I’ve spent some more time reading into CBT technology recently, but I find myself having even more questions, particularly regarding the ground-plane/floor-reflection CBT designs. I mean no offense to anyone, and admittedly, I haven't heard these speakers in person, nor can I fully digest all the academic papers. So, please correct me if my logic is flawed.
As I understand it, this technology simulates a virtual point source located at the center of curvature by arranging drivers on an arc (or using delays) and controls the vertical directivity through amplitude shading.
@Newman mentioned earlier that our psychoacoustic perception of the sound source is dictated by the closest driver. However, I suspect this might not apply to this specific architecture. Since the wavefront is deliberately synthesized to mimic a point source originating from the floor (the center of the arc), I can't imagine why the human ear would localize the sound to the nearest physical driver.
In fact, I stumbled upon a similar technology called
Wave Field Synthesis (WFS), which I believe supports my perspective. WFS also utilizes line arrays with complex delays and attenuation to simulate non-existent, virtual sound sources. The visual animations explaining WFS principles look strikingly similar to JBL's CBT design. WFS was born entirely out of the desire to make driver arrays simulate sound sources at entirely different locations in space.
Therefore, I lean towards the idea that a ground-plane CBT is essentially a speaker that simulates a controlled point source on the floor—not just in its dispersion behavior, but also psychoacoustically. Yet, in the horizontal plane, it diffuses somewhat like a cylindrical wave. Conceptually, it acts like a massive, extremely narrow, fan-shaped waveguide:
View attachment 532538
In conclusion, I have no doubt that CBT is a brilliant technology for sound reinforcement/PA applications. It achieves controlled vertical directivity and lower SPL attenuation (similar to a cylindrical wave) at a fraction of the cost, using a single conventional-sized cabinet. However, for home audio applications, a floor-to-ceiling line array might still be the superior choice.
The CBT concept is simple, it works like this:
Pretend that this is a one-way speaker. It's easier to use a one-way for an example. In this vertical line array, the elements will produce a wavefront that is tall, narrow, and most importantly: it's flat.
Now we take that same array, and we curve it. Again, pretend this is a one way, it's simpler. Same exact idea as the flat array, but the curvature of the cabinet curves the wavefront, and the vertical coverage angle is (mostly) determined by the angle of the arc defined by the array.
The main reason for the shading is simply for even coverage. That's it. Nothing super fancy here; you shade the array so that there isn't an abrupt transition at the edge of that cylinder of sound that the array radiates.
The difference between an unshaded array and a flat array is like the difference between a single light bulb illuminating your wall, and a home theater projector illuminating the same wall.
The light bulb is spherical; because it's spherical, the light is brightest in the center, and as you go further towards the edges, the light slowly fades out.
The projector bulb itself is ALSO spherical! But the projector has a LENS that turns the spherical light output into a FLAT wavefront. The projector takes a spherical wavefront and turns it into a two dimensional planar wavefront.
In other words: the goal of the project is to create a wavefront that matches the screen that it is projected on. If you simply took a light bulb and you removed the LENS from your projector, you would see that the light on the other side of the room looks like the light from a single bulb (because that's what it IS after all.)
Okay I promise there's a point here:
When you look at a CBT, you see a LINE array. But it is a LINE array that's designed to create a wavefront that behaves like a SPHERICAL wave.
Your home theater projector takes a spherical source (a bulb) and uses a lens to transform it into a planar source.
Your CBT takes a line source (it is a line array after all) and with the use of curvature and shading, turns it into a SPHERICAL source.
If that makes sense, then you guys will see why I've spent 15+ years of my life writing about waveformers:
https://www.diyaudio.com/community/threads/square-pegs.217298/
I named the thread "Square Pegs" because we're taking a round peg and cramming it through a rectangular hole. (We're turning a spherical source into a planar source.)
CBT is just the opposite; planar source to spherical wave.
You can see the CBT inside of this submarine, it's in the nose. This is advanced, unclassified Navy technology, carried over to loudspeakers.
The Navy version is
truly spherical. That's why it's a sphere. The Keele and JBL version is a two dimensional slice of a sphere, radiating into three dimensions. All of the software that I'm aware of for CBTs treats them two dimensionally, I'm working to make that 3D. The only exception is Mabat's ATH software, which DOES simulate them in 3D, but cannot simulate the enclosure itself. (That's why I'm doing my thing.)
I'm currently slogging through a piece of software that should be able to generate these in a matter of seconds, if you'd like to follow my work in progress, it's here:
https://www.diyaudio.com/community/...th-loudspeaker-design-can-a-cloud-llm.441785/
