Factory Direct: The EAW 760 Revealed
The curved source - more than a line array, it’s
all in the splay
This Factory Direct was submitted from the loudspeaker manufacturer Eastern Acoustic Works. LSMAG! makes every effort to eliminate any use of marketing inspired hyperbole. Just the facts, is all we ask.
The concert touring EAW KF760 Series would commonly be called a “line
array” system. It is, however, designed and acoustically defined as a
curved source. The KF760 Series includes two products; the KF760 and KF761.
For concert applications, the KF760 is used for short to long throws (70to400ft/20to120m)
while the KF761 is used for near field (10to100ft/3to30m). Normally, both
are used together in each array to provide appropriate audience coverage
for typical venues. In smaller venues a KF760 Series array might consist
entirely of KF761s.
ARRAYS, ACOUSTICS & CONTENT
The KF760 Series was conceived as an array system so the individual loudspeakers
(KF760 and KF761) that comprise the KF760 array elements would be designed
to work together and interact in multiples. This is a far different approach
from designing loudspeakers to work as stand-alone units. In fact, the
KF760 and KF761 both have significant performance limitations when used
singularly.
For this reason, the minimum recommended number of enclosures for a KF760
Series array is four while the optimum recommended complement is eight
or more. The KF760 and KF761 each have a number of unique design elements
that, when used in multiples, address specific issues of uniform, smoothed
frequency response, controlled dispersion, and high output level.
Venue acoustics are an uncontrollable variable. They are primarily addressed
by loudspeaker array dispersion. Special attention is also required to
produce smooth transition and consistent dispersion performance through
the crossover region.
Consistent dispersion control over an array’s full frequency range and
varied dispersion, for specific venues, requires solutions unique to each
range of wavelengths involved, in both the horizontal and vertical planes.
This is no easy task.
Program content governs any loudspeaker system’s frequency response, as
well as the output levels required across the frequency spectrum. While
program content may seem like another uncontrolled variable, hard data
related to program content can be obtained and applied to array engineering.
In regards to the KF760 Series measurements, traditional laboratory input
signals, were the primary engineering tool because of their relevance
to other data and the need to publish usable performance data. In addition,
frequency spectrum data at multiple concert performances was used to evaluate
real-world transducer and system capabilities.
DIVERGENCE SHADING
Traditionally, maintaining a similar SPL over varying distances from a
concert array involves intensity shading. Intensity shading means varying
the output of an array depending on where the sound is aimed.
This is usually done across an array’s vertical coverage by managing loudspeaker
input levels. Equalization and signal delay are often used in an attempt
to reduce destructive interactions between individual loudspeakers within
the array.
Rather than intensity shading, the KF760 Series uses “divergence shading”
to maintain a similar SPL over varying distances. Divergence shading means
varying the dispersion angle depending on where the sound is aimed.
The KF760 Line Array uses two primary design elements in a complimentary
combination to provide the required vertical output variations. The array
is designed to have a tight, narrow vertical dispersion angle for longer
throws, and wider vertical dispersion angles for shorter throws.
Strongly curved arrays serve the near audience, while the furthest audience
members hear only slightly curved or uncurved array sections. As such,
fewer loudspeakers are aimed towards the nearest listeners while more
are aimed towards distant listeners. This is accomplished by varying angles
between the axis of the loudspeaker enclosures.
The initial development work required a single curved source modeling
to determine the required array curvature for design Element #1. Array
analysis was distilled from a carefully selected variety of representative
concert venues. The variable curvature was designed to work within this
range of venues.
Single curved sources were then modeled using a number of identical (and
far more practical) smaller sources and further analyzed in terms of what
was needed for them to function like the single larger source.
ELEMENT #2
In addition to varying overall array curvature by varying the axial splay
angles between sources, more vertical dispersion variability was needed.
This led to design Element #2, which employed loudspeakers with different
vertical dispersions.
