Fine Points of Aiming One-box Line Arrays- Part I

by Richard Honeycutt

Poor speaker aiming is an all-too-common sound-system problem. This article will help you understand the radiation patterns of line arrays.

With single direct-radiator and horn speakers, the problem usually originates in the installer’s LAR (“looks about right”) technique. With one-box line arrays, the problem is more often a lack of basic understanding of the way line arrays radiate and/or the acoustical needs introduced by mounting speakers at or near listeners’ ear height. This is true for both passive arrays such as the SLS LS 6593, the Radia Pro ribbon line models, Tannoy i9’s, or others; and digitally steered models such as the Renkus-Heinz Iconyx or EAW units.

For many years, we have specified coverage by vertical and horizontal angles. This works, within limitations based upon wavelength, for most loudspeakers, and for the horizontal coverage of line arrays. However, it does not work at all for the vertical coverage of a good line array.

An ideal line array is a true line source; that is, a source that radiates sound evenly all along its length, and has negligible width. Perhaps the only products on the market that approach this description are the ribbon arrays marketed by Radia Pro. Other arrays consist of a line of identical drivers spaced at uniform intervals. If the center-to-center spacing of these drivers is less than 1/4 wavelength, the array behavior approximates that of a line array. Obviously, real-world multi-driver arrays cannot maintain such spacing at the highest frequencies.

The horizontal coverage of an ideal line array is omnidirectional. For a real line array, it is determined by the relationship of the driver diameter to the wavelength. Horizontal coverage of 150 degrees up to about 5 kHz, and 120 degrees above 5 kHz, is common.

The vertical coverage can be visualized as the length (height, given the usual mounting orientation) of the array, projected onto the audience plane. Thus an 8′ array will “illuminate” an 8′-high strip of the back wall in an empty auditorium with no seats. The vertical coverage angle thus varies with distance from the array: it can be fairly large if calculated for a point close to the array, and only a few degrees at locations far away. This behavior occurs at frequencies at which the wavelength is less than or equal to the length of the array — a little below 150 Hz for an 8′ array. This same behavior applies for each beam in a multibeam steerable array.

However, this description of line array radiation does not apply to curved arrays such as those now being designed by Don Keele and Marshall Kay. It also does not apply to J-arrays.

Figure 1: 1C16 Single Beam at 250 Hz

Figure 1: 1C16 Single Beam at 250 Hz

Figure 2: IC16 Single Beam 1 kHzFigure 2: IC16 Single Beam 1 kHz

Figures 1 and 2 show a side view of the vertical radiation of an Iconyx IC16, configured for a single horizontal beam, at 250 Hz and 1 kHz, respectively.
Notice that although this 6′ array does provide low-frequency control at 250 Hz, the vertical pattern at that frequency is much wider than at 1 kHz.

Non-ideal line arrays will approximate ideal arrays, but will source sound into “lobes” or beams at different angles from the main beam. This effect is shown for an older IC16 in Figure 3. More recent IC16’s use triple tweeters mounted on the front of each low-mid driver to provide closer spacing and reduced high-frequency lobing.

Figure 3: 1C16 Single Beam at 8 kHzFigure 3: 1C16 Single Beam at 8 kHz

Some speakers that are marketed as line arrays provide only very poor vertical control: I have measured products having vertical coverage angles of 30 degrees or so. Even a 2′ array, providing some control down to about 500 Hz or so, would have a coverage angle well below that value at useful distances and frequencies. “Line array” products such as I measured may be useful for some applications, but should not be used where true line array performance is required. Always look at the CLF or EASE plots before specifying a single-box line array.

In Parts II and III of this blog series, we’ll look at practical aiming considerations.  rh