CBT36K Assembly Manual
Page
86
of 89
14.2.9. CBT36 Enclosure Design:
A most obvious question about the enclosure is why is it circularly curved? The reason can be traced
back to the original military underwater transducer research where they analyzed a spherical-shaped
round transducer. The theory was applied to loudspeaker line arrays by assuming a circularly-shaped line
array. This is the shape that results if several loudspeakers are arranged in a straight line and then
wrapped around a sphere. This is the configuration analyzed in Keele’s first paper of 2000 [4]. The
circular-arc array provides very uniform and well controlled vertical coverage with very wide horizontal
coverage that is independent of frequency and distance and
does not require any complicated DSP
processing
.
So why keep it curved? Three reasons:
Firstly
,
the circular shape dramatically simplifies the required processing to have constant coverage with
frequency. The processing required for a straight-line array to provide the same coverage control as a
circular-curved array is extremely complicated! Each individual speaker in the array would require its own
power amplifier with complex DSP and delay capabilities built in. Furthermore, the required processing is
strongly frequency dependent.
The processing required for a circularly-curved array is extremely simple and is not frequency dependent.
Just a simple frequency-independent amplitude shading adjustment of each speaker is required. In most
cases the processing can be done completely passive with only a single power amplifier required! This is
what’s done with the CBT36 with the exception that two power amplifier channels are required for each
speaker for the LF and HF bi-amplification.
Secondly, the circular shape guarantees circular constant-phase wave fronts in the vertical plane from
points very near the array to points very far away. This means that the vertical coverage of the array is
essentially independent of distance! The coverage of a CBT circular-arc array is so uniform that it
essentially has no near field. Its frequency response is the same at 3 inches away from the surface of the
array as it is at 10 feet away!
Thirdly, the circular shape of the enclosure dramatically increases the strength of the enclosure and
allows thinner materials to be used for the front and back panels which must be bent to conform to the
cabinet shape. The thinner front panel allows the front to be easily bent around the front of the enclosure
even with all the drivers attached. All though thinner, the cabinet still will be much stronger than if the
cabinet were constructed with thicker materials but not be curved.
14.2.10. CBT36 Beamwidth vs. Frequency:
The following graph shows the simulated above-ground-plane beamwidth of the CBT36 with frequency.
Beamwidth is the angle at which the level (SPL) drops 6 dB from a reference direction
(http://en.wikipedia.org/wiki/Beamwidth, note however that for antennas the level drop is defined as 3 dB).
Note how extremely uniform the CBT36 beamwidth is for frequencies above 200 Hz! For the vertical
beamwidth, the reference direction is a line on the floor. Vertically, the graph indicates that the level drops
by about 6 dB at an angle of about 27° above the floor. This narrow vertical coverage greatly minimizes
energy bounced off the ceiling.
Fig. 39. Simulated beamwidth vs. frequency of the CBT36. The graph shows that the
horizontal coverage of the CBT36 is essentially omnidirectional over the range of ±90°. The
CBT36 vertical coverage however is tightly controlled above 200 Hz at a beamwidth between
25° to 30° degs above the ground plane.
