From the summer festival setup on a Stageline SL100 mobile stage to shows at a “shed” amphitheater, to the largest arena productions, vertically arrayed speaker deployments increasingly rule the day in pro audio. Vertical arrays have advantages in deployment, sightlines, and dividing the audience into different coverage zones. Here in the pages of FRONT of HOUSE, I have talked about the physical principles of vertical arrays but not about their practical field deployment for the working technician. We will now remedy this situation, as this month’s tech feature is the first in a series that will detail some real-world idiosyncrasies involved in deploying vertical arrays and practical approaches that the system technician can apply to ensure even coverage throughout the audience area.
To the reader not dealing with vertical arrays every day, we hope this series illuminates some of the complexities behind those “simple” vertical speaker hangs. To the experienced system tech, we ask that you approach the series with an open mind, as we may end up grilling some of your sacred cows. In this first part, we’ll begin with an overview of array behavior, followed by discussing the tools to control the array, and finally we will arrive at the launch pad for investigating practical deployment techniques that can be applied to vertical arrays in the field.
Arrays: An Overview
Today there are a great many manufacturers of loudspeakers that can be configured vertically. The move towards vertically arranged arrays has challenged pro audio manufacturers to bring a new level of integration, mechanically and acoustically, to their products. This has benefited all those who work in the industry with tools that are easier to deploy and integrate better acoustically. But along with the real improvements, some unfortunate terms have come into the industry’s vocabulary regarding vertical arrays. Chief among these is the phrase “line array” — which will be noticeably absent from this series. To be clear, the concept of a line array is a mathematical abstraction useful to learn certain concepts. The term line array is not an adequate descriptor of the behaviors within real loudspeaker systems.
Real arrays consist of physically discrete sources. Some of these sources, like cone loudspeakers, have comparatively low directivity at most operating frequencies. Other sources, like horns, or “plane wave generators,” can have very high directivity, especially at high frequencies where the wavelengths are short. This is not fundamentally different than the behavior of traditional horizontal trapezoidal speaker arrays, although the primary directivity control is now focused in the vertical plane.
What has changed for vertical arrays is the degree to which loudspeaker designers have endeavored to control the vertical directivity to a narrow range of angles at higher frequencies. Then, outside of the frequency range where each loudspeaker cabinet has its own appreciable directivity, the loudspeaker designer has now placed the low(er) directivity transducers close together to facilitate constructive summation between elements. In practice, both horizontal and vertical arrays have different source behavior for different frequency ranges. At high frequencies, the listener experiences sound produced primarily by one or two highly directional sources, while at lower frequencies sound at the listener is produced by essentially all the low directionality transducers in the array.
As an example, users of horizontal “trap” arrays have experienced that the low-mids build up as more boxes are added horizontally. This effect results from the directivity of the array changing in the low-mids due to the additional cabinets increasing the array’s width. As the array gets physically wider, the horizontal coverage of the low-mids narrows, and this results in more mid-bass energy being concentrated in the main array lobe, causing low-mid buildup on the primary array axis. While grabbing the graphic equalizer may fix the low mid tonality issue for the FOH mixing position, it does not influence the underlying change in array directivity that caused the buildup to begin with.
Vertical arrays exhibit the same changing directional behavior as additional cabinets are added to the array, but generally the narrowing of the array in the vertical plane is beneficial to the overall coverage consistency for the audience. Unlike the days of trapezoidal arrays, this change in directivity is generally well quantified for vertical arrays. Modern vertical array loudspeakers either have their response tailored to balance the buildup effect, or the manufacturers offer processing in front of the array to manage the change in tonality. Further, modern array prediction software tools allow the system tech to visualize these behaviors in advance of deployment. What was once a side effect for trap arrays is now a useful pattern control feature.
Tools in the Toolbox
When deploying vertical arrays, there are several tools that can be brought to bear for improving the evenness of array coverage. The first — and often most constrained — is the array’s length and aiming. Determining where the array can be hung, and how deep, sets the timbre for all further array optimization. Beyond the overall array hang, of course, is the choice of inter-box aiming angles, which we will discuss below.
Next in the toolbox is the ability to electronically process the array. At the simplest, all boxes in the array are fed the same pre-processed signal. In more advanced implementations, the array is divided up into sections, each receiving different processing. This is perhaps a controversial decision for some, and some vertical array manufacturers are opposed to processing various regions of the array differently. Let me clearly state my personal conviction that there is much to be gained in segmenting a vertical array into different regions, despite any manufacturer protestations. It is my hope that through this tech series we will make a convincing argument that there are performance benefits to be gained from processing a vertical array in different sections.
Once we consider an array that has different boxes, each aimed differently, and then divided into processing zones, the true complexity of array optimization becomes clear. This complexity is further compounded by the diversity of digital processing we could potentially apply to each vertical zone of the array. This includes various equalization filters, delay, level, crossovers, and all-pass filters. To make sense of these permutations, there is value in stepping back and considering the underlying physical behaviors that we can manipulate.
