One aspect of spe aker design that continues to propel my interest in speaker science is the many different ways there are to produce great sound. Conventional woofers, dome tweeters, folded ribbons, electrostatic membranes, tapped horns, plasma arcs, among many other examples; there are lots of different ways to precisely create pressure waves in the air that we hear as sound, and each has their own advantages. Because of this, the Constant Beamwidth Transducer (CBT) designs by the esteemed audio engineer Don Keele caught my attention. It is a peculiar new design type, a curved line-array, that is backed by a wealth of published data regarding its advantages by Mr. Keele. Eccentric speaker designs are announced frequently in the audio world. We've seen speakers that look more like sousaphones or dwarf elephants at some of the trade shows we've attended, but seldom do they come with the volley of hard data or the engineering chops of a legendary engineer like Mr. Keele. When Dayton Audio announced a new affordable speaker based on the CBT design, I jumped at the chance to get a pair of review units in house to hear what all the buzz was about for myself.

Unpacking and Appearance

The CBT24 speakers can only be purchased from Parts-Express.com and so must be shipped to the buyer’s home. They come well-packed, which was particularly fortunate in my case since the exterior box showed signs of rough treatment during shipping. They come double-boxed and amply cushioned by thick foam packing pieces. The speakers are covered with thick cotton sleeves which should be enough to protect them from moisture and also scuffing from the foam.

The Epique CBT24 speakers do not quite look like any other speaker. Their design is considered a curved line-array: a stack of 24 2.5” drivers fitted onto a narrow cabinet that arcs backward into a curve. Some people will think they look interesting, while they may be a bit too odd in appearance for others. I’m in the first camp; I think they look cool. They do not have a grille, so those who are turned off by the sight of exposed cones will not go for this look. The clean, back-leaning curve of the cabinets and rounded base gives them a futuristic vibe; organic, rounded shapes combined with the right-angle geometry of its edges. With their wide base and five-foot height, the CBT24 speakers are not going to disappear in a normal room.

The finished pair is only available in a smooth, almost satin black finish. It is slicker than matte black but it is not a shiny or deluxe finish. In a darkened room, this finish will allow the speakers to vanish. Indeed, I had trouble taking pictures without using the camera flash. There is also a kit form of the CBT24s available that does not come with a finish. Those who buy the kit form of the CBT24s have bare MDF to deal with and can apply any kind of finish they want- I think silver or white would look very nice for this design. There is a front-facing label on the base that spells EPIQUE in a stylized courier font. The bottom line regarding the CBT24 speaker’s appearance is that personal taste will play a heavy role in the decision to get a pair, since they look so unique. They will inevitably be a conversation piece as much as a loudspeaker.

Design Overview

It only takes one glance at the Dayton Audio Epique CBT24 speakers to see that they are a radical departure from conventional speaker design. As we mentioned before, they are a curved line-array using 24 identical 2.5” full-range drivers mounted in a circular arc. The array is mounted in a sealed enclosure which rests on top of a wide platform that gives it balance and stability. The CBT24 is a one-way system; there is no crossover to a woofer or tweeter. Each driver is given the entire frequency spectrum; however, not all drivers are playing at the same level. The array is divided into three banks of drivers with each bank playing a different level of amplitude. The difference in amplitude for each bank of drivers is important to the design, and we will discuss this aspect more later. The CBT24 is a curved ground-plane CBT array.

An example of conventional line array speakers

Although CBT speakers are considered line array speakers, they are not traditional line arrays at all; there are some major differences. Before we talk about the differences of CBT designs with respect to conventional line arrays, let’s briefly talk about normal line arrays. Conventional line arrays are completely vertical stacks of identical drivers that all play at the same amplitude level. This design has its advantages. A typical line array will project sound with a narrow vertical angle but a wide horizontal angle, which means that the sound field will encompass a wide listening area but not be affected by acoustic reflections from the ceiling and floor as much. Another advantage is that the apparent loudness does not change as much with distance with respect to most other speaker designs; the sound pressure levels fall off at half the rate per doubling of distance (3dB/DD) versus a normal speaker (6dB/DD), so as you move closer or farther from a line array speaker, it does not change in loudness as much.