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The curved source was then modeled using multiple sources with
differing vertical dispersion patterns. It was found that the required
variation in dispersion angles could be accomplished with four different
rates of array curvatures in conjunction with two different models
of loudspeaker. The resulting sound levels from these design elements proved remarkably
consistent over a range of distances without the problems inherent
to intensity shading. |
FREQUENCY DEPENDENT DESIGN ELEMENTS
To avoid discontinuities in both the frequency response and SPL, highly
frequency dependent design elements are used, each functioning over a
limited frequency range. Destructive interaction between vertically adjacent
enclosures is minimized by reducing time arrival differences to the listeners
from multiple transducers and/or enclosures.
At frequencies where physical dimensions prevent this, coverage is limited
to a singular enclosure or transducer. Where there is overlapping coverage
from multiple transducers and their horns, outputs are added coherently.
At shorter wavelengths, transducers are arranged so that sound paths from
vertically adjacent enclosures to any particular listener originate from
a single point of geographic origin behind the enclosures. At longer wavelengths,
adjacent horns acoustically couple to form a unified, coherent wavefront.
All loudspeakers have the same input level to avoid complex and variable
intensity, equalization, or delay settings in the signal processing. At
lower frequencies, where vertical dispersion pattern widens for each individual
enclosure, array length provides the necessary columnar behavior to maintain
vertical pattern control.
At the lowest frequencies, the output of the individual enclosures couples
coherently to both increase the low frequency(LF) output and extend the
LF response. The sonic character of the sound is maintained for distances
as close as around 15ft(4.6m) to as far as many hundreds of feet.
ACHIEVING HIGH SPL
High SPL is accomplished in a way that significantly simplifies the signal
processing, amplification, and the physical setup. High sensitivity transducers
and high power handling are a given for high output systems and the KF760
Series is no exception.
Other techniques used to maximize SPL capabilities while maintaining uniform
frequency response and dispersion include multiple transducers within
each bandpass, horn loading, tightly focused dispersion patterns, wavefront
shaping, multiple enclosure coverage, line array effects, mutual coupling,
and appropriate enclosure splays.
A normal technique for increasing SPL output to reach the furthest listeners
is to use multiple transducers within a single loudspeaker and/or by using
multiple loudspeakers. This effectively focuses more SPL within a given
coverage area.
However, a problem inherent in doing this is that the source elements
must be physically separated. As a result, time arrival differences to
the listeners can create dramatic frequency-specific cancellations. These
cancellations will occur at and above the frequencies where the physical
separation approaches the sound wavelength.
Nonetheless, the KF760 design uses multiple transducers and enclosures
aimed at the furthest listeners to achieve high SPL. However, it does
this in a way to avoid destructive cancellations.
OVERCOMING MULTIPLE SOURCE ANOMALIES
The most difficult potential problem in a properly configured KF760 array
is the long throw section, where enclosures are normally arrayed “flat-fronted”.
In this case, multiple enclosures are at aimed at the same listening area.
Several frequency dependent techniques are used to circumvent multiple
source anomalies. These techniques carry over into the shorter throw (more
curved) sections of the array.
UPPER HIGH FREQUENCIES (HF) >6kHz
Because the HF wavelengths are so short, any small, time arrival differences
to a listener from multiple sources will cause cancellations that severely
compromise the HF response. This is potentially compounded by the fact
that each KF760 enclosure has two HF transducers.
To circumvent this problem, KF760 HF horn geometry is designed to provide
an extremely narrow vertical pattern from each transducer in the highest
two octaves. As the wavefront leaves the enclosure, the vertical pattern
actually has less vertical height than the face of the enclosure with
a nearly, but not quite, flat wavefront.
This combination ensures that, even at long distances, a particular listener
is primarily listening to the highest frequencies from only one, or at
most, two HF transducers. Because flat-fronted KF760s are only intended
for long distance projection, any time offsets must be computed within
this context.
Assume a listener distance of 300ft(100m) and three flat-fronted KF760s.
A worst-case path length distance difference between any two of the six
HF transducers would be 0.17in(4.3 mm). At 10kHz, this distance corresponds
to a phase offset of about 45° and an SPL loss due to partial cancellation
of only 0.7dB.
