Realizing a Full Bandwith Line Array
Full bandwidth line arrays are typically three way systems.
The practice of dividing the band into 3 separate passes is
done to enable the cross-over points to always be substantially
low enough that the radiation from each pass exhibits wave-
lengths that are always longer than the physical device, or driver
spacing. This is relatively easy to achieve for the low frequency
section of any line array and is also easy to achieve for the
mid-band section.
In mid-band sections the mid range devices are 6 inches
in diameter to 8 inches in diameter. The crossover points are
selected so that the device spacing is always small compared
to the wavelength radiated. The problem for a full bandwidth
line array systems is the high frequency radiation.
As mentioned earlier, historical line arrays were excellent
in terms of low frequency and mid-band control of the pattern,
but always suffered from polar lobing errors associated with
the device space “B” being greater than the wavelengths being
radiated. A 16 kHz wavelength is on the order of 3/4 of an
inch and as a consequence device spacing must be comparable
to those wavelengths or shorter, if possible. This was always
a problem in the past because engineering techniques could
not realize spacing closer than the driver diameters themselves.
Even with modern neodymium iron boron based magnetics,
the diameters were always at least 4 inches or greater (for
large format diaphragm devices). That spacing limited good
performance to below approximately 3 kHz, obviously not a full
bandwidth device.
As a practical example, fmax, the maximum high frequency
control based on the relationship between the spacing of the
devices b and the wavelengths is as follows. For base line
arrays where we are interested in control up to 250 hz, the
spacing needs to be at least 4.5 feet. This is relatively easy
to do with 15 inch and 12 inch drivers and as a result the real-
ization of bass frequency line arrays is very straightforward.
For mid-band line arrays, if we are interested in frequencies
between 250 and 1,250 hz, the spacing needs to be 11 inches
or smaller. Again, this is relatively easy to do with 6 inch or
8-inch drivers, and this is frequently the diameter of mid range
devices in both large format and compact line array systems.
Figure 27
shows an Electro Voice Hydra™. This device
basically takes the radiation of a compression driver and acts
to produce both equal amplitude and equal phase sources at
the front of the wave-guide. The full drawing in
Figure 27
is 3
Hydras vertically stacked, thereby generating 21 “point source”
radiating surfaces coupled to a horizontal wave guide with an
included angle varying between 90 and 120 (model dependent).
Figure 28
shows a Hydra without the driver or wave-guide
coupled. Each hydra has 7 output “slots”. The driver is coupled
to the input side of the hydra and the 7 outputs are then
interfaced with a horizontal wave-guide to produce the
required horizontal included angle. The space b for a hydra is
.826 inches, which equates to a wavelength of 16,434 Hz.
Again, it is always best for wavelengths to be longer than that
spacing, so in this implementation, the Hydra presents excel-
lent high frequency control in the 15kHz to 16 kHz range. The
Array Show plot
Figure 29
shows 21-point sources in a vertical
orientation with the exact spacing provided by a hydra.
Figure 27
Figure 28
Figure 29
λ
= 16,434 Hz
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