Q: When is a line array not a line source?
A: When the array is not in a straight line.
Well, duh. Now that seems obvious to the point of being redundant. But there's a critical point lurking underneath this distinction, and it could make a difference in what kind of "line array" loudspeakers you'll want for your church with your next audio upgrade.
After all, if you look at photos of line array systems in use today, you'll notice that usually only the largest systems-in big arenas, stadiums or outdoor festivals-will have arrays that are hung in a straight line. And even with those, the bottom part often curves backward. Beyond that, a number of new loudspeaker systems on the market can only be assembled as a curved arc, not in a straight line. Yet these are called a type of line array. What gives here?
Getting it straight
We'll start by distinguishing between a line source, which is defined by a clear acoustical principle, and a line array, which is a term now becoming somewhat muddled by marketing departments.
To understand how a line source works, you first need to understand how a point source works. In acoustics, a point source radiates sound from a single point in free space. The sound propagates outward in the shape of a sphere. Applying some mid-level math, we can determine that the sound intensity ("volume") for the point source drops by 6 dB with every doubling of distance from the origin. Most people would perceive that as "about half as loud."
Now, let's take a bunch of point sources and put them in a straight line to create a true line source. For example, we could line up eight 6-inch drivers in a row next to each other, all reproducing a 1 kHz test tone. Now, in this case if you take an on-axis sound level measurement at one point, and again at double the distance, the level will drop by only about 3 dB. In other words, compared to the same level from a point source, the sound will propagate further and not drop off as quickly.
The reason is simple, if somewhat counter-intuitive. The sound waves from the eight speakers combine and constructively reinforce each other along the center axis of the array. Further away from the center axis, they interfere destructively, and sound level drops off. Consequently, rather than sound propagating out as a sphere, in this case the sound squashes outward into a shape more like a bagel, with the line array in the middle. And with this particular array reproducing only this single frequency, it would be a reasonably well-shaped bagel.
Lumpy bagels
What we've described here is, of course, the classic column loudspeaker still used for voice reproduction in many churches. It is a true line source, and within limits it works reasonably well. It directs sound outward along its centerline, maintaining a wide horizontal dispersion pattern (usually 100 degrees or more) but with a restricted vertical pattern that keeps sound from bouncing off a reflective ceiling.
That's fine if all you need is vocal range reproduction from roughly 500 Hz to 3 kHz. But even within that narrow range, this particular example will start breaking down at the top end. For a line source to work properly, the space between the individual point sources-in this case, the center of the loudspeaker cones-must be less than half the wavelength of the frequency reproduced. Consequently, at 3 kHz the loudspeaker cone centers should be no more than two inches apart. But here they are four inches apart, which means a 3 kHz test signal would generate significant irregularities, called lobes, in the response pattern. The nicely shaped bagel we saw at 1 kHz would show big, protruding lumps.
At about four feet long (that math is easy), this column would hold its vertical directionality to the lower end of the vocal range, but not much below that. For vertical control at 100 Hz, for example, you would need a line source at least 22 feet long.
From line source to line array
True line source loudspeakers are largely restricted to the column arrays like the above example, and as thoroughly detailed in an article in Church Production's May 2011 issue (www.churchproduction.com/0511_line_array). Some modular systems allow for very long columns that provide directional bass, and most have arrays of small, tightly spaced tweeters for high frequencies up to 8 kHz with minimal lobing.
But, for various engineering reasons, these columns can't produce the higher power levels desired for contemporary music in larger venues. So now we turn to the modular line array system-which is sometimes a true line source, sometimes not, and sometimes both.
The individual cabinets for modular line arrays are two- or three-way loudspeaker systems, and they come in all sizes. Compact systems will have 5.25-inch or 6-inch low-mid drivers, while larger systems will have 12-inch or even 15-inch bass drivers transitioning to mid-range cones and then to high-frequency sections-usually compression drivers with special waveguides, but sometimes "magnetic planar" or ribbon drivers. When you stack these cabinets together in a straight line, with zero degrees splay between them, then you have a full-range system that is functioning, more or less, as a true line source.
More or less? Well, in the real world, making a system that functions perfectly as a pure line source from 30 Hz to 20 kHz involves excruciatingly difficult engineering. High frequencies are particularly daunting, as long arrays of tiny tweeters are impractical for larger systems. Manufacturers instead have created special waveguide that create the very narrow vertical dispersion required for each cabinet, usually between 5 and 15 degrees.
But the end result is close enough, and that's why you'll see the top portions of line arrays used in large venues configured in straight lines. They are using the line source effect to direct a narrow beam of sound to the back balconies. But down at the bottom they usually from a gentle "J" shape to spread the sound more evenly to closer seats.
So, rather than calling them simply "line arrays," some companies will refer to them as "curvilinear arrays" or "articulated arrays," acknowledging divergence from true line source behavior. Nevertheless, these hybrid arrays function remarkably well when configured properly. Ay, there's the rub.
Properly designing a hybrid articulated array is a tricky proposition-one best left to seasoned professionals. First, the array has to be hung in precisely the right place, and the splay angles between cabinets set properly. In many cases, the array will be "zoned" in two or three sections, with levels and equalization set separately for the cabinets in each zone. Most major line array manufacturers supply software programs for doing this, but mastering the intricate procedures takes diligent study and practice.
One good curve
And that brings us to the latest variation on the line array, which of course is not a line at all. It's a curve, or an arc. Needless to say, these arrays do not function as true line sources, though in some configurations and at some frequencies they may take advantage of constructive interference to enhance directional behavior.
These products go under several monikers, including "constant curvature line array," "arrayable loudspeaker," and "point source array." Most will have a 10-inch or a 12-inch woofer along with one or two compression drivers on special horns with a narrow (15-degree to 20-degree) vertical dispersion pattern.
A critical difference between an arc array and a true line source becomes apparent when you add more cabinets to your array stack. With a true line source, stacking up more cabinets will progressively narrow your vertical coverage into a tighter beam. The effect is frequency-dependent, of course, but still that's the general trend. With the arc arrays, the opposite is true. Adding more cabinets extends the arc, and thus adds to your vertical dispersion pattern. So a two-cabinet array may have a nominal 100 horizontal by 30-degree vertical pattern, whereas adding two more cabinets will extend vertical coverage to 60 degrees.
The bottom line is that calculating coverage becomes a much simpler matter, to the point where many churches might be confident in configuring a new arc array system on their own. Again, most manufacturers will offer free software to assist in the job, and it likely will be simpler in this case. Rather than complex array zoning, adjustments for near and far seating areas can be done with simple "amplitude shading" and "frequency shading" controls on the cabinets. Those are fancy terms for cutting the levels and high frequencies for bottom cabinets aimed at nearer seats.
What's your line array?
If your church is looking at a new loudspeaker system, chances are one of these line arrays variants will work for you. In a nutshell:
For small- to mid-sized churches with highly reverberant acoustics and requiring primarily vocal range performance, line source columns are recommended. If music range reproduction is required, look at large-scale line source columns or smaller line arrays.
For larger churches with long throw distances (100 feet plus)-particularly those desiring to avoid putting in delay systems-look at hybrid line arrays with the upper portion functioning as a line source. Seek professional guidance in configuration, particularly when covering a main floor and balcony with one array.
For mid-sized churches with modest throw distances and covering only one level, consider the arc arrays. If the configuration software doesn't leave you flummoxed, have a go at configuring the arrays yourself.
That's the straight skinny on line arrays. I trust this modest advice doesn't end up throwing you a curve.
