CBT stands for Constant Beamwidth Technology - a patented technology from JBL that allows for pattern control consistency by using complex analogue delay and amplitude-tapering to create beams of sound with consistent vertical coverage, without the narrow vertical beaming and out-of-coverage lobing that are typical of other straight form-factor column speakers ...
To simplify; the units are passive column speakers consisting of multiple drivers in a vertical line, this creates a narrow vertical beam of sound.
CBT is a true line-array design, i.e. when multiple drivers are spaced correctly, certain frequencies couple together, creating a single ‘beam’ of sound. This differs to your usual point source speaker which radiates out in all directions. Take the example of dropping a pebble into a pool of water. A point source driver is your single pebble rippling out in all directions; you drop two pebbles equally spaced and the ripples clash, creating both positive and negative interference and start to form a flat, forward wavefront ... 3 pebbles, 4 pebbles...
What’s important here is that we are using this interference to shape and direct the speaker’s sound. What the researchers at JBL found was that when you curve an array of speakers you get a delay and that delay helps further shape the beam as well as preventing some of the lobing issues (spikes of sound where we don’t want them) that straight columns suffer from.
The main issue now was the size and depth of these speakers wasn’t practical for install. The solution was to delay each driver in the circuitry to create a ‘virtual curve’. This is accomplished by having the central pair as point zero, you then step out to the next pair (above and below) and add a slight delay and a little bit of attenuation (calculated from the curve), then you keep stepping out. This is all done passively in the box, so no additional DSP costs; any amp that meets the power requirements will do the job.
The result is the CBT speaker: a shallow, slim column that is very sensitive visually and architecturally but offers powerful, highly articulate, full-range sound with wide horizontal, and narrow, focused vertical dispersion. The range is also IP-55 rated when used with an input panel cover, making them suitable for outdoor install.
The other major benefit is the effect of line-arrays on loss of level over distance. There’s a physics rule called the Inverse Square Law, this states that for every doubling of distance there is a halving of power; for point source speakers this is true, that halving of power being a -6dB drop.
With line-arrays and columns you actually find the drop over the nearfield halves, so you see a -3dB loss over the same distance. What this gives you is consistency of sound. Walking the room with the CBTs on, you will notice very little variation in level from the front right to the back, obviously this is a huge selling point for speech and music.
I’ve just scratched the surface of the technical brilliance behind these speakers, but our colleagues at Harman are currently hosting regular online deep-dive training presentations on many of the JBL lines, including CBT. Check out the Harman University site for more detail on these.
So that’s all well and good, but what are the applications?