CBT36K Assembly Manual
14. APPENDICES:
14.1. A
PPENDIX
1:
D
ETAILED
D
ESCRIPTION OF THE
CBT36:
14.1.1. A Revolutionary New System Based on CBT Theory:
Marshall Kay and Don Keele brought their concept to reality in conjunction with electronics
supplier Parts Express and its Dayton Audio brand of loudspeaker components to create the CBT36,
the world's first broadband constant-directivity (CD) loudspeaker system for the home. The CBT36
system provides extremely even coverage and flat frequency response at all locations in the
listening room. It’s specifically designed to operate over an acoustically reflective ground-plane (a fancy
name for the floor) and does not suffer from detrimental floor-bounce effects.
Licensed under U.S. Patent
numbers 7,684,574 and 7,826,622 from Harman International Industries, Incorporated.
14.1.2. CBT Theory:
CBT, which stands for
C
onstant
B
eamwidth
T
ransducer, is a loudspeaker line-array technology pioneered
by Don Keele in six Audio Engineering Society technical papers, and is based on unclassified military
underwater sound research.
14.1.3. System Description:
Each CBT36 contains 18 each 3-1/2" full-range drivers, used as mid woofers, and 72 each 3/4" diameter
wide-band tweeters, that are crossed over at 1 kHz. Initially, the system will only be available in a bi-
amped version but later a passive crossover may be offered. The drivers are mounted on a front panel
that has a 36° circular-arc with the system standing about five feet tall and a width of about 7 inches, and
a depth at the base of about 25 inches.
14.1.4. Drivers:
The 3-1/2" drivers are Dayton Audio's ND91-8, which were designed specifically for the CBT36 system.
The ND91 utilizes a Neo-Balanced 1" underhung motor that is capable of over 10 mm of peak excursion
and features a shorting ring to help keep distortion under control.
The very-small 3/4" diameter wide-range tweeters are also made by Dayton Audio, and feature a 0.5”
edge-drive inverted aluminum dome that utilizes a high-energy neodymium magnet for low distortion and
high efficiency. The tweeters are mounted on PC boards in groups of four to ease assembly and optimize
center-to-center spacing. Eighteen four-tweeter assemblies (72 tweeters in all) are then mounted on the
36° circular-arc front panel which provides an extremely-even wideband narrow vertical coverage of 28°.
The very-close 0.9” center-to-center tweeter spacing insures that the tweeters operate as a continuous
high-frequency (HF) line source up to 15 kHz to minimize HF grating-lobe problems
(
http://en.wikipedia.org/wiki/Side_lobe
14.1.5. Specifically Designed to Eliminate the Detrimental Effects of Floor Bounce:
The CBT36 has been specifically designed to operate over a reflective ground plane and does not suffer
from destructive floor-bounce effects. The curved-line array of 72 tweeters extends all the way down to
the floor to take advantage of the ground-plane acoustic reflection. Effectively the floor provides a
reflective surface that acoustically extends the array below floor level and thus creates an array that is
over 10 feet tall and controls vertical coverage and directivity down to below 160 Hz!
The system works equally well over a carpeted floor with only a moderate degradation in the HF response
for points close to the carpeted floor. Listening with your ear close to the floor reveals a noticeable HF
rolloff when carpet is present. With carpet, a test mic attached to a real-time spectrum analyzer confirms
that the high frequencies start to be affected within about 8 inches of the floor. Apparently HF sound
grazing the floor is absorbed more than lower frequencies. The real-time analyzer reveals no degradation
of the sound for points higher than about 12 inches above the floor as compared to a non-carpeted floor.
14.1.6. The Restricted Vertical and Horizontal Coverage of the CBT36 Greatly Reduces
Ceiling and Wall Reflections:
The above-floor vertical coverage of the CBT36 is a narrow 28º which is extremely stable with frequency.
This greatly reduces ceiling reflections as compared to a typical box style system. In addition, the
horizontal coverage which is very broad narrows as you go around the side of the system, which also
significantly minimizes side wall reflections.
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