There is no escaping the sea change in professional audio, arguably initiated by L-Acoustics, towards widespread use of vertical loudspeaker arrays, more commonly named “line arrays.” Whether for large touring productions or the new generation of column loudspeakers installed in conference rooms, vertical arrangement of loudspeaker transducers is now firmly affixed in the industry’s consciousness.
A specific class of vertical array, the “constant curvature” vertical array, is a relatively new arrival on the loudspeaker scene. With constant, fixed angles between adjacent cabinets, constant curvature arrays are presented as having the benefits of a line array, along with simpler rigging hardware and aiming requirements. Let’s take a look at how vertical arrays operate and clarify the place of constant curvature arrays in the panoply of vertical arrays.
Divide to Conquer
Since the beginnings of professional audio, we have needed more output, coverage and frequency response than a single loudspeaker transducer could provide. Even with dramatic increases in modern transducer performance, it would seem that combining multiple drivers together — whether for response, coverage or output — will be an ongoing fixture of the industry. Because of the limitations of drivers, much of the effort in professional loudspeaker design has been expended in combining multiple drivers in a single loudspeaker box, and then combining multiple boxes together into an array.
For a substantial fraction of audience layouts, segmenting the audience vertically makes the most sense. Audience members near to the array need comparatively low sound levels and experience only small amounts of high frequency absorption (Figure 1). Audience members farthest from the array require greater output (and more compensation for high frequency absorption) to maintain even coverage in the venue. Splitting an array into vertical zones, where the upper portion covers distant audience members, allows for tailoring the sound arriving at each audience cross-section in a more even manner. Custom control of vertical dispersion and output is the biggest advantage for conventional line array products.
Array Interactions
A second advantage of line arrays is their motivation of the industry to more carefully consider, measure, and optimize the interactions between adjacent loudspeakers. The results have been improved rigging, waveguides, modeling and processing.
In spite of these improvements, loudspeakers will never behave like laser beams. Loudspeaker coverage patterns widen and narrow in a frequency-dependent manner, and the performance of each loudspeaker in an array depends on its neighbors. The variable performance of loudspeakers is a consequence of the huge range of wavelengths they are asked to reproduce. The shortest wavelengths in the domain of human hearing are less than an inch long, while the longest are tens of feet in length.
The physics of acoustic waves dictates that they bend fairly easily around objects whose dimensions are comparable to, or smaller than, the wavelength. This behavior, known as diffraction, has been well understood for over a century, and no product or manufacturer has immunity from its effects. You can illustrate this for yourself by taking a loudspeaker, turning it so the drivers face away from you, and listening to it from behind. The mids, low mids, and low frequencies will still be clearly audible behind the box, as they have bent around the loudspeaker enclosure.
At very short wavelengths, which is to say high frequencies, the loudspeaker’s physical size will be sufficient to provide strong control over how sound travels. A useful guideline is that virtually all individual loudspeakers retain excellent directional control for frequencies above 5 kHz. This is akin to saying that one can treat each loudspeaker as independent from its neighbors above 5 kHz.
As wavelengths get longer, and frequencies get lower, the ability of an individual loudspeaker to provide directional control decreases. Sound bends around the edges of the box, and spills on the features around the loudspeaker. Sound also bleeds onto adjacent loudspeakers in the array. This means that the coverage behavior (and frequency response) of a loudspeaker in the middle of an array, with other loudspeakers above and below, will be different from a loudspeaker near the top or bottom of the array. It also means that processing applied to individual boxes in the array is not independent, and will influence the response of the entire array.
The mathematical simulation of line array behavior is complicated, and becomes even more so when you consider that the aiming, inter-box angles, levels and system processing can vary on a per-box basis. Fully realized line array systems give the system provider many variables to juggle and implement, increasing the risk of poor deployment.
Processing required to correctly influence the overall array response can be very unintuitive. While it is tempting to adjust the processing of the single loudspeaker pointed at a specific audience location, this can rarely be done in isolation without deleterious effects on the overall array response. It should come as no surprise that several line array manufacturers offer comprehensive training and certification programs for aiming and processing of their products.
