Arcline 8/212 User Guide V1.0
Page 53
Spectral balance
At low frequencies where individual cabinets are less directional (see earlier), the full vector
summation occurs nearer the stage. You enter their combined influence earlier because they
are not as directional at LF.
The downside of this is that you run out of cabinets to compensate for radial attenuation
earlier too, so the “line array effect” ceases to apply. Smaller array coverage is also difficult to
adjust using inter-cabinet angles as opening an angle to reduce vector summation too near
the front of the audience would simply create a hole in the mid/HF coverage. And any attempt
to equalise, say, the lower cabinets, simply attenuates the whole LF “point source”.
This equation is often quoted for a straight array of acoustically small sources:
Note that doubling the line length quadruples the transition distance, and vice versa, so don’t
be tempted to reduce the number of cabinets because “it’s only a folk act”. The line array
advantage disappears with reduced array length.
The transition distance equation above assumes omnidirectional array elements. It is only
applicable where individual cabinets’ vertical baffle or horn mouth dimensions are acoustically
small compared with the wavelengths to be projected.
Most modern line arrays (e.g. the Arcline 8) are modular with each element having acoustically
small vertical dimensions below a few hundred hertz (500 Hz and lower for the Arcline 8).
Modular arrays rely very heavily on designers specifying a vertical array that’s long enough for
spectrally balanced coverage to be provided.
Frequency
(C = 343 m/s)
125 Hz
250 Hz
500 Hz
Number of
Arcline 8
Straight vertical
array length
Approx straight-line transition distance
4
1.14 m
0.24 m
0.47 m
0.95 m
8
2.28 m
0.95 m
1.89 m
3.79 m
12
3.42 m
2.13 m
4.26 m
8.53 m
16
4.56 m
3.79 m
7.58 m
15.16 m
20
5.70 m
5.92 m
11.84 m
23.68 m
24
6.84 m
8.53 m
17.05 m
34.10 m
Source: Mark S. Ureda - Analysis of Loudspeaker Line Arrays
Nearfield – farfield transition distances at low/low-mid frequencies
As you can see, the transition distance is almost non-existent for small arrays at low
frequencies. In SPL terms, you end up with a low/low-mid coverage peak nearer the stage
whereas the same length of array provides much smoother coverage at higher frequencies.
11 Appendix A - Line array basics
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