Array Control — Aiming
The first physical behavior that we can control for a vertical array is the physical location and aiming of the boxes. Aiming of the loudspeakers is both a powerful tool and a limited one. The concept that the lows, mids and highs emanate from each vertical array box in a narrow beam pointed at a specific region in the audience is not supported by physics. At high frequencies (i.e., approximately 3k Hz), where sound has wavelength smaller than, or comparable to, the waveguide dimensions of an individual box, the loudspeakers’ inherent directivity is effective at directing sound at the desired location in the audience. However, as frequencies get lower, and wavelengths get longer, each box has less individual directivity, so directional control becomes an ever-increasing function of the total array.
At low and midrange frequencies, as the entire array’s directional behavior comes into the picture, the respective physical position and aiming of each loudspeaker has already been locked in, as the array’s aiming is typically configured based on the necessary high frequency coverage distribution for the audience. Thus, we have an array of individual loudspeaker sources with some fixed physical position that are now radiating sound over wide swaths of the audience area. At each location in the audience, the resultant response then becomes the summation of all the elements in the array at that point for every frequency the loudspeaker system is producing. Due to physical geometry, sound from each loudspeaker in the array will arrive at each audience location with different volumes and at different points in time. It is then that these differences in volumes and arrival times shape the aggregate directivity of the entire array at low and midrange frequencies.
Array Control — Processing
The physical location of each loudspeaker box may not be ideal for creating consistent audience coverage from the array. As such, we can turn to electronic means to manipulate the array’s response further beyond what the choice of aiming allows. Regardless of how fancy the applied processing becomes, it ultimately influences only two physical acoustic behaviors at the audience for each frequency. These are:
• Volume
• Arrival Time
Notice these are the same two parameters that the physical location of each array elements also influences! The difference is that electronic manipulation allows us to shape volume and arrival time in ways that physical position cannot. It is important to remember that electronic processing of the array does not change the directional response of a given array speaker, but rather the aggregate summation of all the array elements at each audience location. We cannot use a processor to magically make a specific box with the array have more midrange pattern control, but we can influence the overall midrange pattern control of the entire array.
Processing Limitations
Except at very high frequencies, array processing decisions cannot be made in isolation. For example, if system techs require more than 300 Hz in the audience area where the third box from the top of the array is pointing, they cannot simply boost 300 Hz on that box in the array and expect to create the resultant boost in that section of the audience. The level of 300 Hz for that audience region can be influenced, but it requires considering the processing of all array elements as whole to achieve the desired result.
Another limitation for array processing is that most of the electronic filters available to today’s system tech are of a type that influence both the volume and arrival time of each frequency. Common equalizer filters, for instance, cause a change in arrival time for a given frequency when the relative volume of that frequency is changed up or down. This effect is not a limitation of the loudspeaker system processor, but instead due to a deep link within the underlying physics.
As such, any equalization change of a specific frequency for a portion of an array will create an accompanying shift in that frequency’s arrival time relative to the un-equalized remainder of the array. However, when carefully considered, this effect can be a powerful tool for shaping the array’s coverage. Later in the series we will also consider delay and all-pass filters, two processing elements available to the system tech that only influence arrival time, and not the volume of a frequency.
Prediction is Your Friend
The interlocking effects of aiming and processing are difficult to tease out mentally without tools to predict how a proposed configuration will behave. Thankfully, vertical array prediction software, as offered by most loudspeaker vendors, is one of the best advances ever developed for the working audio technician. The powerful modern array prediction tools are free, or very affordable, for the technician whose company has invested in a vertical array system.
For this series, Meyer Sound has graciously allowed the use of its MAPP prediction tool to create renderings illustrating how we can influence array behavior. I specifically approached Meyer about MAPP for two reasons. First, MAPP offers a lot of flexibility in simulating not only the response of an array, but also the effects of various types of processing on the array. Second, MAPP gives a transparent presentation of the resulting array data.
Meyer has provided very high resolution input data for MAPP, and MAPP can provide very granular output data that gives a detailed picture of the performance of real arrays. The goal of the series is to present physics principles that can be applied to any vertical array product, and I want to use simulation software that would exposit the underlying physics with clarity.
Coming Attractions
In the second part of this series, we will launch immediately into looking at vertical array behavior, building on the foundations contained in this article. We will discuss selecting aiming angles for audience coverage at high frequencies and investigate how this choice of aiming influences the array’s directional response at high versus low frequencies. We will also explore some electronic processing that can be applied to a zoned vertical array to help make the lower frequency coverage more closely match the high frequency coverage pattern.
Finally I’d like to thank Scott Gledhill, Winnie Leung, Michael Creason, and Dr. Roger Schwenke of Meyer Sound for their participation and support in making this series a reality using MAPP.
