“Let our rigorous testing and reviews be your guidelines to A/V equipment – not marketing slogans”
Facebook Youtube Twitter Google Plus instagram pinterest

Epique CBT24 Loudspeaker Designer Note by Don Keele

by D. B. (Don) Keele, Jr. April 27, 2018

I want to thank James Larson for sharing a preview of his CBT24 review with me. I am speaking here as the designer of the CBT24 and not as a spokesman for Dayton Audio/Parts Express, the seller of the system. They have not seen my response in this designer note.

I didn’t change one word of his review but did make suggested revisions in two areas: one concerning the coverage of the CBT24 with added figures and the other concerning the inherent power roll-off of circular-arc loudspeaker arrays and the design of a special driver for the CBT24 that partially compensates for this roll-off.

I truly realize the massive amount of effort that has to go into a loudspeaker review like this having been the chief loudspeaker reviewer for the now-defunct Audio Magazine in the 90’s. He is to be commended for his efforts. I much appreciate the opportunity to chime in here in a designer’s note to explain the origins of the CBT24 and to share some additional measurements.

The CBT24 came a about in an attempt to greatly simplify the complexity of the Parts Express two-way CBT36K kit system  which is a very challenging kit to put together! The CBT36 has been marketed since early 2012. This array utilizes 72 closely-spaced very-small dome tweeters in addition to its 18 each 3.5” mid -bass drivers. I thought it would make sense to create a less-expensive much-simplified one-way system using only wide-range direct-radiator drivers without tweeters.

However, when I first considered this design I was very apprehensive that eliminating the tweeters would actually work! But it did!

I’ve been preaching for years that the drivers in a properly-designed CBT array need to be spaced at no more than about one-half to one wavelength apart at the highest frequencies to minimize chaotic and rough high-frequency response.

Yes, the CBT24 exhibits chaotic and rough response above 8 kHz, but from a subjective standpoint it doesn’t seem to matter much! The magic of a CBT array is its very broad and even coverage at all locations in front of the array even if the response is erratic at high frequencies. It’s essentially uniformly erratic everywhere and as a result still comes across as sounding and imaging extremely well.

Listening to the CBT24, I was surprised that subjectively I was hardly able to hear any chaotic and rough high-frequency problems even with pink noise at many different locations in front of the system. This design theoretically shouldn’t work very well, but I was pleasantly surprised!

The roughness at any one particular location averages out at the many other locations making the system sound quite good overall, believe it or not. Just judge by Larson’s very positive comments in the listening section of the review, and listen for yourself if you have a chance.

The CBT loudspeaker technology is very well documented in a series of 17 technical papers I’ve authored or coauthored starting in 2000. All these papers are available from the Audio Engineering Society (AES) E-library  which is free for AES members. I’d also like to suggest another easily accessible source for CBT information and technology in a nine-part YouTube video my son and I created called the CBT Chronicles.

Side comment: You’re welcome to skip over the rather-pedantic first parts of the video (Parts 1 - 5)with that crazy Don Keele lecturing about CBT technology and skip directly to the real-time measurement sections (Parts 6 – 9) which compares the coverage with a conventional well-designed three-way system against the CBT36. Highly worthwhile to take the time to view this!

One area that I want to amplify in this designer note is the vastly superior vertical coverage of a CBT ground-plane array as compared to conventional loudspeaker systems. James Larson was only able to include a small part of his measurements on the CBT24 that he accomplished due to space considerations.

Here I have added a series of measurements of a pre-production CBT24 that I accomplished that illustrates the extremely-well behaved vertical coverage of a CBT array. These measurements were not measured anechoically, like the excellent outdoor measurements that James Larson accomplished, but were run in my church’s large-metal-building family center using windowed-measurement techniques.

The following response measurements illustrate the excellent vertical magnitude/phase coverage of the CBT24 not in the conventional manner as a series of responses versus angle but in a series of responses versus height at a location very close to the front of the array. A CBT array has very uniform coverage and response at all points in front of the array including near-far, up-down, and right-left. A CBT ground-plane array easily passes the typical stand-up sit-down test with flying colors but also sounds great for people lying on the floor as well.

These tests are straightforward, unmodified, and honestly illustrate even the warts of the CBT24. The tests were run with the system placed on an acoustically-reflective tiled floor and measured at a series of six vertical heights in the range of 0.25 m up to a height of 1.5 m even with the top of the CBT24.

