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A Free-Standing Circular-Arc
CBT Line Array with Frequency-
Independent Legendre Shading

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A Ground-Plane Circular-Arc
CBT Line Array with Frequency-
Independent Legendre Shading

Brief Overview:

• Provides broadband constant-directivity, beamwidth and coverage.
• Pattern is independent of frequency and distance!
• Requires no DSP, frequency-independent level shading only!
• No nearfield! Simple to implement!
• CBT arrays are revolutionary!
• The CBT line-array concept dictates that multiple loudspeakers be evenly placed around a circular arc.
• CBT is a term originated by the U.S. military in a series of un-classified Naval Research Lab ASA papers published in the late 70s and early 80s.
• Keele applied the technology to loudspeaker arrays in a series of five AES papers between 2000 and 2005 and some of the technology is now in the public domain.
• Keele has been an ardent advocate and evangelist for the CBT concepts for over ten years and believes the CBT know-how can vastly improve the sound field uniformity of existing line arrays and conventional loudspeaker systems.

A Brief CBT History
CBT or Constant Beamwidth Transducer theory is based on un-classified military under-water transducer research done in the late 1970s and early 80s(Navy CBT Papers). This research describes a curved-surface transducer in the form of a spherical cap with frequency-independent Legendre shading that provides wide-band extremelyconstant beamwidth and directivity behavior with virtually no side lobes.

The theory was applied to loudspeaker arrays by Keele in 2000(Keele CBT Paper 1)where he extended the concept to arrays based on circular-arc line arrays and toroidal-shaped curved surface arrays. Keele also extended the concept to straight-line and flat-panel CBT arrays with the use of signal delays (Keele CBT Paper 2). The 3D sound-field of CBT circular-arc line arrays was analyzed by Keele in 2000 (Keele CBT Paper 3) . In 2003 Keele also described the practical implementation of CBT circular-arc line arrays (Keele CBT Paper4). In 2005, he described a design variation of the CBT loudspeaker line array, called a ground-plane CBT array, which is intended to operate very close to a planar reflecting surface (Keele CBT Paper 5).

What Other Audio Pros Are Saying....

Siegfried Linquitz quote: (http://en.wikipedia.org/wiki/Siegfried_Linkwitz)
“D. B. (Don) Keele has done extensive work on line array loudspeakers for sound reinforcement applications, where it is important to control the vertical radiation pattern and to avoid lobes, while maintaining wide horizontal dispersion to cover an audience. (………..) I have heard Monte Kay's CBT center speaker and was quite impressed with its acoustic performance in his home theater application. It seems the CBT in its various forms presents a business opportunity for highest quality sound reinforcement, where especially the mid and high frequency portions of the spectrum must be properly dealt with.” (http://www.linkwitzlab.com/links.htm)

Alvin Foster quote: (Founder and past president of the Boston Audio Society in his introduction to Don Keele’s AES/ASA/BAS Boston presentation of 1/14/2010 titled: (“Introduction to CBT Loudspeaker Arrays”)

“I think you are looking at the future loudspeaker over there. (Pointing to Keele’s ground-plane CBT prototype brought to the presentation for demonstration.) It won’t replace, of course, the traditional two- or three-way loudspeaker box, but it will be in the home of those who care a lot about sound.”

Floyd Toole quotes: (Quotes taken with permission from his book: "Sound Reproduction, Loudspeakers and Rooms" by Focal Press, Copyright 2008, an excellent book!).

Pages 370 – 372:
“Keele culminates a series of papers on constant-beamwidth transducers (CBTs) in a collaboration with Button, in which they examine the performance of several variations of truncated lines: straight and curved, “shaded” (drive power reduced toward the end), and un-shaded (all transducers driven equally), all standing on a plane reflecting surface (Keele and Button, 2005). It is a masterpiece of predictions and measurements that provide many answers and suggest many more possibilities.”

“Figure 18.3 shows a small sample of the informative sound field simulations in the paper. (……….) How did (e) and (f) sound? Excellent—at least that is the author’s opinion from a biased, sighted test. It was distinctive in how little the sound level and timbre appeared to change with location in the room and how the loudspeaker did not get “loud” as one walked up to it.”
(Click to view pages 368-372 which are included on the "Goody Disc")

Page 407:
The Perfect Surround Loudspeaker? “Figure 18.3 showed sound-level contour plots for several variations of truncated, curved, and shaded-line loudspeakers. The two on the bottom (e and f), versions of the constant beamwidth transducers (CBT), were of special interest because they exhibited constant directivity (potentially good sound), and some of the contours held nearly the same sound level over long distances. Inspired by this, Figure 18.21 shows a family of contours taken from the same paper (Keele and Button, 2005) but inverted, placing the loudspeaker at the ceiling interface. The row of “heads” across the width of this imagined room intersect with only one line; they are at a nearly constant sound level from 200 Hz to 8 kHz. And as one moves even closer to the loudspeaker, at the same height, the sound level goes down.” (…………) 

“If a variation of this design or something else entirely, can come close to this performance target in a real room, the result would be a remarkable improvement in spatial and directional effects for the entire room. It is probable that fewer surround loudspeakers would be needed for large audiences.”
(Click to view pages 407-409 which are included on the "Goody Disc")