Roundtable: Inside The Approach
JBL’s development team talks about line array concepts
Pairs of VerTec VT4887 compact line array elements suspended beneath
large arrays of the full-size VT4889 elements. (Radio City Music
Hall, December 2002, Christmas Music Spectacular, ProMix Inc.)
As we’ve met up at various trade shows, the JBL engineering team
and I have had some interesting discussions regarding line array
design and concepts.
We decided to present some of these ideas to you, from the folks
directly responsible. So let’s check in with JBL Vice President
of R & D Doug Button and Director of Tour Sound Engineering Mark
Engebretson about line arrays in general as well as the company’s
VerTec Series. Keith Clark
Live Sound: At the risk of sounding like Jerry Seinfeld, “What’s
the deal with line arrays?” In other words, provide us with an overview
of your particular perspective of the topic.
Mark Engebretson: Line array systems for larger-scale PA applications
are a relatively recent market trend. While the same acoustical principles
govern the performance of all such design attempts, obviously not all
line array system designs that end up seeing the light of day as finished
products are the same.
There are widespread misconceptions about how they work and perform.
Separating fact from fantasy requires taking acoustic measurements under
controlled conditions, and an application of scientific principles to
interpretations made of the resulting data.
Figure 1: Comparison of relative height of a four-box array (compact,
midsize and full-size line array elements), showing distance to
near field boundary at 4 kHz
Prior experience in the setup, optimization and use of traditional
fan-shaped multi-box arrays does not translate directly to line
array systems. Line array deployment and operation is affected by
variables in array size, box splay angles, and the input signal
It’s important the choices made in these realms are based upon reliable
work that has been scientifically developed. Such an approach will
have more validity than either assumptions, or hearsay.
Live Sound: Yes, but aren’t there some basic acoustical truths
that govern the behavior of line arrays? Are all line array systems really
Doug Button: In some ways they are similar, and in other ways they
can differ. In the 1930’s, acoustical pioneer Harry Olson showed us the
narrowing of the beam with increasing frequency of a true line source,
as defined in the classical sense.
Figure 2: Schematic of a typical line array enclosure integrating
MF and HF elements.
At the highest frequencies the beam becomes so narrow as to be
pretty useless for audience coverage. The first inclination is to
curve the array to get better coverage. But Olson clearly showed
the pattern control versus frequency of that is not terribly consistent.
This suggests that neither solution (straight, or evenly curved)
So when we create a high-powered full-bandwidth vertically oriented
system, further array modification through controlled, incremental
articulation is required to provide more ideal coverage.
Live Sound: So the current generation of line array systems are
basically just large, curved speaker columns? And aren’t the recent smaller
line array offerings, like JBL’s VT4888 and VT4887, scaled-down versions
Doug Button: Today’s so-called line arrays are multi-box columns
that purport to simulate a line source. They offer wider bandwidth than
early vocal-range speaker columns. But, low frequencies behave differently
than high frequencies.
If what you want is better performance, the acoustical elements and transducers
you design into that multi-box column, and their geometric arrangement,
determine what the system is, and is not, capable of just like the type
of engine “under the hood” really does matter in an automobile, when you’re
driving it on the highway instead of listening to a car salesman.
Figure 3: Horizontal HF performance, shown 0 degrees to 60 degrees,
with no bandpass integration devices fitted to the system.
If you want viable performance in more compact arrays, you can’t
skimp on the components just because the boxes get physically smaller.
Plus, as we “shrunk VerTec,” we faced a number of mechanical challenges
that led to the development of purpose-built transducers to achieve
our desired goals.
Live Sound: So when you were looking to scale down VerTec, what
factors came into play?
Mark Engebretson: First we had to evaluate the advisability of scaling
down both our acoustical and mechanical technologies. There are clearly
applications for smaller arrays that contain the appropriate number of
articulated elements. As with full-size line array elements like the VT4889,
the challenge for system designer and user alike is to understand how
the resulting array will work.
A graphic can illustrate the principle of “near field extension” with
three VerTec system enclosures. (Figure 1) At 4 kHz, the on-axis far field
begins at 23 meters (75 feet) for an array of four full-size VT4889s.
For the same number of midsize VT4888s, which has an enclosure height
of about three-fourths that of a VT4889, the far field begins at slightly
greater than half the distance, or 12 meters (38 feet).
Similarly, using four compact VT4887s will produce 4 kHz far field characteristics
at 7 meters (23 feet). So “extending the near field” isn’t always desirable.
Since we always want our audience to be in the far field, yet close to
the near field, the choice of what size enclosure to use might be determined
by audience area and venue size.
Live Sound: About the near field versus far field... What about
the reported sound level loss of only 3 dB for each doubling of distance
with line arrays? How do you rationalize this in the design of your compact
Doug Button: Much has been written about the near field versus
the far field. It’s been purported that line arrays create a cylindrical
wave front that at some point in space, transitions to a spherical wave.
