Loud Speaker Arrays - Don Keele - BAES
Channel: Audio Builders Workshop
Date: 2017-12-12

Introduction to CBT Loudspeaker Arrays
By Don Keele

Thursday Jan. 14th, 7pm
Walsh Function Room *
Boston College 



Don Keele has worked for several companies in the area of loudspeaker R&D and measurement technology including Electro-Voice, Klipsch, JBL, Crown, and Harman. He holds three patents on "constant-directivity" loudspeaker horns, has four patents pending, and is a fellow of AES and a member of ASA. For ten years he wrote for Audio Magazine as a Senior Editor performing loudspeaker reviews. More recently, he worked for Harman/Becker Automotive Systems in the advanced technology development group and was a member of the Harman corporate acoustics engineering group working under Floyd Toole. Currently he is semi-retired and heads his own consulting company DBK Associates and Labs. His passion for the last ten years has been to promote the use of CBT (Constant Beamwidth Transducer) loudspeaker technology in the loudspeaker industry. Mr. Keele holds two BS degrees in EE and physics from California State Polytechnic University and an MSEE degree from Brigham Young University where he minored in acoustics. He has presented and published over 35 technical papers on loudspeaker design and measurement methods and other related topics, among them the paper for which he won the AES Publication Award, "Low-Frequency Loudspeaker Assessment by Nearfield Sound-Pressure Measurement". He is a frequent speaker at AES section meetings and workshops, has chaired several AES technical paper sessions, and is a member of the AES review board. Mr. Keele is a past member of the AES Board of Governors and is past Vice President, Central Region USA/Canada of the AES. Mr. Keele received the TEF Richard C. Heyser Award in 2001. In 2002, he received a Scientific and Engineering Academy Award from the Academy of Motion Picture Arts and Sciences for work he did on cinema constant-directivity loudspeakers. 

Come out and hear Don Keele talk about his 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 array prototypes that were constructed for home applications. He will close with several suggestions for possible CBT-based DIY home construction projects. General design guidance will be provided. Floyd Toole, who is an ardent advocate of the CBT technology, describes home theater applications of CBT arrays in his new book “Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms” (Focal Press, 2008). 

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 five AES papers between 2000 and 2005 and most of the technology is now in the public domain. The CBT loudspeaker arrays provide directional performance and sound-field coverage that borders on revolutionary. The CBT arrays provide a broadband 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. 

The CBT line arrays provide a very-tight extremely well-controlled vertical sound-field coupled with a well-controlled very-broad horizontal coverage. Most 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.