At 20kHz, the SPL loss would only be 3dB. However, this loss is a moot
point because air losses will pretty much obliterate information above
10kHz at a distance of 300ft(100m).
OUTPUT LEVEL
A simple equalization boost compensates for the fact that only one or
two transducers are providing the highest frequency information to any
one listener. This boost also compensates for the natural roll-off of
the horns. The required EQ boost does not overtax the transducers because
of the low power requirements in typical program material at these frequencies.
This boost can also incorporate some compensation for HF air losses at
longer distances.
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VERTICAL DISPERSION In the more highly curved portion of an array covering the medium to short throws, highest frequency reproduction is a viable goal. |
In this case, the axii of the HF transducers are more widely splayed in
vertically adjacent enclosures, essentially eliminating overlapping coverage
and the resulting destructive cancellations in the uppermost frequencies.
(see Figure #2)
HORIZONTAL DISPERSION
High frequency horizontal dispersion is controlled in a different way.
The HF horn has a traditional diffraction slot throat that opens to compound
expansion horn. At the uppermost frequencies, this design easily provides
the required horizontal pattern control.(see Figure #3)
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LOWER HIGH FREQUENCIES (1 kHz to 6 kHz) To accomplish this, the spacing of the HF transducers, within and between enclosures, was made small enough to keep time arrival differences within an acceptable range to avoid destructive cancellations. |
For example, a listener at 150ft(50m) on axis of one HF transducer at the top of one enclosure would experience a time arrival difference of only 0.04 milliseconds between it and the bottom HF transducer in an adjacent enclosure.
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At 4kHz this is less than a 45° phase shift between the two transducers
at the listener resulting in nearly coherent addition. This amount
of phase shift results in only about 1dB less level than would be
expected with perfectly coherent addition - well within acceptable
limits. To accomplish this, the HF horn geometry gradually and proportionally broadens the vertical wavefront for each of the two HF transducers at lower HF frequencies so that it emits from the entire vertical mouth area in each enclosure. However, the HF horn is physically too small to provide the necessary vertical dispersion control at these wavelengths. |
By mounting the HF horns coaxially in the mouth of the MF horn, the MF
horn actually becomes part of the HF horns, thus increasing their effective
size at the longer wavelengths. As seen in the cross section, this arrangement
of HF horns within each enclosure is essentially duplicated between enclosures.
Thus the vertical interaction between any two HF horns, whether within
or between enclosures is the same.
VERTICAL DISPERSION THROUGH THE CROSSOVER
At the HF/MF crossover frequency, the HF and MF subsections are both vertically
dispersed using the same horn. Thus by definition, the vertical dispersion
between the HF and MF sections through the crossover region is closely
matched.
Horizontal Dispersion
Examination of the HF horn will reveal that it is not large enough to
provide control of the horizontal dispersion in the HF/MF crossover region.
It actually starts to lose its control above the desired crossover point.
One element of the solution to this problem was carefully aligning the
crossover to allow significant overlap of the MF transducers with the
HF transducers. In this way, the MF transducer carries a good portion
of pattern control burden for the lower HF frequencies.
MID FREQUENCY DISPERSION
The simple key for MF dispersion control is that the MF horn is sufficiently
large enough to provide excellent horizontal and vertical dispersion control
over most of the MF passband. The MF horn essentially takes up the entire
face of the enclosure, except for the thickness of the top and bottom
of the enclosure and the mouths of the LF horns on the sides.
At lower MF frequencies, where individual horns start to lose control,
tight coupling of the enclosure fronts and minimal spacing of the MF transducers
allow the MF horns in multiple enclosures to couple as if they were one
larger horn, maintaining dispersion.
MID FREQUENCIES - 500Hz TO 1kHz
Horizontal Dispersion
Using dual MF transducers on a single horn presented a significant problem
for horizontal dispersion in this frequency range. The physical separation
of the MF horn throats in the horizontal plane within each enclosure is
large enough to cause an off-axis cancellation (i.e. notch) within the
operating passband.
