VerTec® Subwoofer Applications:

VerTec® Subwoofer Applications:
JBL
PROFESSIONAL
Technical Note Volume 1 Number 34
JBL's New Differential Drive® Transducers for
Ver Tec” Subwoofer Applications:
Introduction and Prior Art:
JBL's 18-inch 2242H low frequency transducer represents the culmination of a design
philosophy that dates back to the early days of the company. The original Alnico V
magnet material was eventually replaced by ferrite material, and the motor structure now
incorporates both SFG (Symmetrical Field Geometry) and VGC (Vented Gap Cooling).
All moving parts have at various times have been improved in terms of power handling
and reliability, and adhesives have undergone radical improvements. After benefiting
from incremental changes, the 2242H as it stands today is a workhorse for sound
reinforcement applications, and its 800-watt continuous pink noise power rating places it
in the top ranks of that market.
Unlike installed-venue applications where systems are permanently set in place and
where the sheer mass of a system is of little concern, the tour sound industry carries a
large weight penalty. Because of this requirement, tour sound systems have been designed
to be both rugged and as low in weight as JBL can design them. This is especially true for
line array applications, where there may be large numbers of loudspeaker enclosures in a
single array. The global rise in popularity of the line array has required us to re-examine
our basic technology, and one result of this was the Differential Drive motor structure.
The Rise of Differential Drive:
Light-weight Differential Drive transducers were introduced by JBL in the mid-90s to
take advantage of size and weight scaling and to achieve high performance with high-
energy neodymium magnets. The technology was first applied to the EON series of
portable loudspeakers and later to high-efficiency midrange transducers for the horn-
loaded HLA tour sound system.
The new magnets (technically NdFeB, or neodymium-iron-boron sintered structures),
have a very high value of remanent induction, and as a result only relatively small
amounts of the material are required to produce a desired magnetic field strength in
the gap. This translates directly into small light-weight magnetic structures. The high
remanent induction also permits the transducer engineer to design a motor structure that
has two gaps in magnetic series in which two coils can be placed, while at the same time
retaining a sufficiently high magnetic flux for both coils. Details of the basic design are
given in JBL Technical Note Volume 1 Number 33 (copy available on-line at www jblpro.
com).
The primary result using this new material has been a much smaller coil assembly
producing twice the mechanical force of a single coil. In fact, a pair of 2-inch coils
operating in this manner would be equivalent to a single 4-inch coil, provided magnetic
flux density, total amount of conductor in the gap, and displacement limit are all identical.
A Family of Transducers Designed Specifically for Subwoofer Applications:
Figures 1A and 1B show section views of 2242H and JBL's new 2269H Ultra Long
Excursion drivers at the same scale. Note that both have 4-inch voice coils, but with its
dual coil arrangement, the 2269 has twice the voice coil length immersed in an equivalent
magnetic field.
The cone in the 2269 has greater mass than that in the 2242, and the total peak-to-peak
stroke is longer, as is the linear portion of the magnetic field. Figure 2 shows the 2242
versus the 2269 pressure output with 80 watts fed to each transducer at a di stance of
1 meter. As seen in the figure, the 2269 has 2 to 3 dB lower response in the 80 to 250
Hz region than the 2242, but has increasing response below 80 Hz. At 30 Hz, the 2269
delivers an impressive 6 dB more output than the 2242 for the same input power!
Output power alone is not the only aspect of subwoofer performance to be considered.
Overall system fidelity is also a key aspect, especially in music reinforcement
applications. Considering distortion, the data in F igure 3A shows that at 30 Hz the
level of second and third harmonic distortion for the 2269 is about 35 dB lower than
the fundamental. As seen in Figure 3B, for the 2242 the 30 Hz distortion with the same
drive signal is about 15 dB lower than the fundamental. (Note that distortion levels in
the graphs have been raised by 20 dB for display purposes in Figures 3A and 3B.) One
2242H Section View
Voice coil leads Dust dome Cone
Input — =
terminals Voice coil
Ferrite > E
magnet — ~~ Shorting ring
Magnetic a Cooling vent
circuit
2269H Section View
Cone Dust dome Leads for front coil
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A 7 À If | “Back voice coil
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magnets у a CET
A
7 “
Input terminals
Figure 1. Section view of drivers. JBL 2242 (A); JBL 2269 (B).
