Designing & Deploying Line Arrays

Advancing science in the face of conflicting beliefs and practices is Engineering's greatest challenge

Introduction

The past several years have witnessed unprecedented growth in the complexity of sound reinforcement equipment and technologies. Today we have simulation programs to show complex loudspeaker coverage in three-dimensional spaces, measurement platforms to quantify detailed electro-acoustic performance, digital transport and networking schemes and a host of increasingly sophisticated computer-based control and signal processing devices. Virtually every new active element in the signal path includes some form of on-board DSP.

Against this backdrop of ever-increasing complexity, the purpose of these papers is to provide easy-to-understand information and guidelines that will enable consistent high quality results from line array deployments. As such, they are neither an exhaustive treatise nor are they highly technical. They provide common-sense examination of fundamentals that govern all forms of sound reinforcement aimed at improving sound quality and listening experiences for audiences and audio practitioners.

Occam's Razor

Occam's Razor is a principle attributed to the 14th-century English Franciscan friar, William of Ockham. It states that the explanation of any phenomenon should make as few assumptions as possible, eliminating those that make no difference in the observable predictions.

When multiple competing hypotheses are equal in other respects, the hypothesis that introduces the fewest assumptions and postulates the fewest entities is best. The simpler a theory is, the better. When two theories predict phenomena to the same accuracy, then the one which is simpler is usually the better one. Be skeptical when facts and results are postulated from theories or assumption-based hypotheses.

Break it down and keep it simple!

Line Array Myths

Disruptive technologies develop their own mythos. These are reinforced by popular beliefs that interpret the technology and its meanings to all those affected. Unfettered, these beliefs become the common wisdom.

There has been a litany of misinformation that has been promulgated about line arrays and their deployment. This has led to mixed results at every level. Line array technologies need to be properly understood because they underpin sound reinforcement today and in the foreseeable future.

Following are the ten most widespread line array myths.

Ten Line Array Myths

1. Line arrays are a fad
   Not true. Line arrays offer cogent means to increase coverage and SPL while reducing temporal distortion and the architectural footprint of the loudspeakers. Unless you are a speaker manufacturer without one, what's not to like?
2. J-Arrays improve the vertical coverage
   J-arrays consist of two totally different loudspeaker arrays. They perform poorly because of the withering discontinuity where curved and straight segments join.
3. Down-fills are a good way to cover 'down front'
   Like the doomed pilot who runs out of altitude and ideas, we sometimes run out of both time and viable alternatives. But like the J-Array, using down-fill boxes splices completely different loudspeakers to the main array, creating interference where coverage conjoins.
4. Split processing can optimize J-Arrays
   This bolsters the sales of DSP devices, but we can't fix directivity discontinuities with DSP. Curved and straight arrays have radically different vertical directivity characteristics and should not be connected together.
5. Simulations show the best way to configure line arrays
   Simulation programs are subject to the same human errors and manipulations that haunt every complex software, only more so. Most programs are based upon assumptions and constructions that fail to recognize the effects of discontinuities in array shapes. Use simulations with care, but don't recommend or construct an array that has a physical discontinuity just because a simulation shows appealing coverage representations – it isn't possible.
6. Configurable horizontal coverage improves spatial uniformity
   Configurable horizontal coverage seems like a good idea, but we cannot achieve directivity-matched transitions through crossovers while making waveguide mouth openings smaller to narrow the horizontal coverage. The result is irregular directivity-frequency and inconsistent frequency response in the critical middle and upper middle frequencies. Efforts to 'fix' the frequency response with filters come at the expense of spatial uniformity.
7. Each venue requires its own unique DSP
   This stems from the misconception that observed misbehavior is caused by room acoustics, and that DSP can somehow rescue the day. Both assumptions are false. Most misbehavior is either the loudspeakers or the arrays, and DSP has no effect on room acoustics anyway. Use equalization appropriately.
8. Tried-and-true audio practices perform well with line arrays
   Most 'tried and true' practices don't perform as well as we'd like to think they do, and they are less likely to perform well with line arrays. Best results will always come from careful cause-and-effect analysis before postulating solutions.
9. High sound levels from line arrays are OK
   If the sound leaving the loudspeakers is 'clean' there won't be any distortion, right? Wrong. Three types of acoustic distortion are more significant in live sound than all others and line arrays have a propensity for one of them.
10. Line arrays radiate cylindrical sound fields
    All that can be said in support of this thesis is that the near field of a line array is an interference field that roughly follows the frontal aspect of the loudspeakers. Inverse square law has not been abrogated. The far field acts like that of any other loudspeaker.

All of these myths are fallacious
They are either untrue, or require narrow context setting to be true. Some are the result of faulty understanding of the basic mechanics of how line arrays work and some are deliberate misrepresentations. Nevertheless, they are the cornerstones of line array folklore that are responsible for much of what sounds bad today.

Fortunately, practicing good science will always be easier than fumbling bad science.

Why Array?

Arrays serve to increase, decrease or re-shape coverage and/or increase the sound pressure level. Other than these, arrays have no useful purpose.

Loudspeakers that are physically offset from one-another with conjoining coverage are a source of temporal distortion – combing and time smear in three dimensions that cannot be 'fixed' with one-dimensional solutions.

That some loudspeakers can be mounted closer to one-another, and that some might have less coverage overlap than others only reduces combing and time smear. Minimum temporal offset results from small, tightly packed sources. Small sources have low directivity and 'soft' pattern edges, increasing coverage overlap and temporal distortion. High-directivity sources are big – separating the sources in space and time, which also increases temporal distortion.

Line arrays optimize the ability to conjoin coverage of a plurality of like sources to produce minimal temporal offsets in the direction of coverage, but they cannot repeal the realities of time and space.

Why Line Arrays?

Line arrays enable high sound levels, when compared to traditional multi-way systems. Due to their greater length, line arrays maintain high vertical directivity to much lower frequencies.

• Improved direct-to-reverberant sound ratios in enclosed spaces
• Reduction of atmospheric interference effects out-of-doors

Line arrays can be constructed to provide optimally wide vertical coverage to meet special auditorium needs, e.g., balconies. They can also be shaped to provide tapered vertical coverage for very deep auditoria, long throws and low trim heights.

Line arrays lack one dimension that is responsible for temporal distortion in large sound systems.