Constant Beamwidth Transducers (CBT) Loudspeakers
The Constant Beamwidth Transducer (CBT) loudspeaker technology, invented by D. B. Keele, Jr., represents a seminal breakthrough in achieving superior, consistent sound coverage and directivity. The core innovation of CBT is its ability to produce a controlled sound beam that remains essentially constant over an extremely broad range of frequencies and distance. This addresses a long-standing challenge in acoustic design where traditional loudspeaker arrays suffer from inconsistent coverage and lobing.
Brief Overview & Key Benefits
CBT arrays provide broadband constant-directivity, beamwidth, and coverage. The pattern is independent of frequency and distance. They require no DSP, using frequency-independent level shading only, and exhibit no nearfield effects, making them simple to implement. Keele has been an ardent advocate for the CBT concepts for over ten years, believing the know-how can vastly improve the sound field uniformity of existing line arrays and conventional loudspeaker systems.
Key Benefits:
Simple Implementation: The basic circular-arc array requires only passive components for level shading.
Consistent Coverage: Stable, predictable sound coverage from front-to-back, side-to-side, and top-to-bottom of the listening area.
Minimal Near-Far Variation: Naturally reduces the sound level variation between a listener standing close to the speaker and one far away.
Superior Clarity: The uniform sound field contributes significantly to improved intelligibility, especially for speech, in acoustically challenging spaces.
Key Benefits:
Simple Implementation: The basic circular-arc array requires only passive components for level shading.
Consistent Coverage: Stable, predictable sound coverage from front-to-back, side-to-side, and top-to-bottom of the listening area.
Minimal Near-Far Variation: Naturally reduces the sound level variation between a listener standing close to the speaker and one far away.
Superior Clarity: The uniform sound field contributes significantly to improved intelligibility, especially for speech, in acoustically challenging spaces.
The Core Innovation: From Submarines to Sound Systems
CBT theory is based on un-classified military underwater transducer research (Navy CBT Papers) done in the late 1970s and early 1980s, which described a curved-surface transducer in the form of a spherical cap. Keele adapted this theory to loudspeaker arrays.
The foundational principle for a perfect CBT array requires two main elements:
- Physical Curvature: Multiple loudspeakers must be evenly placed around a circular arc (or a portion of a sphere).
- Amplitude Shading: The electrical signal fed to each driver is “shaded,” meaning its level is gradually reduced (“tapered”) for the drivers farthest from the center. This is based on the mathematical Legendre function (frequency-independent Legendre shading).
This combination causes the array to radiate sound with a highly predictable, constant beamwidth, and coverage pattern, with virtually no unwanted side lobes. The coverage of a CBT array acts more like that of a single, highly-controlled point source than a traditional array. acts more like that of a single, highly-controlled point source than a traditional array.
Original Navy Paper #1 | Original Navy Paper #2 |
Original Navy Paper #3Van Buren et.al. June 1983 |
A Brief CBT History and Technical Papers
The Constant Beamwidth Transducer concept was first publicly presented by D. B. Keele, Jr., in his original Audio Engineering Society (AES) technical paper on the subject in 2000. Keele applied the military research to loudspeaker arrays in a series of five AES papers between 2000 and 2005:
Don’s Original Five CBT Papers
Keele CBT Paper 1 (2000): “The Application of Broadband Constant Beamwidth Transducer (CBT) Theory to Loudspeaker Arrays.” Extended the concept to arrays based on circular-arc line arrays and toroidal-shaped curved surface arrays. | Keele CBT Paper 2 (2002): “Implementation of Straight-Line and Flat-Panel Constant Beamwidth Transducer (CBT) Loudspeaker Arrays Using Signal Delays.” |
Keele CBT Paper 3 (2003): “Full-Sphere Sound Field of Constant Beamwidth Transducer (CBT) Loudspeaker Line Arrays.” Analyzed the 3D sound-field of CBT circular-arc line arrays. | Keele CBT Paper 4 (2003): “Practical Implementation of Constant Beamwidth Transducer (CBT) Loudspeaker Circular-Arc Line Arrays.” Described the practical implementation. |
Keele CBT Paper 5 (2005): “Ground-Plane Constant Beamwidth Transducer (CBT) Loudspeaker Circular-Arc Line Arrays” (with D. J. Button). Described the ground-plane CBT array variation. |
Additional CBT Papers
CBT Paper #06 (Nov. 2010) Performance Ranking of Loudspeaker Line Arrays
CBT Paper #07 (Oct. 2015) Feng, Shen, Keele, & Xia Directivity-Customizable Multi-CBT Arrays
CBT Paper #08 (Oct. 2015) Time-Phase Behavior of CBT Arrays
CBT Paper #09 (Oct. 2015) Implementation of Segmented Circular-Arc CBT Loudspeaker Arrays
CBT Paper #10 (Sept. 2016) Design of CBT Loudspeaker Line Arrays for Sound Reinforcement.
