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AES Section Meeting Reports

Chicago - April 23, 2013

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Summary

Come out and hear Don Keele talk about his loudspeaker line arrays based on CBT technology. He has been an ardent advocate and evangelist for the 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. Don will discuss the background and history of CBT arrays and will describe and discuss the measurement and simulation results of several CBT line arrays including the Audio Artistry CBT36 high-end home loudspeaker which is sold in an inexpensive DIY kit form by Parts Express and finished form by Audio Artistry. The CBT36 system will be setup for listening.

CBT, which stands for "Constant Beamwidth Transducer," is a term originated by the U.S. military in a series of three un-classified Naval Research Lab ASA papers published in the late 70s and early 80s. These papers describe spherical-cap underwater transducers with special frequency-independent "Legendre" shading that provide extremely-uniform broadband coverage without the need for any special or complex signal processing. Don applied the technology to loudspeaker arrays in a series of six AES papers between 2000 and 2010 and some of the technology is now in the public domain. The CBT arrays provide a broadband constant-directivity 3D sound field that is incredibly uniform and well behaved with frequency at all distances from very close to far away. The vertical beamwidth control and coverage of the CBT line arrays often exceeds even the finest constant-directivity pro horns.

The CBT line-array concept dictates that multiple loudspeakers be placed evenly around a circular arc and that frequency-independent shading (simple changes in level) be applied to each speaker. An alternate, but much more complex, CBT implementation method is based on the use of delays to provide the circular curvature of a straight-line array. Circular-arc CBT arrays can be implemented passively and do not require any sophisticated DSP signal processing except for simple level changes. CBT array possibilities extend over the full loudspeaker product range from professional, commercial, consumer, home theater, computer, and multimedia. Note that Harman/JBL has copyrighted the term "CBT" in the loudspeaker industry where it stands for "Constant Beamwidth Technology" and has three patents issued to Keele on some of the CBT concepts

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AES - Audio Engineering Society