Part 3 of “building-blocks” is the broadside Arc-Delay array
Here in part three were going to look at another fundamental building-block, the broadside array and the processing that can be applied to it.
First we should look at the reasons for deploying this kind of array by looking at the traditional L-R split-array, as shown in Fig. 1, and the problems associated with it. The “power-alley” in the center and interference resulting in an uneven coverage pattern are the typical problems this configuration suffers from. – (Hard to believe this was the industry standard until recently.)
Fig. 1: Standard L-R Split-Array Configuration
Anyone who has ever put one of these up, shouldn’t be surprised by the SPL maps presented in Fig. 2a, 2b, & 2c
Fig. 2: Split L-R SPL Maps (a) 31Hz (b) 63Hz (c) 125Hz
But what may come as a surprise, is if we consider the 3D aspect, we are pushing energy upwards towards the ceiling and backwards towards the stage (See Fig. 3) – thanks to Dave Rat for pointing this out. 🙂
Fig. 3: 3-D Radiation Pattern (Dave Rat)
One suggestion is to angle the arrays out approximately 30-45 degrees to spread the energy away from the middle and maybe pickup some extra side coverage (See Fig. 4)
Fig. 4: “Splayed “ L-R Split Array Configuration
Observe what happens in Fig. 5a, 5b, & 5c, especially on the stage with this layout – yikes!
Fig. 5: Splayed Split L-R Array (a) 31Hz (b) 63Hz (c) 125Hz
So it should be obvious by now when two sources are separated by multiple wavelengths you will get interference from their summation. (Equal signals, equal levels, equal delays, etc.)
To eliminate this…what happens if we position a mono center cluster? In this case in front of the stage, as in Fig. 6.
Here in Fig. 6, we see the typical setup for this type of array – makes a great stage extension.
In this example, we are going to place ten enclosures on their ends (portrait mode) in a row in front of the stage, each enclosure dimensions are 112cm (44in) x 60cm (24in) x 102cm (47in). These dimensions are important to calculate the center-to-center spacing of this array for our calculations (more on this later).
Here in Fig. 7, we see the layout, (stage omitted for clarity).
Fig. 7: Broadside Array Configuration
Here in Fig. 8a, 8b, & 8c, we can see the progressive narrowing of the beam-width horizontally as we rise in frequency, which is a typical behavior of this layout. If the venue was long and narrow, this directivity would be of some use, but if we need to broaden the beam-width to increase coverage…..
Fig. 8: Broadside Array (no processing) (a) 31Hz (b) 63Hz (c) 125Hz
In an attempt to broaden coverage we are going to physically shape the array, with the help of some sturdy stage-hands, we shape the array as shown in Fig. 9 and then look at the results in Fig. 10a, 10b, & 10c.
Fig. 9: Physically Curved Array
As we can see this has the unfortunate effect of focusing the energy onto the stage; and, in the upper frequencies it’s softened the middle of coverage. (This is a fine example of Mister Murphy’s first law of “unintended-consequences.”) 🙁
Fig. 10: Physically Curved Array (a) 31 Hz (b) 61Hz (c) 125 Hz
Let’s restore the array to its earlier condition, as in Fig. 7 (flat-wall), and apply incremental delays starting from the center and heading outwards as in diagram 1.
Diagram 1: Delay Tap Layout
The cabinets are wired in pairs from the center out and each pair is connected to a different delay tap, T-0 to T-5. We’re going to use an excel spreadsheet to calculate the delay times to place these cabinets on a parabola to produce 90 degrees of beam-width. Given the physical spacing and dimensions of the enclosures, the values in table 1 were calculated and applied.
Now let’s look at these SPL maps. See Fig. 11a, 11b, & 11c.
Fig. 11: Arc-Delay Processed (90 degrees) (a) 31hz (b) 63Hz (c) 125Hz
We can see two things, 1) the coverage has broadened out, and 2) behind the array as well.
Using electronic delay has bent the array in both directions.
As another example we’ll apply 120 degrees of coverage using the values in table 2 and polars in Fig. 12a, 12b, & 12c.
Fig. 12: Arc-Delay Processed (120 degrees) (a) 31Hz (b) 63 Hz (c) 125 Hz
One caution, if the array gets too long it can suffer from self-interference and fall apart. The same can also happen if the elements are spaced too far apart.
Now that we have three basic building blocks, we can start to combine for more complex arrays and directivity control.
Next up is end-fire vs cardioid.
About Martyn “Ferritt” Rowe
Industry veteran and OSA’s Director of Engineering Services Martyn “Ferrit” Rowe brings nearly three decades of real-world experience in live event technical services. Ferrit most recently came from Martin-Audio as the technical training manager for MLA, and uses his vast knowledge and expertise of the multi-cellular technology to support client projects as well as support and train engineers and technicians.
Ferrit began his career running cables on a Thin Lizzy “Live and Dangerous Tour,” and then taking on the roles of running monitors, front of house, and system technician for some of the most popular acts in music, such Judas Priest, Ozzy Osborne, Black Sabbath, The Police, KISS, The Who, Elton John, Poison, Bon Jovi and Van Halen.
Ferrit’s training career began in 2000 when the first of the line array’s from V-DOSC emerged and became an instructor on the line array theory and continued that path with various systems over the years before joining the Martin-Audio MLA division, and then bringing his knowledge and expertise to OSA International, Inc.