One more advantage is that line arrays typically have enormous dynamic range because of all the drivers used in its construction. A speaker that has 40 tweeters and 20 midrange drivers will naturally be far more capable than a speaker with 1 tweeter and 2 midranges. Furthermore, unlike most high dynamic range speakers, line arrays can have a very wide dispersion, since the driver diameters are usually small and their output is not narrowly directed by a waveguide. Most high-dynamic-range speakers use horns or some kind of waveguide to constrict the acoustic power of the speaker to a relatively narrow area of sound. A line-array can have a similarly huge dynamic range but outputs its acoustic power over a far wider horizontal angle, so they have all the power but without the relatively small sweet spot that horn-loaded speakers can often have.

Curved CBT speakers, and by extension the Dayton Audio CBT24s, retain some of the advantages of traditional line array speakers but also have some very important differences. The most obvious difference is the curved shape of the cabinet. The curve is a constant circular arc that forms a part of a circle around a distant axis in the rear of the speaker. In taking this shape, the CBT24 blends aspects of line arrays speakers and conventional point source speakers. As opposed to conventional speakers, line array speakers emit sound as though it were coming from a two-dimensional line rather than a one-dimensional point. The jargon for these different shapes of sound emission sources is ‘line source’ and ‘point source.’ Line source emission have many of the traits described in the above paragraph: narrow vertical sound dispersion, wide horizontal dispersion, and a lower loss of sound intensity for increases in distance. The CBT24 speakers use a line array form but curved around an imaginary point on the ground plane in the distant rear of the speaker, so it has traits of both line source speakers and point source speakers. In a certain sense, it has the advantages of both without the disadvantages of either.

CBT speakers have the wide horizontal dispersion, narrow vertical dispersion, and enormous power handling of conventional line arrays. One feature of CBT speaker design is that, the higher you listen to the speaker up to the cabinet’s highest point, the less the SPL will change from distance. At standing height let's say about 5 feet, you can walk back and forth from the CBT24 speakers to a distance of a few meters and experience almost no change in loudness. This is as opposed to the -3dB change in amplitude per doubling of distance of line arrays and -6 dB per doubling of distance in conventional home audio speakers. This makes it a great choice for uniform loudness coverage out to roughly 10 feet, after which it does fall off at a -6 dB per doubling of distance. The lower height that you listen with respect to the height of the speaker, the more the fall off of loudness per distance gets to a -3 dB per doubling of distance.

Loudness variation with distance: how a CBT array’s loudness decays with distance from a height at top of the array. Images courtesy of Don Keele.

The uniform loudness with distance is one reason why the prominent acoustician, Dr. Floyd Toole, has advocated the CBT design for surround speakers. In many surround sound systems, most listening positions will be closer to one side of the surround speakers than the others. This means that surround sounds will be much louder for them on one side than the other, thereby weighing the surround soundstage to one direction and degrading the intended effect. But a speaker that does not change loudness with distance, as CBT designs can do (up to a point), will not inordinately weigh the surround sound to one direction or the other, even if the listener is much closer to one of CBT speakers than the other. Everyone gets to experience the surround sound as though they were in the center seat.

One other advantage of this quality of an unchanging amplitude and unchanging frequency response over distance is that CBT speakers are as much a near-field speaker as they are a far-field speaker. Most multi-way speakers need distance for the sound of each driver to integrate. A CBT speaker, on the other hand, will sound the same up close or at a distance. There will not be any tonal change of character no matter how far you are from them. Theoretically, they could be used as near-field monitors.

Another difference that these CBT speakers have from traditional line arrays and also conventional home audio speakers is that, since the sound is being produced as though it is emanating from a ground level point source recessed far behind the speakers, ‘ground bounce’ is not a problem at all with these speakers. For those who do not know, ‘ground bounce’ is when the direct sound of the speakers interferes with the sound from the speaker that is reflected off the floor. This interference can cause dips and peaks in the frequency response in bass frequencies at the listening position, typically around 100 Hz to 300 Hz, when the reflected sound waves cancels out or sums up with the direct sound from the speaker. Since the sound of the CBT24s emulates a point on the ground itself, its sound does project downward to then bounce up and intersect with direct sound. This means they can have a more balanced bass sound than other speaker types.