This problem is acerbated by the presence of the coaxially positioned
HF horn between the throat openings for the MF transducers. This created
an even longer path length difference between the individual MF transducers
to listeners well off-axis.
Two different solutions were implemented to mitigate this problem, both
involving the “wing” positioned horizontally across the middle of the
midrange horn. This wing is effectively an extension of the bottom and
top flares for, respectively, the upper and lower HF horns, providing
good vertical pattern control for the lowest HF frequencies.
This allowed the initial portion of the HF horn flare to be shortened,
effectively reducing the off-axis path lengths from the MF transducers.
The effect of this was to move the cancellation notch up in frequency
and out of the MF passband.
Additionally, the wing provided a slight delay for the vertical center
of the MF wavefront, changing what would otherwise be a spherical wavefront
into a more cylindrical wavefront. This change in wavefront geometry also
reduces the off-axis path length differences between the two transducers
as well as enhancing the vertical pattern control.
MID FREQUENCIES - 200Hz TO 500kHz
Output
This critical frequency range provides the vast majority of fundamental
midrange energy, and requires prodigious output levels. Therefore, multiple
midrange transducers are needed to contribute to the sound for each listener.
The physical spacing of the midrange horn throats is small enough that
destructive interference between the two transducers is not a problem
at these wavelengths.
Vertical Dispersion
Vertical dispersion in this frequency range is controlled using true line
array principles. Starting with horns whose vertical dispersion is already
narrow, the line lengths for typically sized KF760 arrays are long enough
to achieve the desired vertical pattern control.
Destructive interference does occur off the vertical axis, but because
of the narrow individual enclosure dispersion, the attenuation off axis
is considerable. In the critical near-throw areas, these off-axis anomalies
are essentially “swamped” by the output of the KF761s that provide on-axis
coverage to these areas.
Horizontal Dispersion
The horizontal dispersion of these frequencies is accomplished by simply
using a horn that is physically large enough to provide dispersion control.
Because the sides of the MF horn mouths are essentially continuous along
the vertical length of the array, they couple together as one larger horn,
eliminating off-axis lobes in this frequency range. These lobes would
occur if the fronts of the enclosures and therefore the horn mouths were
vertically displaced.
EXTENDED HORIZONTAL COVERAGE
The KF760 model is designed with a nominal 80° horizontal dispersion pattern.
This is sufficient for many applications with typical configurations that
use left/right arrays. A significant attribute of the KF760 is that smooth
frequency response is maintained well beyond its nominal horizontal dispersion
angle. Actual measurements of a KF760 array, taken at 110ft(33m), show
remarkable consistency to more than 60° off-axis, which corresponds to
a 120° coverage angle.
At this angle, the response is within +2.5dB from below 125Hz to 10kHz.
This means that, unlike typical concert loudspeakers, the degree of overlap
between arrays is not frequency dependent. This is especially important
for the lower midrange where severe “build up” can occur because of a
lack of dispersion control. This creates significant frequency response
problems that can require complex signal processing or difficult array
positioning to correct.
This horizontal coverage performance has several important consequences.
For audience areas well outside the nominal dispersion pattern excellent
frequency response is maintained, albeit at reduced level. This creates
a “soft-shoulder” in the coverage that is entirely usable for many applications
to eliminate the need for additional fill arrays.
If side fill arrays are required, as is the case with wrap-around arena
audiences, a KF760 array can transition smoothly to a KF760 or KF750 side
fill array, without having to physically separate them, usually simplifying
the rigging.
When using side fill arrays, the “soft shoulder” can be effectively used
to expand the horizontal coverage. Using a 10° “soft shoulder” allows
the coverage of four KF760 arrays to extend some 280°. This is well beyond
the 240° that normally would be assumed using the nominal 80° dispersion
pattern.
With typical left/right arrays, there will be a smooth transition between
them because of the lack of side lobes.