The response graphs illustrate the very uniform magnitude and phase responses of the system up to about 5 kHz at all these heights. At higher frequencies, the response becomes increasingly erratic, but as pointed out above is “uniformly” erratic at all locations in front of the array.

The magic of a CBT array is its very broad and even coverage at all locations in front of the array. The roughness at any one particular location averages out at the many locations making the system sound quite good overall.

The measurements are illustrated in the following six figures. Please refer to the figure captions for detailed descriptions. All the measurements were taken with the Dayton Audio measurement system with a “Blended” gate width of 5 ms and 1/12th-octave smoothing.

 Fig . 1 Larson-Keele CBT24 Review.JPG

Fig. 1. Measurement location: The CBT24 array was measured in my church’s metal family center building. The room is quite large and as you can see the tables were still setup! The tiled floor provides an excellent ground-plane acoustically-reflective surface for the array to sit on.

 Fig . 2 Larson-Keele CBT24 Review.jpg

Fig. 2. Measurement mic locations: This figure illustrates the six test-mic locations that were used in the following measurements. Measurements were made at six vertical height in the range of 0.25 m (10” above the floor) up to 1.5 m (60” even with the top of the CBT24) in steps of 0.25 m (10”). Note that the test microphones were located very-close to the front of the array and are essentially even with the front bottom of the array. Very few conventional speaker systems measure well under these conditions.

 Fig. 3 Larson-Keele CBT24 Review.jpg

Fig. 3. Magnitude frequency response vs. height, un-normalized with miniDSP EQ: This figure illustrates the responses at each of the six test mic locations. Levels have been preserved for a 2.83 Vrms input. The level closest to the floor is highest at about 104 dB SPL, while the level drops to about 85 dB SPL at a point even with the top of the array. The responses are quite even up to about 6 kHz but get fairly erratic at higher frequencies.

 Fig. 4 Larson-Keele CBT24 Review.jpg

Fig. 4. Magnitude frequency response vs. height, normalized at 1 kHz with miniDSP EQ: This figure shoes the exact same curves of Fig.3 but normalized in level at 1 kHz. This graph clearly shows that the response curves are very similar to each other up to about 6 kHz and deviate at higher frequencies.

 Fig. 5 Larson-Keele CBT24 Review.jpg

Fig. 5. Phase frequency response vs. height with miniDSP EQ: This graph illustrates the exceptional phase linearity of the CBT system at all six height locations up to 5 kHz. The rapid changes in phase at higher frequencies are due to “phase wrapping” between ±180° of the data. This shows that the system has extremely well-behaved minimum-phase behavior (just about linear phase) up to about 5 kHz at all these heights. The CBT24 will actually reproduce decent 1 kHz square waves at all these heights! See the following figure.

 Fig. 6 Larson-Keele CBT24 Review.jpg

Fig. 6. Square wave response vs. height: These four OmniMic oscilloscope images illustrate the CBT24’s reproduction of a 1 kHz square wave. The test conditions for this test were slightly different than the previous three figures. The square wave responses were measured at a distance of 0.5 m from the base of the array at heights of 0.25 m, 0.5 m, 0.75 m, and 1 m. This is a rare test result for any speaker system! Usually, if a typical speaker system can reproduce square waves, it is at a magic on-axis spot where the system’s phase response is equalized to be linear. Unlike any other speaker system, the CBT24 can reproduce square waves at most points in front of the array in its coverage region.

Note: If a loudspeaker system can successfully reproduce accurate square waves, this shows that the system is essentially linear phase, time aligned, and will be able to faithfully reproduce the waveshapes of music and program material played by the loudspeaker.

Confused about what AV Gear to buy or how to set it up? Join our Exclusive Audioholics E-Book Membership Program!

 

Recent Forum Posts:

67-79 posts on March 29, 2019 11:04
Hi, im still on the market in audio. I spend 50$ per month in audio approx. I discovered this line array presentation for the first time today. Thanks, i will surrely experiment something in the coming weeks or maybe never but i will surrelly try to listen them soon.
honyewl posts on April 29, 2018 05:34
Hi James,

Thanks for taking the time to explain your idea of using 2 CBT24 horizontally for center channel. Fascinating!

