in consumer applications however. 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

A8D6AD9D-3034-4BF0-AC39-8B2B04582234.jpeg
 
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Absolute

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Heavily considering this type of speaker even though my gf tells me it doesn't matter what speakers I buy as long as it's not those bananas... There's one one theoretical question remaining in my head that I just can't seem to shake, and that's the issue of impulse response.

CBT will have a series of impulses with gradually decreasing spl since the distance from each driver will be different. If we are sensitive to impulses, this will smear the sound, and you will of course have the same effect with regards to first reflections.
If we're not sensitive to impulses, then time alignment and acoustical absorption of early reflections will make no difference whatsoever because that's all impulses in the same way.

People and science doesn't really have a clear answer for me, so I'm bothered to the bone.
 
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Rick Sykora

Rick Sykora

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Heavily considering this type of speaker even though my gf tells me it doesn't matter what speakers I buy as long as it's not those bananas... There's one one theoretical question remaining in my head that I just can't seem to shake, and that's the issue of impulse response.

CBT will have a series of impulses with gradually decreasing spl since the distance from each driver will be different. If we are sensitive to impulses, this will smear the sound, and you will of course have the same effect with regards to first reflections.
If we're not sensitive to impulses, then time alignment and acoustical absorption of early reflections will make no difference whatsoever because that's all impulses in the same way.

People and science doesn't really have a clear answer for me, so I'm bothered to the bone.

Cannot answer your impulse response question either, but can tell you that the ones I have (CBT24s), articulate and image well. Not only can I localize individual instruments, I can do so well outside the sweet spot. Have heard some expensive traditional speakers, but all of them had a limited sweet spot.

My CBTs are smaller than the open baffle speakers I was considering, so my wife is happy. Currently, they do not seem to scale well, but expect it will get better with time. I would like smaller ones as surrounds.

There really is no substitute for a listening experience IMO. By initiating this thread, am hoping to find other owners that might be willing to share the CBT listening experience with others. :)
 

Bjorn

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Heavily considering this type of speaker even though my gf tells me it doesn't matter what speakers I buy as long as it's not those bananas... There's one one theoretical question remaining in my head that I just can't seem to shake, and that's the issue of impulse response.

CBT will have a series of impulses with gradually decreasing spl since the distance from each driver will be different. If we are sensitive to impulses, this will smear the sound, and you will of course have the same effect with regards to first reflections.
If we're not sensitive to impulses, then time alignment and acoustical absorption of early reflections will make no difference whatsoever because that's all impulses in the same way.

People and science doesn't really have a clear answer for me, so I'm bothered to the bone.
Quite the contrary, this is actually what CBT resolves vs a traditional line array.
When the magnitude response is equalized flat with a minimum-phase filter, the resultant phase is substantially linear phase over a broad frequency range at all these diverse locations. This means that the CBT array is essentially time aligned and linear phase and as a result will accurately reproduce square waves anywhere within its coverage
https://www.keele-omholt-technologi...t.-2015-Time-Phase-Behavior-of-CBT-Arrays.pdf
 
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Rick Sykora

Rick Sykora

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The idea seems pretty cool and all...

But am I the only one cringing at the acronym? :eek:

Apparently not me ;), but the term came from the navy research folks and guessing they were not concerned! This “unfortunate” abbreviation stuff happens all the time so context is key...

Years ago, I had a comparable experience when my marketing department insisted some new automation software be named Connected Components Workbench. I pointed out it was too long and would get shortened to CCW. Despite being an established term in another context, they stuck with it anyway. Turned out, few people really picked up on it. Suspect the situation is comparable for CBT too as JBL stuck with it as well :).
 

maverickronin

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Quite the contrary, this is actually what CBT resolves vs a traditional line array.
https://www.keele-omholt-technologi...t.-2015-Time-Phase-Behavior-of-CBT-Arrays.pdf

Skimming the paper it looks like all those pretty wavefront graphs were from a simulation of a 180 degree arc of 112 point sources.

What kind of results do you get from a 30 degree-ish arc (I'm just taking a wild guess from the weird perspective on those pics...) with a reasonable number of real drivers?

Still looks better than a standard line array though.
 

Bjorn

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Skimming the paper it looks like all those pretty wavefront graphs were from a simulation of a 180 degree arc of 112 point sources.