CBT Paper #11 (Sept. 2016) Use of Ground Plane CBT Arrays for Sound Reinforcement
CBT Paper #12 (Sept. 2016) A Ground-Plane Comparison Between B&W801 and CBT36
CBT paper #13 (June 2019) Circular-Arc Line Arrays with Amplitude Shading (June 2019)
CBT Paper #14 (Oct. 2017) Constant Directivity Circular-Arc Arrays of Dipole Elements
CBT Paper #15 (Oct. 2017) Implementation of a Dipole Constant Directivity Circular-Arc Array (Manke Eng Brief)
CBT Paper #16 (Oct. 2017) Design and Implementation of a Practical Long-Throw High-Q CBT Array.
CBT Paper #17 (Oct. 2017) Design and Implementation of a CD Two-Way 12” Woofer Wedge Loudspeaker System
CBT Paper #18 (Oct. 2018) Modeling Point-Source Loudspeaker Arrays in AFMG Programs
Ground-Plane CBT
A significant design variation pioneered by Doug Button of JBL and D. B. Keele, Jr. is the Ground-Plane Constant Beamwidth Transducer array.
- This design leverages the acoustic reflection of a flat, hard surface (typically the floor) to effectively double the length of the loudspeaker array.
- The array is constructed as a half-sized circular-arc CBT and placed directly against the floor.
- The reflection creates a perfect mirror image of the array, forming a complete, much longer CBT array (e.g., a 5-foot speaker on the floor behaves like a 10-foot speaker).
- This acoustic doubling extends the vertical pattern control down an octave lower in frequency and provides highly uniform coverage, dramatically reducing destructive floor-bounce effects.
Enthusiast Systems
World-class engineer D. B. Keele, Jr., in collaboration with Marshall Kay of Audio Artistry, brought the technology to the enthusiast market:
Audio Artistry CBT36: A large, two-way, floor-standing system available as a kit from Parts Express. It used a physical 36° circular-arc design intended for ground-plane operation, featuring eighteen 3.5-inch mid-woofers and seventy-two 0.75-inch tweeters.
Epique CBT24: Later co-designed with Lou Melillo for Parts Express as a simplified, less-expensive one-way CBT system using only wide-range direct-radiator drivers.
Dayton Audio Epique CBT24
Commercial Evolution:
The Straight-Line CBT
Don’s co-author, Doug Button of JBL, pioneered the adaptation for commercial products. This design implements the CBT principle using:
- Straight Column Enclosures: Instead of a curved cabinet, the drivers are mounted in a straight, vertical column .
- Analog Delay Banks: The required curvature and shading are simulated electronically using analog delay circuits and fixed resistors. Each driver in the straight column receives its signal slightly delayed and at a reduced level relative to the central drivers.
This implementation maintains the constant beamwidth advantage while offering a slim, architecturally unobtrusive design. The CBT implementation of of the modern column design is what made the CBT loudspeaker line, patented by JBL, an immense commercial success in demanding environments like lecture halls, houses of worship, and transit centers. JBL has also registered “Constant Beamwidth Technology” has a working trademark. CBT as the acronym for “Contant Beamwidth Transducer” is in the public domain as used by the Navy and scientific community.