May 2013 Issue
Tech Feature

Subwoofer Arrays in the Real World

Fig. 1: The central cluster is one of the simplest approaches to subwoofer placement, and is predictable, yet non-directional.Over the past two years of FRONT of HOUSE, we have covered substantial ground with respect to subwoofers. Whether on the specifics of setting up a basic cardioid array or on how to design your own vented box, we have presented several nuts and bolts articles on the ever-important bottom-end that keeps sound reinforcement exciting.

This installment is very much in keeping with the previous practical articles on subwoofers. I am suppressing my inner egghead tendencies towards discussing topics like “phaser summation in the far-field,” or “radiation impedance,” and instead will look at a number of subwoofer configurations as they were implemented in the field of real gigs. We will briefly overview each configuration and discuss the implementation, and the goals behind the implementation.

It is my hope that the readers will come away with a sense that many different clever subwoofer arrays have been implemented on real gigs, provide effective low frequency control and can be useful arrows in the sonic quiver when circumstances direct that way. Interested readers are encouraged to reach out if they’d like more technical references for creating these arrays.

» Funny Looks

My first attempt at directional control of subwoofers came in 1999, with the goal of reducing the spill between two stages. What I ended up trying out would today be called an endfire array. I remember getting quizzical looks for both the layout of the subwoofer cabinets and my request for additional DSP channels, which were not as cheap and plentiful back in those days. It was only after turning the system on, and seeing that it worked like the theory said, that minds were set at ease. Even today, the “it looks wrong” factor can be a big impediment to otherwise realizing gains in performance and control.

We have been well-conditioned to stereo configurations of main speakers, and pro audio people can be nervous to move out of the left/right sub stack configuration into something more adventurous. If your production company, band, or venue has been looking to try some of these more advanced configurations, let this article be comfort to see that others have blazed the trail, had successful gigs, and lived to mix another day.

» Guiding Principles

Before diving into directional bass arrays, here are some essential principles for understanding how the arrays operate:

• Subwoofers aren’t very directional, so whether you’re standing in front of one or behind one, their response is similar. Turning a sub around backwards is simply a straightforward way to produce physical separation in space between the drivers.

• To produce directional response from a subwoofer array, you must physically space the boxes in some fashion. Only then can you delay certain boxes, change levels, and sometimes switch polarity. The core of directional control is physical spacing the cabinets followed by signal processing; this processing will be delay at a minimum and sometimes also include changes in level and polarity.

• The larger an array is physically in any given direction, the more inherent directivity it has in that direction. A long line of subwoofers (e.g. stacked along the entire stage front), with no further processing, will have a more concentrated and narrow response in the direction where the line is long (i.e., perpendicular to the stage). Sometimes this is a desirable effect, like when two stages are adjacent and you are trying to minimize the spill between them. It can also be a disadvantage in a situation where the coverage area is much wider than the stage width. While directional subwoofer arrays are normally designed to have narrow directivity, one can use processing to broaden the coverage of an array that is large in one particular direction.

• Control of the array can be influenced in each of the three dimensions by different array sizes and placements in each dimension. For instance, if an array is large horizontally, but small vertically, it will have narrower coverage in the horizontal and broader coverage in the vertical.

• Design of these arrays necessitates the availability of additional DSP, and amplifier channels, as each component of the array requires specific processing for the directional control to work properly.

• Directional control is most effective in the “far field,” which means at a distance far enough where the loudspeakers are similar in level to each other. If you stand very near the array, so the volume is dominated by a specific loudspeaker, then the array’s directivity control will be found less effective.

• Simple arrays, like cardioid and endfire, can be set up (and visualized) without array modeling software, but more complicated arrays, with directivity in multiple directions, are best predicted in software before implementing them in the field.

• Arrays need some space surrounding them to allow for the sound from each speaker to interact. One rule of thumb is to keep them at least a meter away from any solid boundary.

With these principles in mind, we now move on to a series of arrays implemented in the field, provided to me courtesy of some of the participants on soundforums.net.

» The Central Cluster

Shown in Fig. 1, the first image — a center cluster of subwoofers under a mobile stage platform — is courtesy of Tom Bourke. This is the most basic break from the left/right placement of subwoofers underneath the main speakers. This is something that can be easily implemented without any additional processing or delay lines.

The center cluster provides more even coverage in the middle of the audience than a left/right pair, which will have “power alley” areas and other areas where the subs are partially cancelling each other out. A very wide center cluster can have overly narrow horizontal coverage, but a small cluster like this one is a simple and effective improvement in low-end evenness for those small gigs. This array does not provide any directional control behind the boxes, so the amount of bass leaking behind this mobile stage is about the same being projected into the audience.