Hon Yew
shadyJ posts on April 28, 2018 19:39
honyewl, post: 1245408, member: 85573
Hi James,

I bought a pair of CBT24 in September last year. I have a 5.1 home theater setup - for both stereo/surround music and movie with a fixed projector screen. The CBT24 replaced my previous conventional 2 tower speakers. The Pros list in your review captured perfectly how giddily ecstatic I have been with them: “Excellent.., Extraordinary.., Superb.., Remarkably smooth..”!

However, as you rightly pointed out, there's no center. And trying to match them with “…preferably one that can actually keep up with them” has been a frustrating experience. My existing center (which I used to like a lot) now just sounds, well, underwhelming. I have also tried a center from a respected brand with concentric design, but again, it just couldn't keep up.

So I did something that I never thought I would do - I removed the center and settled on a 4.1 configuration! Except for listening positions way to the side of the movie screen (92“ diagonal), the phantom center works remarkably well.

I do want to explore having a CBT24 as a center. But not sure how it will work with overhead projector. If I used a drop down acoustic transparent screen, only part of the speakers is behind the screen - not sure if this will screw up the CBT characteristics. Perhaps a big enough screen that will ”cover" all three CBT24 - and the screen will have to reach the floor? Anyway, appreciate if you or Don have any advise and comments on this.

What speaker configuration did you use for testing the movies? Do you add a CBT24 central?

Thanks again for the nice and informative review.
Glad to see you are enjoying your CBT24s as I did with mine!

Per your questions, as you say, the way in which you outline the possible use of a CBT24 as a center does negate a couple of their advantages, but I think it would probably still work OK and sound fine. If you used a screen that reached to the floor, that would work to solve that problem, but that might make for a sub-optimal viewing angle. Depending on your viewing position, that still might be doable though.

One interesting idea might be to take two CBT24 speakers, omit the base, and attach them at the bottom for a larger arc, and then use that horizontally. This way, the shading between the drivers will be consistent for that kind of use. Here is a pic of what I am talking about:


Of course, if you elevate that off the ground, you do get floor bounce, but that isn't the end of the world for the sound. And that would be one hell of a center, it would easily keep up with your left and right fronts. You would want an outboard two-channel amplifier for a center like that, and use a splitter on your center channel pre-out. Since you can not buy these things individually, you would have a spare CBT to do this with anyway. The caveat with a center like that would be it would have wide vertical dispersion, and not so wide horizontal dispersion. This might not be a bid deal because the response of the vertical dispersion of a CBT24 is so uniform, and the pasting it into a center like that makes it so wide that even though it won't shoot out sound at a wide angle, it will still cover a large area. I will see if I can rope Don in here to comment on this potentially dumb idea.

By the way, for my own surround sound listening, I just used my Infinity Primus p362 center. That isn't a big fancy center, but it is a real over-achiever as far as center speakers go. I have yet to see a center speaker that measures better.
honyewl posts on April 28, 2018 11:06
Hi James,

I bought a pair of CBT24 in September last year. I have a 5.1 home theater setup - for both stereo/surround music and movie with a fixed projector screen. The CBT24 replaced my previous conventional 2 tower speakers. The Pros list in your review captured perfectly how giddily ecstatic I have been with them: “Excellent.., Extraordinary.., Superb.., Remarkably smooth..”!

However, as you rightly pointed out, there's no center. And trying to match them with “…preferably one that can actually keep up with them” has been a frustrating experience. My existing center (which I used to like a lot) now just sounds, well, underwhelming. I have also tried a center from a respected brand with concentric design, but again, it just couldn't keep up.

So I did something that I never thought I would do - I removed the center and settled on a 4.1 configuration! Except for listening positions way to the side of the movie screen (92“ diagonal), the phantom center works remarkably well.

I do want to explore having a CBT24 as a center. But not sure how it will work with overhead projector. If I used a drop down acoustic transparent screen, only part of the speakers is behind the screen - not sure if this will screw up the CBT characteristics. Perhaps a big enough screen that will ”cover" all three CBT24 - and the screen will have to reach the floor? Anyway, appreciate if you or Don have any advise and comments on this.

What speaker configuration did you use for testing the movies? Do you add a CBT24 central?

Thanks again for the nice and informative review.
gene posts on April 27, 2018 00:14
Don Keele's Designer Note and Review Feedback
Post Reply