Designing & Deploying Line Arrays

Things you should know before opening your laptop...

Mark Engebretson
Vice President R&D, Chief Systems Architect QSC Audio Products, LLC

Part I - Line Array Architecture

How Line Arrays Work

CONVENTIONAL HORNS

Expanding wave fronts propagate normal to the boundaries of the horn.
At the mouth, high frequency wave shapes from adjacent horns will create destructive interference.
The practical upper frequency limit for summation is reached when the rise (a) of the resulting circular segment at the mouth equals a quarter-wavelength.
This makes conventional horns unsuitable for wide-range line array use.

LINE ARRAY WAVEGUIDES

Line array waveguides use various methods to normalize the propagation distance from the throat to the mouth.
In this illustration we see that each waveguide has been segmented into four identical small apertures, each equidistant from the inlet aperture. The result is minimal rise at the mouth approximating a planar wave shape.
This enables adjacent loudspeakers to sum without interference at high frequencies.

Segments & Line Arrays

Concert Line Arrays consist of Segments

Segments have Coverage Limitations

Segment Coverage Limits are frequency-dependant

3 Line Array Segments:

Straight vs. Curved Line Arrays

Straight and curved (arcuate) arrays have very different characteristics.

A single straight element narrows monotonically in its vertical coverage with frequency. At a given frequency it will have a given directivity and resulting vertical coverage. Two such elements stacked vertically will have twice the directivity and one-half the vertical coverage. Four elements will have four times a single element's vertical directivity (one-fourth the coverage), and so forth.

Curved arrays (arcuate or constant radius) will begin with two elements for wider vertical coverage – whatever that might be – and lower vertical directivity. Doubling the array size while retaining a constant inter-element splay angle will result in vertical coverage that is twice that of 'wider' and vertical directivity that is one-half that of 'wider', etc.

Vertical Coverage

Line Array Types

Straight Arrays

Straight arrays are best characterized by 'column' loudspeakers, wherein all of the array elements are oriented in a continuous, straight vertical line. Vertical directivity in straight arrays is a function of array size and frequency, with vertical coverage being inversely proportional to their product, and narrowing monotonically as frequency increases.

Curved (Arcuate) Arrays

An arcuate array is curved by virtue of its diverging segment axes. When assembled from independent cabinet-type segments, the inter-box splay angles are held constant, which results in a constant effective radius of curvature. A relatively large array assembled from segments takes on the effective vertical coverage of its cumulative segment-to-segment splay angles and maintains constant vertical coverage between the overall size-imposed break point frequency and the frequency where beaming of the individual segments takes place.

J-Shaped Arrays

J-arrays join two totally different loudspeaker arrays – a straight segment intended to cover distant seats, and a curved segment disposed to cover seating near the stage. J-arrays inevitably perform poorly because of the withering discontinuity where the curved and straight segments join.

Spiral Arrays

Spiral arrays do well what the J-array was envisioned to do. Employing arithmetically-incremented inter-element splay angles to decrease the effective radius from the top to the bottom of the array, spiral arrays provide constant vertical frequency-coverage graduating from high-to-low directivity. They are easily implemented and readily compensated.