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Center Clusters and Exploded Clusters: The Debate Continues

David Kennedy • September 2019Tech Feature • September 11, 2019

Last month (FOH, Aug. 2019, page 28) we examined the history of center speaker clusters and the key manufacturers that developed/offered the large HF horns and bass boxes (installed flying junkyards), as well as the later trap (trapezoidal) cluster enclosures. These early center clusters were good at speech-type P.A. (public address) — primarily for communicating clear/intelligible speech — and provided natural sound localization to central sound sources — with emphasis put on uniform sound coverage across a venue seating, per Don Davis, an author and pioneer on installed center cluster design/engineering.

But, good as the vintage center loudspeaker clusters were for speech system installations, and as dominant as they were in the arena and theater markets, the early Altec clusters of a half century ago did not sound very good for large music groups. The bigger the band (with more complex music/signals), the more apparent were artifacts such as distortion and ringing, even if the modest power handling was not exceeded. Fig. 1 shows an example of a center cluster using the later/improved JBL Bi-Radial horns.

Fig. 1: Classic center cluster design by Sound Concepts, LLC for Hope Church (Mason, OH), which combines JBL Bi-Radial horns with Renkus-Heinz components.

‡‡         Loudspeaker Cluster Design Issues

We also covered how the height of a center cluster is critical, as too high an elevation will cause a delay and lack of stage localization. A low ceiling is also a limit that can eliminate a center cluster from consideration. Array/cluster height and under-array lobing will determine feedback stability and how far out into the room you will be able to cover well. Array/cluster design issues include: budget, ease of rigging, program material, ceiling/proscenium/set height, coverage angle, sound imaging, sound level and spacing of arrays (venue size).

‡‡         Stereo Clusters

Last month’s article about center loudspeaker clusters did not refer to most modern live concert events, nor center-fill speakers used in Left/Right arrays. One-night concerts and temporary event setups can’t (and don’t need to) be as optimized for uniform sound coverage, and typically have less need for localization of center sound sources such as speech or solo vocals through a central array/cluster. Live rock systems are more about high SPLs, excitement/impact, spaciousness and wide bandwidth, and of course quick set-up; so stereo arrays/loudspeakers are the default for most rock concerts, as per tech riders.

Fig. 2: PreSonus/WorxAudio central X5i-P line array at First United Methodist Church, Wabash, IN.

‡‡         Center Line Arrays

A compact center line array can work well for moderate sized venues — assuming there is adequate ceiling height. Successful examples of this are numerous. Among these are the PreSonus X5i-P one-box, center line array installed by Indiana-based CSD Group in the nearby First United Methodist Church (Fig. 2), along with other cost-effective projects, including a more esoteric, custom-center line array with ribbon tweeters and cardioid subs, that the author designed for an SDA church in Escondido, CA. Note that array and cluster are synonymous, with array being the newer term.

‡‡         LCR Systems

LCR (Left, Center and Right) systems are the norm in live theater, cinemas and high-end churches. Almost all national touring musical theater shows carry LCR — ideal when panning wireless mics worn by actors as they move across a stage, and for panning instruments between channels — to provide more natural sound localization to the live sound sources. With LCR systems, the total-direct SPL received from all three clusters should be as nearly uniform as practical over the audience — with each of three clusters covering all of the seating area. In the mid-80s, I hired Parnelli Award-honoree Mark Engebretson — a mentor and pioneering engineer for over a dozen pro sound companies — to design superior-sounding loudspeaker systems, based on TAD drivers, for very satisfied clients with smaller LCR projects.

Fig: 3: Here, a JBL PD Series exploded cluster (not LCR) outperforms the original center cluster in the Community Pentecostal Church in Ottawa, Ontario, Canada.

‡‡         Exploded Clusters

As venue sizes have grown considerably over the decades, the required coverage angle, spacing of arrays/clusters (that can cause long delays) as well as higher SPLs, have presented significant challenges in very large venues. This makes center clusters unsuitable in the most massive installations. It is also difficult to get enough gain before feedback when a center cluster’s height is restricted. So, as venues got wider, and a larger sound field was needed, sound designers began installing what came to be known as exploded clusters. Note that Fig. 3 shows three pairs of speakers in an exploded cluster — not LCR.

Fig. 4: Five Tannoy arrays in an exploded mono cluster in the Casa de Dios, a 12,000-seat mega-church in Guatemala. The system was installed by MGA.