What kind of results do you get from a 30 degree-ish arc (I'm just taking a wild guess from the weird perspective on those pics...) with a reasonable number of real drivers?

Still looks better than a standard line array though.
Your assumption is wrong. Please read the paper.
 

Absolute

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Quite the contrary, this is actually what CBT resolves vs a traditional line array.

https://www.keele-omholt-technologi...t.-2015-Time-Phase-Behavior-of-CBT-Arrays.pdf
Keele's own conclusion is what worries my feeble little mind;

"This paper explored the time and phase response of circular-arc CBT arrays through simulation and measurement. The paper showed that although the impulse response of the CBT array is spread out in time, it’s phase response was found to be minimum phase at most locations within the array’s coverage area."

While I don't particularly worry about phase, I worry about the impulse response. I know we like smooth frequency and phase response, so that's covered in a CBT, but the same sound being transmitted a number of times spread out in time seems iffy when I bother my stupid brain with it.
Non-issue? Perhaps @j_j have some opinion on the matter?
 

amirm

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It took me a while do even figure out what you were referencing. At first, I was like "What is wrong with cognitive behavioral therapy?"
I have a reverse problem always thinking of CBT speakers when my doctor talks about CBT! :)
 

j_j

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Your ear responds sustantially in a minimum-phase fashion, so the first attack will have more effect than stuff in the next 10 milliseconds.

Having said that, you WILL overlay the original acoustics with the room acoustics when you have an omni speaker.

A speaker that has a more directed response (AT ALL FREQUENCIES ABOVE BASS) that is roughly even in directivity over frequency can avoid this overlay, at a risk of having problems in the bass if not corrected, and also having a problem in listener position. Late responses (over 25 milliseconds or so, less than 1 second or so) may be masked by most, but not all sources.
 

Absolute

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Your ear responds sustantially in a minimum-phase fashion, so the first attack will have more effect than stuff in the next 10 milliseconds.

Having said that, you WILL overlay the original acoustics with the room acoustics when you have an omni speaker.

A speaker that has a more directed response (AT ALL FREQUENCIES ABOVE BASS) that is roughly even in directivity over frequency can avoid this overlay, at a risk of having problems in the bass if not corrected, and also having a problem in listener position. Late responses (over 25 milliseconds or so, less than 1 second or so) may be masked by most, but not all sources.
So if I translate this into stupid-language so I can understand it, you're basically saying that the next 5-10 ms of identical but softer impulses will not affect us much?
Does this mean that you wouldn't worry about the impulse response of a cbt or line array?
 

Bjorn

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Keele's own conclusion is what worries my feeble little mind;

"This paper explored the time and phase response of circular-arc CBT arrays through simulation and measurement. The paper showed that although the impulse response of the CBT array is spread out in time, it’s phase response was found to be minimum phase at most locations within the array’s coverage area."

While I don't particularly worry about phase, I worry about the impulse response. I know we like smooth frequency and phase response, so that's covered in a CBT, but the same sound being transmitted a number of times spread out in time seems iffy when I bother my stupid brain with it.
Non-issue? Perhaps @j_j have some opinion on the matter?
It basically acts like an IIR filter. I wouldn't worry about it. It's minimum phase behaviour, will reproduce square waves at most locations and has contant directivity. Combine that with very low distortion in a good 2-way design, avoidance of floor reflections and minimization of ceiling reflections greatly it's a very unique speaker. IMO it's much closer to the illusion of live music compared to traditional speakers.
 

j_j

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It basically acts like an IIR filter. I wouldn't worry about it. It's minimum phase behaviour, will reproduce square waves at most locations and has contant directivity. Combine that with very low distortion in a good 2-way design, avoidance of floor reflections and minimization of ceiling reflections greatly it's a very unique speaker. IMO it's much closer to the illusion of live music compared to traditional speakers.

For many rooms, good pattern control with frequency that pushes a zero at the floor, ceiling, and walls is also good. But to some extent this depends on listener preference.
 
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Rick Sykora

Rick Sykora

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From a review Revel thread that was going off road, thought I would cross post this as it is a great learning opp to burn some time while hunkering down...

Don Keele's CBT Chronicles:

 
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