Understanding Line Array Systems
By
John Murray
Behind the buzz, there are a lot of factors at work in line arrays. An explanation and comparison of current models.
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At last count, I found at least 19 companies offering line array loudspeaker systems that are more than simple column designs. Rather than discussing over a dozen different product types, I thought we might approach the subject by defining the technological terms of line arrays. This way, we get a better grasp of the issues involved with line array systems and will be able to discern both the similarities of, and unique differences between, the products being supplied by manufacturers today. |
This discussion can’t be contained in just a few paragraphs, so
we must start with the more basic issues of line arrays and then follow
with more esoteric topics that build on these basics.
A LITTLE HISTORY
Line arrays have been around for over a half of a century as column speakers,
and other than those made by Rudy Bozak here in the US, most were voice-range
only. Their application was generally for highly reverberant spaces, where
a narrow vertical dispersion avoided exciting the reverberant field, provided
a higher Q (narrower dispersion pattern) and, as a result, improved intelligibility
of the spoken word.
Never losing popularity in Europe as they did in America, it’s no
wonder that L-Acoustics V-DOSC loudspeakers from France were the first
to show the concert sound world that more level and smoother frequency
response can come from fewer drivers in a line array. After everyone realized
that for a given listening area, the drivers have no destructive interference
in the horizontal plane and combine mostly inphase in the vertical plane,
the race was on.
CYLINDRICAL WAVEFORM
Basically, a line of sources will create a wavefront of sound pressure
that is loosely cylindrical in nature at a particular range of wavelengths
(frequencies). Its idealized shape is actually more like a section of
a cake, and the wavefront surface area, as it expands only in the horizontal
plane, doubles in area for every doubling of distance. This equates to
a 3dB SPL loss of level for every doubling of distance.
SPHERICAL WAVEFORM
An idealized point source, imperfectly represented by a loudspeaker or
nonlinear cluster of loudspeakers, radiates in a spherical waveform rather
than cylindrical. This wavefront expands to four times the area with each
doubling of distance, which equates to a 6dB SPL loss for every doubling
of distance. This is commonly known as the inverse-square law, and it
applies to all point-source radiant energy. Hence the big advantage for
a line array is that for a given number of transducers, the resulting
long throw level can be much greater than for a non-line array, or point-source,
loudspeaker system.
INTERFERENCE PATTERN
This is the term applied to the dispersion pattern, or response balloon
of a line array. It simply means that when you stack a bunch of loudspeakers,
the vertical dispersion angle decreases because the individual drivers
are outof- phase with each other at positions off-axis in the vertical
plane. The taller the stack is, the narrower the vertical dispersion will
be and the higher the sensitivity will be on-axis. In the horizontal plane,
an array of like drivers will have the same polar pattern as a single
driver. Some believe that the horizontal pattern is wider than for a single
driver, but they are mistaken, likely fooled by the fact that the level
is louder off to the side due to the higher sensitivity of multiple drivers.
However, the actual polar pattern remains the same as for a single driver.
ARRAY LENGTH
In addition to the narrowing vertical coverage angles, the array length
also determines what wavelengths will be affected by this narrowing of
dispersion. The longer the array, the lower in frequency (longer in wavelength)
the pattern control will occur.
CRITICAL DISTANCE
There is a limit to the 3dB per doubling loss, and it’s at this
point where the array is far enough away to appear to be more of a point
source and its level begins to attenuate according to the inverse-square
law at 6dB per doubling of distance. The transition between these two
regions is known as the critical distance for the line array. The region
closer than critical distance, and the region beyond it, is termed as
the Fresnel and Fraunhofer regions, respectively, so named by Christian
Heil of L-Acoustics. Unless you’re a true math dweeb, near-field
region and far-field region roll off the tongue a bit easier.
The critical distance for a given line array length varies inversely with
wavelength (frequency). This was also discussed in depth in the last issue.
Shorter wavelengths (higher frequencies) have much farther critical distances
than longer wavelengths (lower frequencies). In theory this means, at
greater distances, a line array will maintain more high-frequency content
than low. However, air attenuation of the highs will counteract this characteristic.
ARTICULATED ARRAYS
Articulated is the ten dollar term for curved. This describes the very-popular
J-Array shape that most manufacturers currently offer, save one. To date,
the Duran Audio Intellivox system is the only line array that covers from
extreme near-field to far-field seating with a straight-line dead-hang
approach. (Talking about articulated arrays with your clients is what
gets your day rate increased and your job title changed from “sound
tech” to “audio engineer.”)
SPIRAL ARRAYS
This is also a term for curved arrays of a particular type. Spiral arrays
describe a curve that is increasing in the rotational angle from one end
to the other, just as the common J-Array does from top to bottom.
