Zenith D952X - Page 22

SECTION FOUR
THEORY AND APPLICATION
ALLEGRO SPEAKER SYSTEM - MODEL D9013W
Speaker system, Model D9013, is of the bass reflex type.
Figure 1 is the schematic, while Figure 2 illustrates the cabinet
design. As you will note, the enclosure has two openings in the
front, one for each speaker. In the past, most bass reflex
enclosures had a port on the front, with the remaining walls
being solid. Model D9013, as illustrated in Figure 2, has a port
in the rear panel of the enclosure, instead of the front.
It is interesting to note that a bass reflex speaker enclosure can
be described as a tuned enclosure in which air in the port will
resonate with the air in the main area of the cabinet, at a given
frequency. This frequency determines the effective low fre 
quency cutoff of the system (cabinet and enclosure com 
bined). Below the selected frequency (45 Hz in the D9013W)
the response drops very rapidly (approximately 18 db/octave).
It could also be described as an acoustical phase inverter. That
is, at some frequency, within its normal operating range, the
air in the port is moving in an outward direction (to the back),
while the speaker cone is also moving in an outward direction
(to the front). These two movements would occur at the same
instant.
A basic advantage of the bass reflex system, as compared to a
closed box enclosure, is that in a bass reflex system the
designer can either extend the frequency response, while
maintaining the same level of efficiency; or retain the
frequency response while raising the efficiency approximately
3 - 4 db. In designing the Allegro, it was decided to improve
the frequency response beyond that of an earlier model
(S9017).
Sound level of signals radiated thru the port (in the 40-70 Hz
range) is comparable to the sound level radiated by the woofer
(in the range of 70 Hz to 1 kHz). This requires that the 2-5/8 ”
diameter port must “ pulsate ” air at a much higher velocity
than the woofer, which is approximately 8 ” in diameter.
Several factors must be considered in order to maintain the
required port velocity.
A. The woofer uses a highly efficient magnetic structure
making it comparable to a powerful electrical motor. This
forces air, in the port, to move at high velocities, even
though the air in the box is attempting to stop motion of
the speaker cone.
B. The internal air pressure in a bass reflex enclosure is much
higher than that found in a conventional closed-box
enclosure, therefore mechanical construction of a bass
reflex enclosure must be more substantial than that of an
air suspension type. All panels, walls and joints must be
extremely rigid to withstand these air pressures.
C. Speakers (and other components) must be securely fastened
to prevent air leaks. Leaks or loose components can result
in losses which cause a deterioration of performance.
In order to effectively increase cabinet volume, and also to
dampen internal resonances of the enclosure, three acoustic
pads of variable density are placed adjacent to the internal
walls. The area of each pad that has the highest density is
adjacent to the wall surface. These pads must not obstruct
the port.
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FIGURE 1 - MODEL D9013 - SCHEMATIC