The CBT design’s most notable feature is in the acronym for which it is named: Constant Beamwidth Transducer, hence CBT. Let’s explain what ‘Constant Beamwidth Transducer’ means, and we will start with the phrase ‘Beamwidth’. In audio, beamwidth is the point that occurs off a speaker’s direct axis where the amplitude is halved (i.e. it has fallen by 6 dB). Most speaker designers try to get this to occur at the same distance for all frequencies. In other words, they don’t want the speaker’s off-axis sound to change at different frequencies. The more uniform a speaker behaves at all angles and distances, the more balanced and neutral it will sound. So a goal for high-fidelity speaker designers is to make a loudspeaker’s beamwidth remain invariant with frequency, i.e. constant. For those not steeped in audio, the last word in the CBT acronym, transducer, is just another name for speaker, in this case.

A 3D illustration of a CBT speaker’s dispersion pattern. CBT speakers hold this pattern at a very wide range of frequencies. Image courtesy of Done Keele.

The CBT design accomplishes constant beamwidth by arranging drivers on a circular arc surface and then attenuating some of the driver’s output by different levels according to a specific mathematical expression called a Legendre function. It was based off of naval research that looked into how underwater transducers could produce acoustic signals where the angle of the projected sound did not change with frequency. Audio scientist, Don Keele, tried to use this research to invent a new type of horn geometry for loudspeakers in the early 1980s but was not able to apply it to horns. He returned to it again in the late 1990s with the idea of applying it to line arrays where he found it could be successfully translated into a loudspeaker that had a highly uniform dispersion patterns, just like the original research suggested would work for an underwater spherical array. Mr. Keele continued to evolve the CBT design and has published many papers on it, some of which are posted on his website. For those who really want to hear about the inception and story about the design from the maestro in his own words, Mr. Keele has a 90 minute lecture on YouTube about the story of CBT design that he has called the CBT Chronicles.

Dayton Audio ND64-16 driver. Image courtesy of Dayton Audio.

The above discussion of CBT speakers will all apply to the particular speakers under this review, the Dayton Audio CBT24 Epique speakers. CBT speakers can be made from multi-way systems, but this incarnation only uses one driver model, the Dayton Audio ND64-16, a 2.5” full range driver.

Don Keele mentions an interesting fact about the CBT24 and ND64-16 in personal correspondence.

All curved and circular-arc line arrays (not straight line arrays) exhibit a high-frequency power roll-off due to the varying spacing between the drivers and the listener causing progressively increasing attenuation as frequency increases. The power roll off is 3 dB/octave or 10 dB/decade above a certain frequency depending on the size and configuration of the array. In the specific case of the CBT24, this roll-off commences above approximately 300 Hz. If the array is implemented with a driver that has a flat half-space frequency response the array will exhibit this inherent roll-off. In order to make the array’s frequency response flat, this roll-off must be compensated with high-frequency lift EQ or by other means. For the CBT24, a special version of Dayton’s ND65 driver (that has a flat half-space response) called the ND64-16 was designed which has a rising +3 dB/octave frequency response from roughly 300 Hz to 6 kHz. Use of this special driver compensates the CBT24’s falling response and makes the system roughly flat from 100 Hz to 8 kHz without EQ.

Dayton Audio recommends that the CBT24 speakers be used with an equalizer to flatten the response above 8 kHz. Their recommended equalizer is the MiniDSP 2x4 HD, and Dayton Audio has provided equalization curves for the MiniDSP 2x4 HD which help to flatten out the response. These files are available on the CBT24 product page.

MiniDSP EQ compensation file for flattened response on the CBT24.

Listening Tests

In my approximately 24’ by 13’ listening room, I set up the speakers with stand-off distances between the back wall and side wall, and equal distance between speakers and listening position, with speakers toed-in toward the listening position. Listening distance from the speakers was about 10 feet. Amplification and processing was handled by a Pioneer Elite SC-55 and a MiniDSP with a compensation curve loaded for the CBT24. A Hsu Research VTF-15h mk2 subwoofer with an 80 Hz crossover frequency was used to supplement the bass.