When I heard about exploded clusters in the late ‘80s, I was concerned about using them, as most of the system examples for installation projects and textbooks I had studied had center clusters or LCR, and most of the sound systems I designed used center clusters. But, larger/wider and height-restricted venues required a fresh approach, so I collaborated with innovative sound engineers Michael Garrison and Brian Roggow of MGA (mganow.com), to design better-sounding exploded clusters into several large churches using Tannoy components. The small-exploded clusters worked well, if kept within the agreed SPLs and in treated rooms. See five exploded, waveguide-loaded Tannoy (high-SPL) point-source arrays in an MGA-designed 12,000-seat megachurch in Fig. 4.

Note that “mono split cluster systems” may look almost like stereo, but they are not, as each of the two clusters is only covering half of the seating area well, if that. When an ideal LCR system (we’ll look into these next month) is too costly, a venue is too curved/wide, the ceiling is too low and/or panning talkers as they move about a stage is not important, an “exploded” LRLRL System (of exploded speakers) is a reasonable alternative. But be very careful that all the loudspeakers are not used for speech at the same time. Users tend to pan the talkers between the console L/R outputs. However, in such systems the redundant loudspeakers (creating overlapping coverage) can actually reduce speech intelligibility — especially in venues with high reverb times — by introducing too much delay between the arrays. Only the three “L” arrays should be used for talkers.

‡‡         Clusters vs. Line Arrays: The Experts’ View

Another pioneering sound engineer is Chris Foreman, who worked for several key manufacturers (including Altec, Community and JBL), through the live-sound industry evolution of cluster design. He “wrote the book” on sound system design, and is still a prolific tech writer in the industry.

“There are several advantages to exploded clusters,” he said. “But these apply to specific applications, so they’re not set in stone: In a shallow, fan-shaped space with a wide stage, an exploded cluster is usually the best choice (three to five locations can cover the listeners evenly). Each cluster is relatively small, so the visual impact is reduced. Lobing and comb filtering can be controlled by minimizing overlap. This is not true stereo (nor LCR) and isn’t meant to be — but it works and can be very effective. In this same type of space, an exploded cluster is superior to left/right line array hangs as well. Line arrays are likely to have trouble filling the center-front of the venue. And, exploded clusters are less visually obtrusive.”

Foreman added that “a small central cluster, supplemented by a distributed group of arrays above the audience areas, can be a very effective way to cover an arena-type space. The entire system can be reconfigured with delays for different event types. Central arrays have their uses and advantages. However, it is very difficult to design a good central array using available loudspeakers today. Almost nobody makes separate horns that are big enough to control midrange frequencies, so component clusters that existed in the Altec days are practically impossible. Small horns are a problem with arrays of point-source boxed speaker systems as well. And, horn-loaded woofers must be custom-built. Finally, today’s designers tend to put line arrays everywhere I’ve suggested for exploded arrays. But a well-designed array of point-source boxes can offer more horizontal coverage flexibility, reduced comb filtering, a smaller visual footprint and better sound quality — and will almost always cost less. Use line arrays for their greater throw and greater vertical coverage control. Use well-designed arrays of point-source loudspeakers elsewhere.”

“Monaural exploded clusters have some shortcomings, as do all other currently available loudspeaker array schemes,” explained MGA owner Michael Garrison. “But in my experience, this relatively cost-effective design approach can provide excellent results in a broad range of settings — particularly in wide, fan-shaped rooms. We have deployed many three- to five-facet main arrays in facilities with seating capacities ranging from 300 to 12,000. The larger rooms naturally require a ring (or two) of supplemental delay loudspeakers. But in all cases, our clients have been enthusiastically satisfied with the performance and the value of their systems.”

John Hardwick, of MBF AV & Acoustical Consulting
(mbfava.com) suggested that “much of what has driven us away from center cluster design is the semi-educated consumer fueled by the clever/glitzy marketing of media and large audio manufacturers set the stage for the trend, as well as the increased quality of automotive and home sound systems that provide more enveloping sound fields. The fact that quality has been subjugated by MP3 sound files and ear buds has only cemented the acceptance of poor-quality, highly enveloping sound.”

“Don’t know if it is a desire for larger stereo sound, since so many of these systems, especially line array solutions, are not what I would call ‘stereo,’” said Jerrold Stevens, sound consultant at Marsh/PMK (marshpmk.com), “as the left and right arrays do not typically cover the entire seating area. It is difficult to get low-frequency vertical pattern control when center cluster height is restricted.”

