This is a simple graphic representing
two radiating point sources, like ripples in water. It is
a 2-D picture of spherical sound radiation. These waves
radiating spherically are diverging, losing energy
according to the law of inverse squares. With each
doubling of distance, the energy is reduced to 1/4 (-6db).
Imagine the wave is a balloon. As it goes out farther,
the circle is larger and the rubber gets stretched
thinner. Lines of intersection are "nodes."
This is where waves interfere constructively, making the
wave stronger. There are also "antinodes" half-way
between nodes, (which are harder to see in this simple
model) where waves cancel each other out.
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When we overlay three different wave
patterns, the information appears more chaotic. At each
frequency there are still nodes and anti-nodes. Sometimes
they line up, most obviously in line with the acoustic
center, equidistant from each source. But elsewhere nodes
are in different locations. This would create areas where
some waves are quite strong and others fairly weak.
Imagine the further complications in 3 dimensions and
with uneven spacing.
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| This is a simple drawing of a conventional speaker
array. It is a collection of point sources unevenly
spaced in 3 dimensions. For example, there is the
distance between high frequency drivers vertically, which
differs horizontally and diagonally. Visualize
overlapping wave patterns from each source interacting.
The wave interaction from combined sources becomes hard
to predict, particularly around crossover frequencies
where waves originate from more than one size of driver.
There is a tremendous amount of undesirable interference.
This means the listening area would have several hot
spots and dead spots at each frequency. |
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Many address this problem with horn designs,
controlling the sound by focusing the speakers in
different directions. This reduces overlap at higher
frequencies, but there are still unwanted nodes in the
mid-bass, because those waves are less directional. Sound
level is limited in each audience area to that generated
by one set of drivers, losing energy over distance by the
law of inverse squares. The sound has to be loud in front
to reach the back.
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What makes a line array
different? Let's examine 4 point sources, evenly
spaced. The nodes form a linear front, represented by the
horizontal lines of intersection appearing in the model.
The waves keep each other from losing energy by
preventing the divergence which normally occurs in the
spherical radiation of waves. Sonically, the moving high
pressure zones (waves) are adjacent (coherent), so they
transfer energy to each other instead of dissipating.
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| In speakers, a line array is a column of closely and
evenly spaced drivers. This produces a "ribbon"
of sound waves which propogate through the air as a
cylinder, rather than a sphere. In a well tuned line
array, this is true for a broad frequency range. This
evens out nodes and antinodes in the listening area.
Everyone can hear all the drivers together, evenly, and
the sound is nearly as strong at the rear of the
listening area as the front. Some call this the extension
of the near-field, because the coherent sound doesn't
lose energy according to the inverse square law. Volumes
are reduced in front of the cabinets while remaining
constant and stable throughout the listening area. |
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