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) .

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.

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)

“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.”