 

Q&A: Whatever Happened to Center Speaker Clusters?

Regarding center array/clusters, I spoke with experts from the manufacturing and production communities. They included: Marc Lopez of d&b audiotechnik, John Mills of EAW, Scott Sugden of L-Acoustics, Bob McCarthy of Meyer Sound, Pete Till of Yorkville Sound, Rocky Giannetta of Layer8 and Robert Scovill of Avid Technologies.

Has live music reinforcement and the desire for a larger stereo sound overshadowed the need for a center sound source for speaking and solo vocals?

Marc Lopez: The need for a center cluster should be driven by the content of the particular venue or facility. From the d&b perspective, the content and applications have not changed so much over the years, and the typical restrictions of budget and sight lines are still present. We do observe that there is some greater significant influence from the touring side for a Left-Right music system that tends to affect whether a center cluster is specified or even considered.

John Mills: As we all know, a proper LCR system can get very expensive which is why, I think, when line arrays entered the market, we saw a shift to “stereo” because the sales pitch for at least three line arrays would be more than most wanted to swallow at that point. And it was much easier, as said last month, to deploy a line array than all the design, complex installation, and the DSP matrixing needed for true LCR clusters.

Scott Sugden: The inclusion or omission of a center cluster is really a function of the production’s direction; different types of content will influence that need. For example, it is exceedingly rare to see a center cluster style system on most traditional pop music tours and, to the opposite, rare to see a theater show in NYC or London without a center cluster. In small theater spaces, the center cluster is usually capable of achieving coverage for most of the venue and ensures good SPL propagation for any mono signal.

Bob McCarthy: The center cluster has been dead for live music reinforcement for the 40 years I have been in the industry. All its inherent built-in advantages combined do not outweigh the inalterable fact that it shrinks the music down to a single point source: the polar opposite of a symphony spread across a stage. Simply put, musicians (beyond a soloist) are physically spread panoramically across a stage: They are low vertically and wide horizontally. A band mixed down to center cluster is neither, since it must be placed high vertically to be clear of the proscenium. Its sonic image is disconnected from the band and has all the auditory excitement of a totally static, pinpoint sound image. It is not about stereo (not likely possible over large spaces). It is about having sound spread horizontally across left and right (even if it is >90% bused mono). Left/right systems (which I call them rather than “stereo”) can be placed low enough to stay linked to the band and stage. The potential loss in intelligibility is mitigated by the fact that modern systems have so much lower distortion and superior directivity control than grandpa’s center clusters.

Pete Till: If it is important for the client to have a clean stage, and omitting the center cluster reduces set up and tear down time, many sound professionals are happy to leave the center cluster out of the equation. However, it’simportant to be aware of the real benefits a center cluster provides. A dedicated single source for vocals — for both live sound and solo/vocal-only applications — can provide unparalleled intelligibility due to a lack of comb filtering from multiple sound sources and modulation effects from multiple channels.

Robert Scovill: In houses of worship, spoken word and music production have to co-exist. This is a considerable challenge IMO because stereo or even “split mono” as you refer to it is really undesirable for spoken word in that kind of setting, for all the intelligibility and localization challenges you mentioned. But, by the same token, a mono center cluster optimized for spoken word is far from optimum for complex music production. So, here’s the rub, for even moderately sized venues, stereo for music production also suffers from the localization and intelligibility challenges for anything panned to the center of the mix.

Is the omission of center speaker clusters largely due to budget and/or video/set sightline issues?

Marc Lopez: The new d&b audiotechnik A-Series is quite compact, accommodating a sightline-friendly center cluster application.

John Mills: I believe both. Video is king these days and a center video screen just cannot be occluded by a center line array.

Scott Sugden: Without a doubt, use of a center cluster would require the inclusion of the concept in the visual design of the show. It is not an impossibility — merely something that requires early input and planning within the creative team. This is not just true in the context of a center cluster, but also anytime audio wishes to depart from the convention of boxes on each side of the stage. Depending on the production, a center cluster can be an improvement.