ARITHMETIC SPIRAL ARRAYS
Mark Ureda, consultant to JBL, mathematically determined that spiral arrays
that increase their angle of curvature in even increments perform better.
For example, at the top of a line array, the splay between cabinets is
0 degrees. Going down the array, the element boxes are successively splayed
at 1 degree, 2 degrees, 3 degrees, etc. Or it could go in increments of
2 degrees (i.e.: 2 degrees, 4 degrees, 6 degrees, etc.). These are arithmetically
increasing spiral arrays.
LOBES
Lobes describe all the acoustical energy that emanates from a loudspeaker
or group of loudspeakers. The specified coverage angle of a horn is its
main lobe. Spurious lobes are those that emanate out in a non-useful direction
from the source.
STEERING LOBES
Much ado has been made about lobe steering. Visions come to mind of FOH
guys moving loudspeaker coverage around with a joystick. Lobe steering
is generally done by incrementally delaying drivers in a line array. This
can only be done when the sources, (the drivers), are about 1/2 wavelength
apart for a given frequency, and only in the direction of the line array’s
axis. For typical live sound HF drivers with a 9-inch diameter, this means
that they cannot be positioned close enough together to steer anything
above 750 Hz. However, using adaptive apertures to mimic a long line of
smaller sources enables some steering at shorter wavelengths.
SIDE LOBES
Side lobes are artifacts of line arrays. They are called side lobes but
actually emanate from the ends of the array, at the top and bottom, as
a typical line array is viewed in use. They are caused by the individual
elements being in-phase at a particular angle and wavelength at some off-axis
position from the array’s main lobe. It is possible to eliminate
side lobes, but there are limits and consequences to side-lobe elimination
in line arrays.
GRADIENT SIDE LOBES
This is a synonymous term for side lobes. Gradient describes how these
lobes occur at particular angles or grades with respect to the line array’s
orientation. Professional progress terminology tip: use gradient side
lobes rather than side lobes in your technospeak. Chicks dig it.
DRIVER SPACING
Another of the fundamental parameters of line arrays is the spacing between
individual elements. The accepted limit is that for good line array behavior,
the sources should be no more than 1/2 wavelength apart for a given frequency.
This means that loudspeakers reproducing longer wavelengths can be spaced
farther apart without any deterioration in performance. But since 1/2
wavelength at 15 kHz is just under one-half of an inch, HF devices can
never be close enough. One manufacturer maintains that because of this,
line arrays do not really work at very high frequencies. However, I disagree,
because even at very short wavelengths, the 3dB loss per doubling of distance
still holds true, and this is what defines the line array effect. (In
my humble opinion.) What does result from driver spacing of more than
1/2 wavelength is more pronounced gradient side lobing.
LOGARITHMIC DRIVER SPACING
Duran’s Intellivox Series line array loudspeakers employ the logarithmic
driver spacing technique. This provides denser driver spacing at short
wavelengths and economizes on the number of drivers needed for longer
wavelengths by spacing them in larger and larger logarithmic increments.
ISOPHASIC APERTURES
Isophasic aperture is my current favorite high-tech term. It describes
the phase characteristic of the slot that loads the horn bell of
some line array box HF sections. The perfect line array driver,
particularly for very short wavelengths, is a ribbon driver like
those used by SLS Loudspeakers. Compression drivers are more rugged
and capable of higher output levels than a ribbon driver, but they
do not have a linear phase signal at the mouth of a horn.
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Ideally, the signal at both the top
and bottom of the driver’s horn mouth would arrive in-phase
with the signal at the center of the horn mouth to mimic the ribbon
driver’s characteristic. Since the center of the horn is closer
to the driver’s diaphragm than the top and bottom, the more
central paths to the horn from the driver must delay the signal
to arrive in phase with the longer paths to the top and bottom of
the horn. There are two ways to accomplish this. |
Perhaps the most interesting technique for an isophasic device is the
patented mid-high frequency aperture by Adamson. It employs the longer
path length method, and utilizes directional vanes to prevent excess vertical
dispersion as well. This approach is used for both the highand mid-frequency
sections of their line array systems. The mid-frequency energy exits via
two vertical slots on either side of the high-frequency exit slot. The
paths of the mid-frequencies curve around the HF chamber housing. All
slots are isophasic.
With the slots of the MF section on each side of the HF slot, diffractional
problems of each slot on the other could be very problematic. However,
Brock Adamson came up with a unique solution: overlapping the crossover
points between the mids and highs. This provides in-phase pressure
fronts from the other slots to prevent diffractional interference
in the frequency range where it would be a problem.
