CBT36K Assembly Manual
14.2. A
PPENDIX
2:
CBT
T
HEORY OF
O
PERATION
:
14.2.1. Background:
The CBT36 is based on unclassified “CBT” military under-water sonar research done by the U.S. Navy.
CBT, which stands for “
C
onstant
B
eamwidth
T
ransducer,” is a term coined by military researchers in a
series of three papers published between 1978 and 1983 [1-3] (see Bibliography at the end of this section
for reference numbers in brackets). This research resulted in under-water transducers that exhibited
extremely even coverage independent of frequency and distance. The CBT36 is the result of applying this
research to high-end home loudspeaker systems.
14.2.2. Application to Loudspeakers:
Don Keele applied this theory to loudspeaker arrays in a series of six pioneering Audio Engineering
Society papers written between 2000 and 2010 [4-9]. Don is a designer and engineer that’s been involved
with “Constant Directivity” professional loudspeakers and horns since the early 70s. These systems are
designed to have extremely-even coverage and flat frequency response over wide angles both
horizontally and vertically. His goal was to design a line array loudspeaker system for domestic and home
theater environments that exhibited the same characteristics.
14.2.3. Design Goals:
The design goal of the CBT36 was to create a no-holds-barred high-end line array loudspeaker for home
use with extremely even coverage at all points in the listening room and with broad-band constant-
directivity (CD) behavior. This type of speaker would have no preferred so-called “sweet spot” listening
axis and associated listening distance for optimum sound. It would sound good not matter where you
listened to it; whether sitting down, standing up, laying on the floor, listening directly in front of the system
from six inches away or 12 feet away, or listening to the system way off to one side.
An additional goal of the system was to completely eliminate the deleterious effects of floor bounce and
create a system that would work located on a hard reflecting surface such as a tiled non-carpeted floor
without exhibiting frequency response comb filtering or other aberrations.
14.2.4. CBT36 Design:
The design of the CBT36 may be summarized in three design targets:
14.2.4.1. Extremely Even and Uniform Frequency Response:
The CBT36 has very even frequency response from listening points ranging from up and down, side to
side, and from near to far.
Up - Down:
The frequency response is quite uniform from listening locations on the floor to points significantly
higher than the array itself, and all points in between!
Side to Side:
The CBT36 has extremely even and wide, but well-controlled, horizontal coverage that extends over
very-wide angles from ±90° from directly in front of the system.
Near and Far:
The CBT36 has extremely even and flat frequency response from points even as close as 3” (75 mm)
from the front panel of the system to points 10 to 14 ft (3 to 4 m) away. The CBT36 can be used as a
perfect near-field monitor because the systems can be placed as close as 2 ft (0.6 m) from each other
and listened to from locations only 2 to 3 ft (0.6 to 1 m) away!
14.2.4.2. Eliminates the Deleterious Effects of Floor Bounce:
Most speakers exhibit comb filtering effects due to the sound of the speaker bouncing off the floor. The
floor-bounce effects depend highly on the distance and height of the listener. The vertical coverage of the
CBT36 is essentially perfect from points on the floor to above the array and from distances from very
close to far away. This is true because the system is a ground-plane design specifically intended to
operate over a reflective surface [8].
14.2.4.3. Compensates for near-far variation of sound level:
The CBT36 compensates for level variations with distance as compared to traditional box-style speaker
systems. At seated height, the level only decreases 10 dB from directly in front to 10 ft (3 m) away and
stays relatively flat! For standing listeners, the sound level of the CBT36 hardly changes from listening
points directly in front of the array to points 10 feet (3 m) away!
14.2.5. Brief Review of Military Underwater CBT Technology:
The original CBT military under-water sonar research [1-3] was applied to so-called “spherical-cap”
(
http://mathworld.wolfram.com/SphericalCap.html
) underwater transducers with special frequency-
independent “Legendre” shading. This shading provides wide-band extremely constant beamwidth and
directivity behavior with virtually no side lobes. The technique works without the need for any special or
complex signal processing. The shading is just a simple level adjustment of the individual elements that
make up the transducer.
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