3
2242 and 2269: 80 watts input (at 1 meter)
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2242
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100 200
500 1K
Frequency (Hz)
2K
Figure 2. À comparison of on-axis output of JBL 2242H and 2269 drivers at
80 watts input.
can clearly see that the 2269 has been designed primarily as a subwoofer driver for
applications where it is used in the range of 20 to 25 Hz up to about 100 Hz.
Figure 4 shows the relative output levels of each of these drivers when normalized for
the same amount of total harmonic distortion at 30 Hz. Note that at 30 Hz the 2269 has a
clear 10-dB advantage over the 2242.
VerTec Subwoofer System Applications:
All VerTec subwoofer models feature Differential Drive transducer technology and have
been specifically designed for use with VerTec VT4887A, VT4888, and VT4889 full
range line array elements.
The VerTec VT4880 subwoofer, a full-size arrayable system element, uses the 2258
Differential Drive transducer (a modern dual coil version of the 2242), while the more
recent model 2269 driver, shown in Figure 1B, is used in the VT4881 A compact and
VT4880A full size models, both recent additions to the VerTec system product family.
Figure SA shows a section view of the 2258, while F igure SB shows the on-axis response
4
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1k
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3rd
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Fundamental (at 1 m)
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Frequency (Hz)
2242H driver at 80 watts (B). (Distortion
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Figure 3. Fundamental and distortion response of 2269H driver at 80 watts (A);
fundamental and distortion response of
raised 20 dB)
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135 JBL 2269 and 2242 drivers at relative levels for same harmonic distortion
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7 Nog | Deep ine 2242 distortion ( —
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95 1775
20 30 40 50 80 90 100 200
60 70
Frequency (Hz)
Figure 4. A comparison of output levels of JBL 2242H and 2269H drivers
at the same distortion level.
at 10 dB below rated power along with 2nd and 3rd harmonic distortion (As previously
noted, the distortion components have been raised 20 dB for display purposes.)
Basic Design Parameters:
Table 1 shows selected Thiele-Small parameters for the 2242, 2269, and the 2258 drivers.
The 2242 and 2269 have 4-inch voice coils while the 2258 has 3-inch coils.
T-S parameters 2242: 2258: 2269:
Moving mass: 158 grams 168 grams 294 grams
О, 0.275 0.36 0.36
9 mm 10 mm 19 mm
AES Rated power: 800 watts 1200 watts 2000 watts
Free-air resonance: 35 Hz 33 Hz 28 Hz
Table 1. Properties of three 18-inch low frequency drivers.
2258H Section View
A Cone Dust dome
\ | Z
= 7
Input
terminals
x,
A i
— E <=
Magnetic + 1h > Front voice coil
circuit 7
NIB Back voice coil
magnets
B
130
125
120 Fundamental (at 1 m) A
~~ ~~ L
115 TTY
wd a
о N pd
m 110
o
A À
105
A harmonic A
à A 1 } A
2nd MAN no HO
= Ё t + Ei t
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re Y | y AA * i ! Y i 1
“. i # 4 % I i
\ И NS i ! 1 р Eo | / \ 7 | ! i f | N : ; ! ‘ vi
>. / af EUA / Yi \ } Ed fi i $
= р HP a IE e y 4 | y 4 A A в i
90 1 17 Le : (Lv / ‘ ZE E 1} 4 fr :
20 50 100 500 1k 2k
0
Frequency (Hz)
Figure 5. Section view of the JBL 2258H driver (A); fundamental and distortion
response at 80 watts (B). (Distortion raised 20 dB)
The value of Q_ indicates the degree of damping in the overall electroacoustical
performance of a driver. A lower value of Q, indicates a higher degree of damping,
while a higher value indicates a lower value of damping. For subwoofer applications, we
want to increase the driver's low frequency output without rai sing the level of its driving
signal, and increasing Q, is one direct way of doing this.