Music Listening

One recording that I listened to on the CBT 24 speakers was an outstanding choral album of compositions from the period of 1500 to 1558, or the years of the lifetime of Charles V. The album is titled “Music for Charles V, Holy Roman Emperor” and is performed by the London-based Chapelle du Roi, a choir specializing in the performance of sacred renaissance music. Charles V was a great patron of music, and much of the music from his time has managed to survive the centuries to now. Listening to “Music for Charles V” on the CBT24s proved to be a captivating experience. The music is lovely as is the performance, and the production captured these performances with utmost clarity. The soundstage as relayed by the CBT24s was expansive and wide but did not sacrifice a strong center image. Singers of each vocal range had a well-defined position within the soundstage, so the placement of the performers, whether tenor, baritone, or soprano, occupied a distinct point in space. The imaging was so vivid that it seemed as if I was sitting in a front row seat at St. Jude’s Church in Hampstead where this performance was recorded. Voices were rendered naturally as was the acoustic space of the church. All the singers were crystal clear, no matter how many performers sang simultaneously. There was no muddiness or confusion even in dense passages. I am not sure how much better a loudspeaker could reproduce this album, but I have no complaints about the CBT24’s performance on “Music for Charles V.”

Another album I listened to was Enya’s 2005 recording “Amarantine.” I chose this for its emphasis on a single voice rather the plethora of voices in a choir, and also a single voice that I was familiar with. The sound of “Amarantine” is very typical of Enya: lush instrumentation, sonorous vocals, and a mix of energetic, flowing tracks and gentle ballads, all drenched with a Celtic, new-age flavor. As with other Enya albums, production is top-notch. To be sure, Enya’s recordings often have a great deal of studio trickery involved in creating their signature sound, but what a sound it is: sweeping, dramatic, nearly cinematic. The large soundstage presented by the CBT24s was a perfect marriage with the large sound of “Amarantine.” In fact, I had to check if the surround speakers were on at one point in an atmospheric track on this album. As with “Music for Charles V,” the soundstage was wide, almost larger than life, and not with any lost definition in the imaging. Enya’s voice had precise center imaging when called for but also a tremendous panoramic sweep in choral crescendos. Instrumentation had their moments as well, with pianos, violins, and a polyphony of strings given their chances to shine on the CBT24s amidst Enya’s vocals. For this album, the CBT24s hit all its marks and provided a balanced presentation where I heard nothing that I felt was out of place or otherwise lacking.

For something on the orchestral side, I picked the album “Orchestral Suites,” which is a recording of three orchestral suites by Bach from the Tafelmusik Baroque Orchestra. Tafelmusik Baroque Orchestra, or more simply Tafelmusik, is Canada’s orchestra on period instruments, and all members are specialists in historical performance practice and perform on original instruments or modern replicas faithful in design and construction to the originals. In “Orchestral Suites,” Johann Sebastian Bach’s intricate and dynamic suites are given a more authentic treatment to their original performance than they are normally played with. This album, made in 2002 at the Grace Church On-The-Hill in Toronto, is an exquisite and forceful recording, and it is a great candidate for demonstrating a loudspeaker’s ability to recreate a well-recorded musical performance. The CBT24s did a marvelous job with “Orchestral Suites.” Imaging was outstanding, and each instrumental section had a clear location within the soundstage. The various instruments sounded vivid and lifelike. The CBT24s gave the soundstage a three-dimensional quality. One aspect about high-fidelity loudspeakers that I always find thrilling is the ability to create the illusion of any sonic event before me, and the CBT24s did just that; they placed a celebrated orchestra right into my family room, or at least an aural facsimile thereof with a high degree of verisimilitude. It is uncanny how these oddly-shaped boxes magically generate such a complex and lively scene in an empty room. “Orchestral Suites” was an absolute pleasure to listen to with the CBT24 speakers. They proved to do as well with instruments as they did with human voices.