FREQUENCY TAPERING
The term “tapering” is also commonly called “shading.”
They are essentially interchangeable. One of the first tricks used
to take advantage of the line array effect was frequency tapering.
My earliest exposure to this technique was the Electro-Voice LR-4B
column speaker. For low/mids, it used 6-inch by 9-inch cone drivers
that had lowpass filters at successively lower frequencies for speakers
placed farther out to the ends of the column. This resulted in a
longer column at longer wavelengths and a shorter column at shorter
wavelengths, producing a similar dispersion pattern and critical
distance for all frequencies, which in turns provides a more balanced
frequency response at all listening distances.
AMPLITUDE SHAPING
Another tapering/shading technique is amplitude shading. This is
used in many current line array products to accomplish front fill
coverage where the bottom hook of a J-Array covers the extreme near-field
listeners. This technique is simply lowering the volume of the loudspeakers
covering the nearfield seating with respect to the longthrow loudspeakers
higher in the array.
DIVERGENCE SHADING
Some line array systems offer more than one choice for vertical
dispersion of the individual box elements in the array. They do
this as a solution to cover the near-field and extreme nearfield
seating in most venues. EAW has gone one step further by offering
two different models, matching the vertical dispersion and output
level so that the drivers produce equal mouth SPL throughout the
array. They avoid any amplitude shading for the drivers covering
the closer listeners by increasing the coverage angle of those box
elements. Why is it important to avoid amplitude shading?
According to David Gunness, EAW director of research and development,
whenever two wave fronts with different pressures are combined,
there will be a discontinuity at the juncture of the two. This discontinuity
will be audible as though it were a separate, non-coherent source
(delayed loudspeaker). The result is transient smear and uneven
frequency response. Divergence shading provides a wave front whose
curvature varies, but whose pressure magnitude does not. Therefore
there is no introduced time smear to the signal.
HORIZONTALLY SYMMETRIC ARRAYS
The majority of available line array systems are horizontally symmetric.
Ideally, each band pass is a 1/2 wavelength wide strip that runs
the entire length of the array. The advantage is that it avoids
horizontal lobbing at the crossover-frequency band. It also requires
symmetric pairs of inner mid and outer LF drivers flanking the HF
sophistic ribbon.
The drawback to this approach is that for the mid-drivers to be
within 1/2 wavelength of each other, they must be incorporated into
the bell of the HF horn. The normal 90-degree angle causes reflections
between the MF drivers and the discontinuous horn walls cause HF
problems as well.
HORIZONTALLY ASYMMETRIC ARRAYS
EV, Meyer (on their smaller system), and NEXO have opted for an
asymmetric design. This approach avoided the mid-frequencies in
the horn bell problem and contends with the horizontal lobbing at
crossover problem inherent in asymmetric designs. Choose your poison.
CARDIOID AND HYPERCARDIOID LF SECTIONS
Line arrays have great directional control in the vertical axis. Subwoofer
systems, by nature of the very long wavelengths involved, do not have
any directional control unless arrayed.
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Even then, because of the omni-directional
nature of each element in the array, there is no front-to-back directionality.
This causes muddiness on stage and low-frequency feedback problems.
Enter cardioid and hypercardioid low-frequency sections. |
FIR-BASED VS. IIR-BASED DSP FILTERING
IIR (Infinite Impulse Response) filters in a DSP processor act just like
analog crossover and equalization filters. Their amplitude and phase characteristics
are in a fixed relationship. So much boost or cut produces an exact corresponding
change to the phase response.
FIR (Finite Impulse Response) filters are able to manipulate phase independently
of amplitude and correct for distance-related cancellations between drivers
if each driver is under individual DSP control. Some systems, like Intellivox,
employ separate DSP processing and amplification for each driver in the
array. These types of systems will define the next big step forward in
loudspeaker technology.
GET LUCKY?
So, the next time you want to impress the ladies at the local hall, tell
‘em “We’re gonna hang a logarithmicspaced, articulated
spiral array in a horizontally asymmetric configuration employing frequency
tapering and divergence shading, which will include isophasic high-frequency
and mid-frequency apertures, hyper-cardioid low-frequency transducer sections,
is controlled by finite-impulse response filtering digital signal processing,
and works well with a psychoacoustic infector.” You might just get
lucky…
Live Sound Technical Editor John Murray is a 26-year industry veteran
working for EV, Midas, MediaMatrix and TOA. John has presented two AES
papers, chaired three Syn-Aud-Con workshops and is a member of the TEF
Advisory Committee and ICIA adjunct faculty. If you have a question you’d
like to ask John, e-mail him at jmurray@livesoundint.com
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