KF761 DESIGN
The KF761 behaves similarly to the KF760, but with several important differences
for its intended application. In the KF761, the HF and MF sections use
the same physical horn.
Throughout the HF to MF frequency range the size of this horn is adequate
to provide the needed vertical and horizontal pattern control. Tight packing
the enclosure fronts provides the same continuous horn mouth concept used
on the KF760 for the lower horn frequencies.
Keeping with the divergence shading concept, the shorter throw distances
for the KF761 required the output to be lower than a KF760. To accomplish
this without resorting to input level adjustments, the KF761 is designed
with a wider horizontal and vertical coverage.
The KF761s also forms the portion of the array with the highest amount
of curvature. Both these elements reduce the output for a given audience
area compared to a KF760 without the necessity of changing the input levels.
Because the KF761 is used for short throw distances, geometry dictates
that a wider horizontal dispersion is needed to provide coverage for the
same audience width as the longer throw KF760s. In addition, the listening
characteristics of the KF761 had to be engineered for listeners possibly
as close as ten feet away.
Vertical and horizontal Dispersion HF Section
Because the output requirements for the HF are relatively low and the
possibility of destructive interference between multiple transducers over
short distances is much higher, a single HF transducer is used in the
KF761. The vertical dispersion for the entire HF range is controlled such
that any listener in the near field of a KF760 array is addressed by only
one KF761 HF transducer.
Vertical Dispersion MF Section
Like the KF760, the KF761 midrange has two MF transducers mounted on a
large horn to provide the necessary pattern control. However, they are
slot loaded in the sides of the horn rather than coupling through a horn
throat. This design means the transducers use the horn more as a wave-guide
while still providing the sensitivity required for its intended application.
Horizontal Dispersion MF Section
The dual MF transducers in the KF761 are spaced such that off-axis, the
path length differences are small enough that cancellations are avoided
within the MF passband. In addition, the horizontal dispersion is extended
by the simple technique of line-of-sight.
As you move off-axis of a KF761, the physical location of the midrange
transducers is such that one remains visible to listeners beyond the typical
physical horn wall limitation. This provides essentially direct radiation
to these areas.
KF760 INTEGRATION
At the highest frequencies, no integration between the KF761 and KF760
is desired because single transducer coverage is the norm in this range
of frequencies for both models.
In the lower HF range, the KF760 and KF761 use different designs, however
they are complementary in their behaviors for both the vertical and horizontal
planes. In this frequency region the KF760 has design elements that allow
two HF transducers and HF horns to behave as one. In the KF761, there
is only one HF transducer and horn.
In the MF frequency range, the KF760 and KF761 MF horns are the same width
and have similar expansion profiles. This largely prevents dispersion
discontinuities between them. However, the MF horns in the KF761s are
somewhat shorter so their rate of attenuation off-axis in the horizontal
plane is less than the KF760, thus enhancing and extending the horizontal
pattern for near field coverage.
In the lower midrange and low frequencies, the KF760 and KF761 designs
are nearly identical. By definition, this assures the KF761 integrates
well with the KF760 as well as other KF761s.
KF761 SONIC CHARACTER
Many concert loudspeakers are not suitable for nearfield listening. They
are usually voiced too aggressively, particularly in MF and HF frequencies,
in order to provide long distance projection.
While equalization can help, it cannot change this characteristic. The
design intent for the KF761 was to “soften” the listening characteristic
compared to the more aggressive sound of the KF760s required for long
distance projection.
The basic design elements to accomplish this were using a single HF transducer
on a larger horn with wider dispersion and slot-loading of the MF transducers.
These design elements “tone down” the KF761 to the appropriate, non-aggressive
sonic character needed for close-in listeners.
KF760 & KF761 - MID TO LF TRANSITION
Because of the longer wavelengths, maintaining pattern control commensurate
with the higher frequencies through the mid to LF crossover is a problem
area in typical loudspeaker arrays. Direct radiating LF transducers simply
do not allow this kind control.