So the CBT24s have proven they can do elegant and subtle, but how about brash and loud? To see what they could do with something on the opposite end of the musical spectrum, I threw in a hard drum’n’bass mix, since music scarcely gets more rambunctious than that. I decided to use a CD of various drum’n’bass tracks mixed by Calyx and Teebee that came with a drum’n’bass magazine called ATM back in 2007. This music is a relentless onslaught of fierce percussion, blaring synthesizers, and brutal bass. At loud enough levels, it can be a near constant tax on any audio system, and it is an effects-heavy music that just doesn’t sound great on a wimpy pair of speakers. The design of the CBT24s do not suggest wimpiness, but I decided to give them the go-over nonetheless. I had to be sure, so I loaded the CD and cranked the volume. As one might have guessed, these can handle loud. A lesser loudspeaker might be pushed into layering gobs of distortion in these tracks at these levels, which can end up sounding obnoxious, but the CBT24s remained clean and composed. These speakers laughed off what I considered to be ‘loud'. Even with 120 watts per channel using an outboard amplifier I did not have near enough power to make the CBT24s break a sweat. If you are looking for a party speaker or head banging speaker, the CBT24s are a great choice. They have enough dynamic range for pretty much anyone.

The one caveat I should mention here is that the CBT24s do not carry that extreme dynamic range in bass frequencies. I was using them crossed over to subwoofers at 80 Hz. While they can be made to play bass frequencies that subwoofers normally cover, they aren’t able to do that frequency band extremely loud. If they are run full-range and pushed hard in bass frequencies, they will audibly distort. However, when used with a subwoofer covering the lower bass range, they have a tremendous amount of headroom. Those 24 2.5” drivers provide an incredible amount of displacement potential for mid and treble frequency output, but it has to be remembered that for every octave lower in the frequency spectrum, a sealed loudspeaker must quadruple the air displacement in order to maintain the same sound pressure level. The combined surface area of its cones is about the same as a single 12” cone, and its drivers have a 2mm Xmax (one-way distance of linear throw)- that is a lot of woofer for mid and treble frequencies, but that doesn’t add up to a tremendous amount of displacement for deep bass. The CBT24s should be used crossed-over to a subwoofer for bass frequencies.

Movie Listening

One film that I watched using the CBT24s was the recent WW2 epic ‘Dunkirk,’ Christopher Nolan’s acclaimed depiction of the massive retreat of allied soldiers off the coast of France in 1940. As with Mr. Nolan’s other films, ‘Dunkirk’ has a superbly dynamic sound mix. The sounds of aerial dogfights, torpedoed ships, sniper fire, and bombing raids are all rendered in a seemingly realistic manner, and it is all underlined by Hans Zimmer’s throbbing musical score. This is a movie where the scale of the sound matches the scope of the movie; it looks and sounds enormous. For this movie I decided to crank the volume and see what the CBT24s were capable of. Even pushed to reference levels, the CBT24s were not challenged. Every aspect of the soundtrack was reproduced with startling resolution. I would say it sounded authentic, but I have no idea what WW2 battles really sounded like, so I will just say it sounded like what I would expect a WW2 battle to sound like: frightening! The CBT24s gave ‘Dunkirk’ the grand presentation that it deserved. Even at extreme loudness levels they remained crisp and clear. Those looking for an Imax-type ‘big sound’ can certainly find it with the CBT24s.

Another movie that I watched with the CBT24s was the popular 2016 superhero film “Wonder Woman.” I had not previously seen it but guessed that it would be a good movie for demonstrating how a more conventional big-budget sound mix would perform on these speakers. The sound mix of “Wonder Woman” is very polished, as would be expected of such high production values. Dialogue, effects, and music had a perfect balance on this sound mix, or at least they sounded that way on the CBT24s. Particularly memorable was the musical score by Rupert Gregson-Williams; the electric guitar theme overlaying the pounding orchestral percussion was one of the better superhero scores in my opinion. The theme as reproduced by the CBT24s gave “Wonder Woman” some truly electrifying moments. This is an epic score that deserves a correspondingly epic sound system, and the CBT24s did not fail in this regard.

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