A continued resurgence in the popularity of passive loudspeaker columns has encouraged manufacturers to improve the art by providing more than just a straight column of closely spaced mid-range drivers. The ability of columns to project sound with wide horizontal and narrow vertical coverage can outperform traditional point-source enclosures, especially in large, reverberant spaces. At the same time, their tall, slim profile helps them blend into the architecture and virtually disappear.
JBL Professional’s CBT 70J-1 and 70JE-1 provide a sophisticated solution that’s easy to use and achieve successful results. The CBT 70J-1 column employs four 5-inch woofers with a column of 16 one-inch soft-dome tweeters axially mounted in front. The companion CBT 70JE-1 low-frequency extension cabinet adds another four 5-inch woofers to improve pattern control from 800Hz down to 400Hz.
Both 500W, 8-ohm enclosures have screw terminals on the rear and a simple piece of zip-cord can jump them together. I personally prefer an NL-4 Speakon connector, but we quickly added one by cutting a short cable in half and stripping its raw leads, and for many installations the terminal strips are preferred by most others. Together, the pair of enclosures makes a good match for amplifiers producing about 1,000W to 2000W at 4 ohms.
Although an optional MTC-CBT-SMB1 portable tripod speaker stand mount bracket attaches to the back of the CBT 70J-1, using the combined CBT 70J-1 and 70JE-1 system is not recommended due to the long array’s offset center-of-gravity being unstable with today’s smaller speaker stands. The supplied two-part Swivel-Tilt Wall Bracket does an excellent job of permanently mounting the two-box system to most walls.
The Swivel-Tilt bracket has two parts, a swivel bracket that attaches to the wall and a tilt mechanism that attaches high, middle or low to the CBT 70J-1 by itself, or to the thick connecting plate when both enclosures are employed. Tilt is easily set up or down at 5, 10 or 15 degrees, as well as no-tilt, and these angles are represented in the CBT Calculator software.
JBL’s CBT Calculator is similar to VerTec’s Line Array Calculator, but designed for their CBT products. While the CBT is far simpler and less configurable, being able to investigate it before making decisions is a great comfort and design tool. It runs on most Windows computers.
Experimenting with the Calculator, it becomes obvious that the CBT 70J-1 operates successfully at a variety of heights, allowing its architectural placement to be dictated in part by aesthetic considerations.
In the Box
One mechanism behind CBT is its use of a passive network of inductors and capacitors designed for flat group delay over a wide bandwidth. Small amounts of delay and attenuation are accumulated down the passive network that provides micro-delay tapering and amplitude shading of the high-frequency drivers.
The second mechanism at work is the unique high-frequency baffle supporting the 16 tweeters. The variable-width HF baffle slightly slot-loads the 16 drivers to widen their VHF dispersion, while breaking up baffle diffraction, and the notches in the baffle provide the four woofers behind with wider horizontal dispersion at their higher frequencies.
The CBT 70JE-1 four-woofer extension cabinet attaches to the top of the CBT 70J-1, using a thick coupler plate that’s included, providing a well-controlled progressive line array with asymmetrical vertical coverage. This coverage produces a higher concentration of sound with a tighter pattern projecting to the far areas of the listening space, and a lower concentration with a broader vertical pattern projecting toward the near area. This results in more even front-to-back SPL levels than would be the case from traditional speakers or columns that project symmetrically.
Threaded points on the coupler plate accept the provided swivel-tilt wall bracket, as well as points for OmniMount 60.0 and 120.0 Series compatible wall-mount brackets. Twelve M6 mounting points accept eyebolts for suspension or swivel brackets for wall or pole mounting.
Self-Evident Truths
JBL’s CBT line of columns is named for their Constant Beamwidth Technology. In 2000, Don Keele created a concept with the idea that a constant beamwidth array can be created by bending an array into an arc and then gradually amplitude-shading its drivers from the middle towards its ends to eliminate side lobes, creating a perfectly constant beamwidth.