Bob McCarthy: The omission is because almost no mixers want them. Touring mixers have show files with L/R and sub outputs. Center cluster mains are not on the rider. The budget becomes an issue also because the center cluster has to cover the entire room. I can’t tell you how many times I have seen center systems that cover under 2/3rds of the hall or have inadequate power. The left array doesn’t have to cover to the opposite side wall — but the center needs to reach both. The vocal has to be on top. So, when the center doesn’t reach these places, the next step is adding vocal into the L and R to fill those areas in and we have lost the game again. And if you make the center cluster wide enough to cover the whole room, look out for the mother-of-all rear wall reflections coming back at you (especially in a radial room). A pinpoint source has a major downside when it focuses back on you.

Pete Till: I believe a combination of sightline issues and setup time are the main reasons we haven’t been seeing as many center clusters today as we have in the past.

Rocky Giannetta: The omission is largely due to people misunderstanding image localization, and that a center channel is needed to create that image, when left/right speakers are used in all but the smallest spaces. Sometimes there are sightline issues, but more often it’s just the users thought that stereo is better, or the only loudspeaker solution is a line array, and that of course presents sightline issues in many spaces.

Has there been a lack of low-profile center cluster products available?

John Mills: Yes, especially in larger rooms that really need the long-throw capabilities of a line array.

Bob McCarthy: It is not about products. Making them low-profile won’t bring the image down. It is about applicability to the program, placement, panoramic width, integration with L and R.

Scott Sugden: The capability of the array is a function of its size: the longer the line, the more capable it is at providing more uniform SPL reproduction and tonal balance across the audience, as well as better gain before feedback on stage and a reduction in the energy introduced to the reflective surfaces of the venue. The combination of the practical design requirements and the needs of productions for the best sound quality over the audience have made the choice to use larger center clusters more logical. It is still possible to use a smaller format system, albeit with a compromise to the desires or needs of the modern production.

Robert Scovill: Low profile meaning “small and powerful” yet able to cover a large geometry? That’s a big order to fill even today. But if you’re talking voice only for that speaker system — then there are a LOT more possibilities at your disposal to get the job done. Especially so for arenas, etc. But once you add a live music component, even in a fan shaped room, by modern standards of music’s dynamics, frequency range and SPL measured against coverage capabilities, an adequately effective solution is way more elusive.

Are more new center speaker cluster solutions now available?

John Mills: The EAW ADAPTive line fills the above video/set issues very well. Because of their ability to steer straight down, the trim height is less important than that of a traditional array. So, you can place them almost as high as necessary to clear any video or set issues. Because it hangs straight, you can place two, three or more columns right next to each other to get a wider coverage pattern. A two-column wide array of ANNA can cover 180 degrees, so it is an ideal choice for a true center cluster. In the case of our ANYA system, a two-column wide approach would give 140 degrees. The length of our arrays is determined by two factors, the lowest frequency you wish to be able to control and the desired SPL.

Marc Lopez: The new d&b audiotechnik A-Series Augmented Array is a very flexible solution for mid-size venues offering both horizontal and vertical configurations, adjustable splay angles and Midrange Directivity Control for making sure the vocal range is smooth across the listening area.

Scott Sugden: To have the most pristine vocal clarity from a center cluster, a line source array or single point-source should be used and clusters of point-sources should be avoided. Customizable horizontal control can be especially useful in a center cluster for venues that are narrow but deep. Stability in the response in the coverage areas as well as stability in the cancelation areas is paramount to good gain before feedback. Many loudspeakers can make great center cluster options; from L-Acoustics we have long-throw solutions available that are often used as center clusters like Kiva-II and Kara. Some venues would be better served with medium-throw solutions like the A Series in a horizontal array configuration. In smaller venues, a simple short-throw point-source is often the right choice for a center cluster.

Pete Till: Yorkville Sound has the solution with a cost-effective approach. We have just released the Synergy Array Series, an array-able cabinet that helps solve some of the issues associated with center clusters. The SA153 is a very compact, high-output cabinet, making sightlines less of an issue. The cabinet provides precise directivity control and is array-able in both the horizontal and vertical planes, so virtually any desired dispersion can be achieved, without the typical comb filtering effect. I believe we are going to be seeing more center clusters in the near future.

‡‡         More to Come

Some of the replies that I received to my questions above — from live-sound array factories — are more related to our next article on LCR and immersive systems, so they will be deferred until next month — see you then!

David K. Kennedy, a 40-year live-sound veteran and author, operates David Kennedy Associates, consulting on the design of architectural acoustics and live-sound systems, along with contract applications engineering and market research for speaker manufacturers. He has designed hundreds of auditorium sound systems for churches, schools, performing arts centers and AV contractors. 

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