Don Keele describing CBT Loudspeaker Arrays.
Boston AES Meeting
January 2010
with Don Keele
On Thursday January 14, 2010 the Boston Sections of the Audio Engineering Society, the Acoustic Society of America and the Boston Audio Society held a joint meeting featuring guest speaker Don Keele who presented "An Introduction to CBT Loudspeaker Arrays".
Don started by describing how Constant Beamwidth Theory was first developed by the military for underwater transducers. He reviewed examples of loudspeaker types and discussed the directivity characteristics of each. He explained that "it is typically very hard to provide a speaker with constant directivity, particularly in a domestic environment. The main issue with designing loudspeakers is to not have the polar pattern of your radiating device change over your frequency range".
To create a CBT loudspeaker line array, several identical drivers are equally spaced around a circular arc and a special frequency-independent shading is applied. Legendre Function Shading is the process of sending different amounts of level and frequency to small groups of transducers in the array. The shading attenuates the arc's outside speakers as compared to those mounted near the center of the arc. Don described an alternate version of the array that is designed to operate over an acoustically reflective ground plane. Here the array height is effectively doubled due to the acoustic reflection, and the array's low-frequency beamwidth control cutoff is extended down by an octave.
The ground plane arrays on display at the event were created using clear PVC piping originally manufactured for the medical field. The pipe was cut into smaller sections to simulate a bend instead of having to actually bend the pipe. He convincingly proved the effectiveness of his CBT arrays upon sharing the comparison of frequency response results both with and without shading.
Examples were given of how arrays have been implemented in the live sound reinforcement field and focused on two specific installation examples; a home theater, by Monte Kay, and a Church by his brother Marshall Kay, who was in attendance at the event. Answering a few questions about the process Marshall commented that "although there is some initial research needed, it can be a very cost effective alternative to buying from a manufacturer". Don concluded his presentation by encouraging attendees to attempt to build their own CBT arrays and shared sources for where the necessary parts can be purchased. Keele emphasized that two Harman/JBL patents are pending on the CBT concept that may soon issue, and that this be taken into account in any home construction project.
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. 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. 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.