Fig 2: Reversing the direction of one sub box in a ground stack (in this case, the middle enclosure) can result in a cardioid response. Fig. 3: A cardioid array created by reversing the direction of the bottom enclosure in a ground stack configuration.» Two Takes on Cardioid

Fig. 2, which comes courtesy of Robert Caprio, shows Cee Lo Green’s side-fill rig for a series of performance dates in Las Vegas. Here the central subwoofer in a stack of three is turned around to implement the classic cardioid coverage pattern. Fig. 3, courtesy of Bennett Prescott, is the same type of cardioid array, but with the bottom box instead reversed.

Theoretically, reversing the bottom box, which is closest to the floor boundary, gives the best overall cardioid pattern. In reality, though, both arrays are similar in coverage. This is a classic example of how production realities can get in the way of what is theoretically ideal. Cee Lo Green was a guest artist in the theater, and his monitor rig needed to be struck quickly. The end result was reversing the middle box and leaving all three subwoofers on the dolly board for easy movement.

Both of these arrays achieve the same directional control objective, which is reduction of bass behind the array by at least 15 dB across the entire low-end range. This provides much needed relief for Cee Lo’s monitor engineer, and helps Prescott manage the on-stage bass level in the theater. Notice that in both Figs. 2 and 3, Prescott and Caprio have left sufficient clearance around the arrays to allow the cardioid pattern to be formed effectively.

Fig. 4: An endfire subwoofer array configuration.» Classic Endfire

Prescott also provided Fig. 4, which is a picture of the stage right endfire sub array from an outdoor show in New Jersey. This particular endfire array is three rows deep, and provides at least 12 to 15 dB (or more) of rear-facing isolation across the sub band. The endfire configuration provides a broad horizontal coverage pattern out in the audience area, while still providing useful reduction in sub level on the stage behind the arrays. Endfire arrays offer more isolation as you increase the number of rows in the array, and require a fair amount of depth.

Subjectively, endfire arrays are typically slightly preferred in overall sonics to the cardioid array. Also visible in this photo is the cardioid side-fill subwoofer, and a supplemental center sub cluster to help even out the horizontal coverage in the audience.

Fig. 5: An endfire subwoofer array placed as a center stage cluster.» Center Endfire

David Sturzenbecher from Absolute Productions provides an interesting take on the endfire array for an event in South Dakota, as shown in Fig. 5. Here the 16 subwoofer
boxes were placed in a two-deep endfire center cluster, eight in each row. The subwoofers of the back row were tight packed to provide a wider boundary for increased directivity, and the subwoofers in the front row were spaced slightly in the horizontal to shape the forward coverage and insure the rear rows sound would not be influenced by a large boundary towards the audience. They achieved 10 to 12 dB of isolation on stage behind the subwoofer cluster, and also achieved a more narrow horizontal bass distribution due to the width of the cluster. This is a nice configuration for the situation where you want isolation on the stage and a narrow main coverage lobe. It would be useful in the context of two adjacent stages at a festival.

Fig. 6: The innovative ICAD approach creates a directional response simply by moving the center sub enclosure(s) back by 1/4 wavelength (at 80 Hz), or approximately 3.5 feet.» Hybrid Arrays

The last two figures are examples of what skilled systems techs can produce when they move beyond simple cardioid and endfire configurations. Fig. 6 comes from Brandon Romanowski. He has dubbed this the ICAD array, for the Indigo Concert Audio Department array. This array likely started as a basic endfire configuration, but by spacing the subwoofers both vertically and horizontally, he can control the horizontal directivity and rear rejection simultaneously. This is a clever design that modeling software can really help confirm before implementing it in the field. More advanced hybrid arrays are typically tweaked in the modeling software to nail down all the processing levels and delay times.

Fig. 7: Another hybrid technique involves combining two cardioid subwoofers: one front- and one rear-facing.The hybrid approach in Fig. 7 was first configured using the software EASE package before being implemented in the field. It comes courtesy of Michael Smithers at Eclipse Audio in Australia. In the photo are two cardioid subwoofer pairs (one forward-facing SB1000 and one rear-facing). The two pairs are then configured into a “V” that faces towards the audience. The back of the “V,” where the two rear facing subwoofers are closest, provides a consistent location for the rear rejection of the cardioid array. At the front of the “V,” the spacing between the subwoofers influences the width of the horizontal coverage. Thus this array shapes both the rejection behind, and the width of the horizontal coverage. With the local residential area in clear view within the photo, this is a nice “win” in regards to noise complaints for the production company.

» Just the Beginning

This article includes less than half the photos that were culled from various gigs where people implemented low frequency control. The ones chosen here were by no means the biggest or fanciest arrays people had implemented, but represent a reasonable cross-section of types of arrays employed in the field today.

Hopefully, what may seem like strange configurations spotlighted in this article may open a few eyes in terms of what can be possible for low frequency control. Whether in the simple/no extra processing center array or simultaneous control of directivity in multiple directions like the ICAD array, there are many creative and useful ways to lick the problem of low frequency control