Constant Beamwidth Transducer (CBT) Speakers

[Newman]

I see a lot of people referring to the elimination of the floor bounce. The CBT's (Half CBT)shown don't e eliminate the ground bounce but utilize it, actually need it, for proper response.

I get your meaning, but the point being made is that the problematic floor bounce is when a fully formed wavefront travelling from the speaker to the listener is destructively interfered with by a delayed bounce off the floor. This destructive floor bounce is eliminated.

For this to work, the floor needs to be highly reflective at all frequencies emitted. Carpeting or rugs destroy the design.

The acoustic center is on the floor in this case.

The psycho-perceptual (apparent) source of the audio is the nearest drive unit. (Keele, Sept 13, 2012)

 

[Jaxx1138]

I get your meaning, but the point being made is that the problematic floor bounce is when a fully formed wavefront travelling from the speaker to the listener is destructively interfered with by a delayed bounce off the floor. This destructive floor bounce is eliminated.

For this to work, the floor needs to be highly reflective at all frequencies emitted. Carpeting or rugs destroy the design.


The psycho-perceptual (apparent) source of the audio is the nearest drive unit. (Keele, Sept 13, 2012)

That is correct. I wouldn't say they are acoustically perfect but they can be very impressive and can assist in solving some acoustical problems in specific applications.

 

[lilin]

My question is, if positioning them in reverse, so with the curving at the top instead not the bottom, will be an issue when you still sit directly in front of it and not having them above tilted downwards (where the curving makes sense to reduce the level for people standing nearer). With this the positioning on stands would be more robust and someone may use the extension module as a stand as well. Practically making a floorstander with that (originally the extension module is meant to be added at the top).

Thanks for the info! I guess inverting the speakers shouldn't be a major issue. considering that the floor is usually much closer to the listener than the ceiling, placing the broader dispersion section at the top might actually be a better way, especially considering the stability advantage.

In China, CBT series speakers are surprisingly cheap. I'll probably buy a pair of used 50LA speakers soon to try them out.

If possible, I'd like to use the CBT 1000 for my LCR. Although I won't need that much SPL, their off-axis behavior seems better. I just don't know if my budget allows it.

 

[lilin]

Their sweet spot is quite large vertically(about 20 degrees of the ends.) and horizontally(about 120 degrees ). You can build a full CBT array that doesn't use the floor as reinforcement. As long as the curve and shading effects are implemented correctly. I don't remember how much of the dB gain over distance is retained from the line source inverse square of 3dB vs 6dB from a true line source.

Yes, the JBL CBT speakers create a pancake of sound. As mentioned before, I used them in a church to bypass the podium microphones and the first several rows of pews that didn’t need reinforcement. In a home, this would minimize floor and ceiling reflections but would also be more demanding of listening height on a smaller room.

Thanks for that! It seems that with some careful aiming—perhaps a slight upward tilt—I should be able to align that vertical beam to cover the intended listening height range effectively, even in a smaller room.

 

[Paffi]

Thanks for the info! I guess inverting the speakers shouldn't be a major issue. considering that the floor is usually much closer to the listener than the ceiling, placing the broader dispersion section at the top might actually be a better way, especially considering the stability advantage.

That's what I thought too. The idea of using the extension module as a stand is very intriguing.

My idea is to place diffusors at the ceiling, where the curved part is aiming too, maybe that will enhance spaciousness. But that's far away from the specified use case of the CBT 70. Regarding your planned purchase: I wouldn't buy the 50 because these are broadbands. At least compare them to the 70 or 1000.

 

[Jaxx1138]

It looks like I phrased it wrong at the end of my statement. Inverse square in a mathematically perfect distribution of sound is -6dB every doubling of distance from a point source or spherical wave front and a -3dB for every doubling of distance on a line source or cylindrical wave front. Since the CBT is neither but a hybrid that simulates part of a spherical wave front. They do project well , I can attest to that. Through an experience of demonstrating them in a very long room (around 40 feet) . It was many , many years ago when I measured some of the first prototypes. I am not good at expressing things on these type of forums. Better at speaking. I hated writing papers. Floyd Toole always told me if you can't explain it to a lay person without relying on a bunch of mathematical formulas that you just spit out then you most likely do not understand it yourself. (You still need to know the math)

 

[Newman]

...sound is -6dB every doubling of distance from a point source or spherical wave front and a -3dB for every doubling of distance on a line source or cylindrical wave front. Since the CBT is neither but a hybrid...

Just in case readers think you are suggesting something between -3 and -6 dB, the 'Keele CBT' concept has an amazing attribute of horizontal isobars for the first few metres from the speaker. Meaning, 0 dB per doubling of distance.

This offers the opportunity to use them for side and rear surrounds with off-centre listeners, uniquely creating no disparity in left-right loudness compared to a listener on the centreline.

cheers

 

[Jaxx1138]

Just in case readers think you are suggesting something between -3 and -6 dB, the 'Keele CBT' concept has an amazing attribute of horizontal isobars for the first few metres from the speaker. Meaning, 0 dB per doubling of distance.

This offers the opportunity to use them for side and rear surrounds with off-centre listeners, uniquely creating no disparity in left-right loudness compared to a listener on the centreline.

cheers

This is very true. They do many things very well in an unorthodox way. They perform well on desktops (scaled down), or in large rooms with relatively low ceilings as a PA or background music system(upside down against the ceiling).

 

[lilin]

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:

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.

 

[Newman]

@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.

You can see in my post at the top of this page that it was Keele himself who provided that information. It was definitely with reference to this speaker architecture.

cheers

 

[lilin]

You can see in my post at the top of this page that it was Keele himself who provided that information. It was definitely with reference to this speaker architecture.

cheers

The physics behind CBT are indeed incredibly complex, and there are likely still many nuances I haven't fully accounted for. If I ever get the chance, I will definitely hear these speakers in person.