One key to achieving a good mid to LF transition in the horizontal plane
is having excellent pattern control to a low enough frequency for the
mid frequency section. This is achieved in the KF760 Series simply by
the large width of the mid frequency horn and the fact that the MF horns
in the lower frequency range couple together to function as a larger horn.
The other key is providing pattern control for the upper frequencies of
the LF section. This is achieved in the KF760 Series with the spacing
of the LF horn mouths in each enclosure. The mouths operate similar to
a dipole radiator in that the spacing between them causes intentional
off-axis phase cancellations.
This effectively provides attenuation proportional to the off-axis attenuation
of the mid frequency horn. The result is a smooth crossover transition
throughout and well beyond the nominal horizontal dispersion angle.
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KF760 & KF761 LOW FREQUENCIES |
The outputs from these slots add coherently to extend the LF response.
This is readily evidenced by the fact that the 3dB down point for a single
enclosure is at 80Hz, for a four enclosure array (the minimum required)
it is at 60Hz, and for an eight enclosure array (a typical array size)
it is just above 40Hz. Within the nominal dispersion angle in the horizontal
plane, the left/right spacing of the LF slots in comparison to the wavelengths
reproduced allows nearly coherent addition.
Given the typical length of most KF760 arrays, the curvature of the array
also contributes significantly to the pattern control at low frequencies.
This prevents excessive LF buildup near to and almost under the array,
while focusing more energy to the longer throws.
Compared to traditional systems, this helps maintain the frequency response
over varying distance to very low frequencies. Again, this is achieved
with the physical design of the loudspeakers rather than complex signal
processing.
KF760 & KF761 SIGNAL PROCESSING
The signal processing, specifically parametric equalization and crossover
settings, for the KF760 Series has been carefully engineered by EAW. The
signal processing serves three primary functions: crossover filtering,
transducer equalization, and transducer signal alignment.
The factory preset signal processing either does not change or requires
only minor changes with variations in the size of an array and its configuration.
In other words changing a KF760 array’s coverage and output levels is
not done with the signal processing but by changing the number of enclosures
and the array curvature. Standard 1/3rd octave equalization, rather than
changing the signal processing, is the recommended way to provide “house
tuning”.
A full KF760 Series array would typically use eight channels of signal
processing. One 3-way output for the KF760s, one three-way output for
the KF761s, one output for the HF transducers for any flat-fronted KF760s
(for long throw air loss compensation) and one output for subwoofers.
Crossovers
Beyond normal band-stop functions, the crossover settings for the KF760
and KF761 are carefully engineered to provide proper control of the dispersion,
frequency response, and overall output through the crossover regions.
The precise crossover settings are dependent on the physical spacing of
the transducers, the dispersion characteristics of the horns, and the
output capabilities of the transducers.
For example, in the KF760, signal delay is used to align the HF and MF
wavefronts to provide coherent addition through the crossover point. However,
near the crossover region, the HF horn lacks sufficient size to provide
horizontal pattern control. The crossover filter parameters chosen actually
extend the pattern control of the MF up into the HF passband.
Because of the intimate effect of the crossover on the array performance,
any change in the crossover settings will adversely affect the dispersion,
frequency response, and output through the crossover region.
Subwoofers
While a full KF760 Series array has substantial output down to 40Hz, subwoofers
are normally used for popular music reinforcement. Subwoofer choice can
be based on any of various criteria ranging from truck pack to sonic requirements.
However, a natural complement to a KF760 Series array is EAW’s KF940 subwoofer.
The LF sections of the KF760 and KF76 have similar output to size ratios
to and draw part of their acoustic design from the KF940.
SUMMARY
The KF760 and KF761 use unique acoustical design elements within each
enclosure to:
1.) Operate as individual sources, or as one larger sound source, depending on the frequency range.
2.) Provide acoustical integration between different models KF760 and KF761.
3.) Provide the ability to change the primary variables of vertical dispersion and throw distance without depending on signal processing.
4.) Provide scalability, so that frequency response, horizontal dispersion, and output over distance remain consistent regardless of the array size.
September/Ocrober 2001 Live Sound International