This description of wavefront behavior in a line source is one many can
easily visualize, but it’s largely an over simplification with some underlying
A more accurate way to describe the so-called near-field region is that
it’s an interference field. According to classic line array theory, look
at the sound pressure level (SPL) versus distance of a line source 4 meters
(approximately 12 feet) tall. This shows a transition to the near field
at about 100 meters (300 feet) at 10 kHz, 10 meters (30 feet) at 1 kHz
and 1 meter (3 feet) at 100 Hz.
Figure 4: Horizontal HF performance, shown 0 degrees to 60 degrees
this time with R.B.I.s in place.
If we move toward the array, SPL doesn’t increase as much at higher
frequencies. This is because the near field is actually full of
So the notion there is some form of gain that ‘projects’ the near
field and ‘increases’ the high frequencies is misleading. The SPL
drops off more slowly with line arrays because the interference
diminishes as we move away from them.
Live Sound: O.K. regardless of size, how were your basic line
array designs derived?
Mark Engebretson: A logical configuration for multi-band line array
elements in a horizontally oriented enclosure was suggested by Joseph
D’Appolito in 1983. His configuration placed the high-frequency section
in the middle of the system, flanked on either side by devices that cover
frequencies immediately below.
About 1992, Christian Heil introduced a variant of this D’Appolito configuration,
using compression drivers for high frequencies. Subsequently, L’Acoustic
began to market systems based on this design. An additional twist with
these was the addition of midrange devices to reside inside a waveguide
that was energized by a slot-aperture high-frequency section. This allowed
the effective lateral dimension of the midrange devices to be smaller,
enabling wider horizontal dispersion. (Figure 2)
Figure 5: Cross-section of a NDD loudspeaker motor. Note the long
voice coil former to accommodate the dual voice coils fitted to
the twin-gap magnet design.
To further this idea and offer a degree of additional control,
all of our models include a pair of R.B.I. units (Radiation Boundary
Integrators). They’re fitted over the midrange devices and serve
several functions, not the least of which is to smooth the transition
from the output of one set of bandpass devices to another.
The accompanying figures show how the addition of these units to
the system resolves some significant issues. This is especially
important in the critical mid- to high-frequency crossover region.
(Figure 3 and 4)
Live Sound: JBL continues to develop its own transducers, which
is now somewhat a rarity. Can you talk about the advantages of this approach,
and perhaps more specifically, the role your dual voice coil design plays
in the VerTec design?
Doug Button: The intent of the dual voice coil is to increase power
handling, lower power compression, resulting in more dynamic headroom.
But it’s not as simple as it might seem, just adding an extra voice coil.
Like many other inventions, one can wonder, if it’s seemingly so simple
and offers obvious benefits, why isn’t everybody doing it? As early as
the ‘50s, patents appeared describing a loudspeaker with two voice coils
wound in opposite directions, spaced apart axially, and connected in series
on the same coil former.
Figure 6: JBL 2435 beryllium-diaphragm neodymium-magnet compression
driver, performance contrasted with another available compression
Although early designers may not have directly considered it, two
coils with twice the surface area can handle twice the input power.
This idea appears to have been ahead of its time, as the early designs
never really found commercial applications.
It’s also important to mention that once we had this basic motor
design worked out, we were able to scale it up in size... or down.
For VerTec, we’ve developed cone drivers ranging from 8-inch to
12-inch models up to 18-inch low-frequency elements used in different
VerTec systems. (Figure 5)
Live Sound: And what challenges were faced with the high-frequency
Doug Button: Most compression drivers have near-maximum theoretical
efficiencies in the midrange, but usually fall short at high frequencies
due to excessive mass in the mechanical assembly.
Because all large-format (Ž 3-inch) domes have break-up modes in the usable
bandpass region, it’s possible to have efficiencies greater than the theoretical
envelope at higher frequencies by designing in resonant behavior. We sought
high efficiency and smooth response, with no ringing, and true pistonic
motion to at least 15 kHz.
Figure 7: Laser-scanned image of the 2435 drivers 3-inch diaphragm
(left) at 14.5 kHz compared to displacement of a titanium diaphragm
at the same frequency and input voltage.
Further, we wanted to balance the various parameters to achieve
maximum efficiency in a specified bandwidth. We chose 3 kHz to 20
kHz as the bandwidth we wished for maximize efficiency. Using our
controlled testing, it’s interesting to see the performance of this
driver, called the 2435 (Figure 6).
It’s also useful in gaining more understanding to see data on the 2435’s
diaphragm under load. (Figure 7) You’ll see that it exhibits pistonic
characteristics, even at frequencies as high as 14.5 kHz. However, the
traditional diaphragm has moved into a serious break-up mode.
So we ended up with a more robust design that has less harmonic distortion.
We’ve taken all that we learned on the HF section for the full-size VT4889,
and applied that to the newer drivers used on the midsize VT4888, for
Live Sound: What’s next from JBL in line array systems?
Mark Engebretson: It’s no secret that we’re working with Crown
and dbx on the DrivePack series of electronics modules, which will enable
our line array system products to be self-amplified with integrated digital
signal processing. Beyond that, let’s just say our R&D program continues
to take input from professional users regarding the challenges they face
in portable sound reinforcement.
September 2003 Live Sound International