Keele and Doug Button later showed that short time delays could be used instead of physical arcing, and the mostly-straight column employs individual group-delays to each driver. This prevents the coverage from narrowing vertically at the extreme sides of the coverage area, where a physical curve would disappear, whereas the delay-taper maintains its effect. The smaller diameter of the CBT 70J-1’s 5-inch high-power woofers helps maintain wide coverage as well as inter-driver coupling up to the crossover point. The CBT approach is also employed in JBL’s CBT 50LA-1 and CBT 100LA-1 columns.
CBT 70J-1 has tight vertical coverage for the area above its aiming axis, but a broader downfill below, providing excellent front-to-back evenness of coverage. Downfill is achieved through a slight downward curvature of the bottom half of the array, combined with the delay beam forming, which overcomes the inherent limitations of the directivity of the HF drivers themselves.
Broad EQ
There are two switches set low on one side of the CBT 70J-1. The first allows a choice between a “Speech” and a “Music” EQ mode. The cabinet has a slightly contoured response, with 5 and 10 kHz presence peaks that are helpful. When the CBT 70JE-1 is added, it produces a modest 125Hz bump, whose warmth benefits both music and announcements.
In Music mode, the midrange is reduced, with its lowest response around the 1200Hz crossover. The Speech mode is a 5 dB upper-midrange lift four octaves wide centered at 2kHz. This helps speech intelligibility and improves sensitivity — and maximum SPL capability — by 5 dB in the midrange. Faced with these two choices, it’s better to surgically remove offending frequencies than accept a broad mid-range reduction, unless the application is simply unprocessed background music or perhaps surround channels.
The second switch changes the passive delay network between a “Narrow” 25° vertical coverage pattern (that actually begins widening below 2000 Hz) and a “Broader” 45° pattern (that is consistent to 400 Hz). Unless you have a room without a balcony and perhaps a back wall that you want to stay off of, the broader setting is better, since the 45° vertical dispersion is closely matched for all frequencies above 300 Hz.
After Smaart measurement and listening, we cut 1000Hz and 2200Hz, as well as 250 and 160Hz each by 3 or 4 dB to smooth the response and adjust to our live concert hall. The horizontal off-axis response of the CBT 70J-1 is remarkably smooth throughout the mids and highs, and EQ adjustments translated well across the entire venue. Obviously, the cabinet loses control below 300Hz, and the lower EQ filters helped in the lowest octave where it becomes nearly omni-directional, muddying the live room somewhat. The cut near the crossover isn’t surprising, and the higher 2.2kHz filter reduces the shrillness accompanying many voices.
Jacksonville’s 1,800-seat Robert E. Jacoby Symphony Hall at the Times-Union Center for the Performing Arts is the classic “shoebox” design seen in prominent U.S. venues including Boston’s venerable Symphony Hall and Seattle’s newer Benaroya Hall, with a balcony at the back.
Most of Jacksonville Symphony’s concerts are “straight,” with no need for a sound system, other than for pre-concert repertoire discussions, pre-show announcements and comments from the maestro on stage. This “public address” is important to audience, sponsors and conductors, and the previous pair of Bose Panaray 402 speakers was nearly omni-directional between 2 and 6 kHz and generally unintelligible, especially to senior patrons.
The CBT 70J-1/70JE-1 combination would also perform well in traditional churches and synagogues with capacities of one or two thousand with live acoustics, as well as large lecture halls, auditoriums, gymnasiums and, of course, train stations and airports. Anywhere the spoken word needs to carry a large, wide reverberant space. Naturally, they would sound even better outdoors, and they are designed to withstand the elements.
The CBT is rated IP-54 (per IEC529) and is further upgradable with an inexpensive watertight panel cover that JBL makes. It is UV, moisture and salt-spray resistant. Its drivers are weather-treated, the fiberglass reinforced ABS cabinet is good for outdoor applications, external screws are stainless steel, and the painted aluminum grille resists rusting. The CBT 70 is available in a choice of black or white.