The value of x... indicates the maximum amount of linear displacement (on one side of
the rest position) that the driver can safely accommodate. In subwoofer desi gn we need
as much linear displacement capability as we can get in order to achieve hi gh levels at
very low frequencies. The difference between the 2258 and 2269 drivers is almost double
the linear displacement — a significant design breakthrough.
The rated power handling capability of the 2269 is 1.5 times that of the 2258, accounting
for a 2-dB increase in output capability, relative to the 2258.
The lower free-air resonance of the 2269 relative to the 2258 contributes to a lower
tuning frequency alignment for a given enclosure volume — an important factor in
maintaining a workable enclosure size.
All of these factors, and others, play off against one another in the actual design process,
and the designer's primary job is to optimize all of the trade-offs.
VT4880 and VT4880A System Performance Differences:
Response curves of the two dual 18-inch models are shown in F igure 6, where the
bold curve shows the response of the VT4880 and the dashed curve shows that of the
VT4880A. For line array applications, subwoofers are normally crossed over at about
80 Hz, and the increase in low frequency response smoothness and extension of the A-
version is readily apparent.
It should be noted that the VT4880 and VT4880A both use the same-sized enclosure, and
the interior parameters of the central port aperture are identical.
VT4880 and VT4880A ground-
plane curves run at 2.83 V, shown
lowered 3 dB for 1 W sensitivity
Solid curve: VT4880
Dashed curve: VT4880A
Frequency (Hz)
Figure 6. A comparison of VT4880 and VT4880A system fundamental output over the frequency
decade from 20 to 200 Hz. (Ground plane measurements adjusted for 1-watt sensitivity)
Compared to the VT4880, the mechanical and acoustical differences are:
VT4880: VT4880A:
Frequency range (-10 dB): 26 - 160 Hz 25-160 Hz
On-axis response (+3 dB): 28 - 75 Hz 28 - 120 Hz
Sensitivity (1 W @ 1 m): 98 dB 95 dB
Nominal impedances: 2 x 8 ohms 2 x 8 ohms
Continuous power rating; 2400 watts 4000 watts
Table 2. Properties of VerTec VT4880 and VT4880A models
However, frequency range and response information given above does not tell the
whole story. With 2269H woofers, when arrayed, the VT4880A makes a much stronger
statement between 20 - 40 Hz than does the VT4880, and it can be driven harder (with
more power applied), effectively boosting the "foundation octave effect” even more than
what would be observed on a side-by-side enclosure or group of enclosures loaded with
2258H woofers.
If both 2258-loaded and 2269-loaded enclosures are driven with the same bandpass
signal (to 80 Hz) at moderate levels, the enclosure loaded with 2269H drivers will appear
9
louder due to increased power in the first octave. At the same time, if the subwoofer's
low pass filter settings are raised (above 80 Hz to 100 Hz, for example), the enclosure
loaded with 2258H's will seem to get louder or " punchier" at moderate levels due to their
increased driver sensitivity above 80 Hz, as compared to the results observed with the
2269H's. As significantly more power is applied to each system, the VT4880A will once
again appear to be "fuller" or louder, due to its ability to generate sustained high-volume
air movements at higher power levels. At the same drive level the VT4880 enclosure
will begin to exhibit power compression, and cone travel mechanical limits will become
evident.
In summary, 2269 Ultra Long Excursion woofers mounted in VT4880 enclosures will
work very well, essentially the same as in VT4880A enclosures. The VT4880 and
VT4880A have different frequency response profiles and sonic characteristics, providing
system designers flexibility and options when determining what kind of subwoofer and
low bass performance characteristics may be desired for specific musical reasons. The
user now has the choice between added "punch" above 60 Hz or low frequency extension
below 60 Hz, as needed.
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