Constant Coverage Arrays
Enter the constant coverage/constant curvature vertical array. While one could implement this type of array using line array boxes, more common is a new class of boxes with fixed coverage angles. These loudspeakers lie somewhere between a full-fledged line array and the classic “column” loudspeaker.
Constant coverage arrays offer a compromise between performance and complexity of implementation. Constant coverage vertical arrays typically offer a single nominal vertical coverage angle. This angle is usually narrower than a traditional, horizontally-arrayed trapezoidal loudspeaker, but broader than a line array enclosure. Angles in the vicinity of 15 to 20 degrees are common. Accompanying the single, fixed vertical coverage is a singular flyware angle. This reduces the cost, complexity, and weight of the flying hardware. It also removes one element of difficulty from the array’s deployment.
JBL’s VRX series, the prototypical constant coverage array, offers the ability to perform in-box tapering of the high frequency level in the vertical plane. Many other products in the space offer similar functionality. This zoned tapering is effective at very high frequencies where the loudspeaker has good pattern control, as discussed above. This tapering can help compensate for distance-dependent high frequency losses without the need for additional processing and drive channels.
The fixed flying angle between boxes allows the manufacturer to provide optimal high frequency arrayability. Their waveguide(s) can be tailored specifically for the flying angle, providing smoother coverage at box transitions. There is a psychoacoustic advantage to managing inter-box transitions in the vertical plane, as human hearing is much less sensitive to the artifacts of where the boxes overlap in the vertical plane than in the horizontal.
Constant coverage arrays have limitations, of course. For instance, adding an additional box can result in over-coverage of the space. This results in spilling too much sound onto the stage, or onto the back wall of the venue. Further, constant coverage arrays generally cannot be hung “deep” enough to provide directional control at low frequencies.
Other limitations include less output, and fewer high frequency drivers to overcome air absorption, than a conventional line array. This limits constant coverage arrays to short- and medium-throw applications, similar to those one would cover with a conventional horizontal array.
Limitations aside, there are plenty of situations where a single conventional loudspeaker doesn’t have enough output, and a line array is the wrong product for the job. Constant coverage vertical arrays stand in this gap, ready to win bids or finish the gig.
Array Implementation
In regards to output, the constant coverage vertical array is in competition with a multi-box horizontal array of conventional trapezoidal boxes. The decision to use either type of array boils down to coverage requirements, flyability, sightlines, logistics and client desires. Both types of systems can be the right tool for the job.
A factor that simplifies array setup is the broader high frequency vertical coverage angle of the constant coverage array. This makes the array more forgiving in the precision of its aiming requirements compared to line arrays. Of course, this wider coverage must be balanced against potential to bleed undesirably within the venue.
Generally, if the array must be ground stacked, a constant coverage array will be of questionable added value, unless the venue has a tall, steep audience area. Similarly, if the necessary horizontal coverage is very broad, a conventional array may be the more appropriate tool.
When logistics allow for a constant coverage array, the following guidelines will help you implement a constant coverage array:
• Pick the number of boxes needed to cover the venue vertically, from slightly above the back row to the stage lip.
• Check for lobing immediately below the array that might cause feedback.
• Tailor the levels of the HF waveguides to be highest at the top of the array, and lowest at the bottom.
• Amplifier gain tapering of each box in the array is acceptable, but generally of little effect for these short arrays. However, this may improve midrange coverage evenness.
The Bottom Line
Constant coverage vertical arrays are comparatively new arrows in the quiver of pro audio solutions. They provide the opportunity to divide the audience vertically at high frequencies, but with greater design simplicity than a full line array. Typically, they can be used with fewer processor channels than a full line array. In short and medium throw environments, where one might process each zone of a standard line array in similar fashion, the constant coverage array can produce similar results with a lower cost basis.
Further, constant coverage arrays can be lighter, smaller, and provide lower visual impact on the venue than a trap array or conventional line array. There are many smaller venues and installations where these are very valuable traits.
Finally, constant array cabinets can provide good flexibility for the rental house. They can be used as front fills, delays, a powerful speaker on stick and as a medium-sized flown system — many different gigs from one type of box.
Phil Graham is the senior engineering consultant of Passband, llc. He has been implementing advanced arrays for more than a decade. Email him at: [email protected].
