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Acoustic Expertise: Worship Facility Structure

Oct 10, 2011 10:06 AM, By Bob McCarthy

Understanding worship shape to fit your sound design.


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Horizontal speaker configuration examples for 28-meter, 90-degree fan-shaped room. A single 90-degree system can cover the entire space but must be placed at the focal point of the radius. This is not practical.

Horizontal speaker configuration examples for 28-meter, 90-degree fan-shaped room. A single 90-degree system can cover the entire space but must be placed at the focal point of the radius. This is not practical.

Imagine a scenario where the architects in charge of designing a new house of worship asked what the best room shape would be for the sound system. Yes, this is far-fetched but worth considering as an exercise. What would it be: a shoebox, a fan shape, a square, an oval, or perhaps something exotic? It's not so simple because there is no single best shape for sound in general or for HOW sound in particular. Instead, we need to clarify first what the priorities are for the sound system. Is clarity of the spoken word the overriding priority or is an immersive musical experience also of great importance? The best shape for one is not necessarily the best for the other.

Back in the real world, we are faced with these same choices, but from the other direction. The building shape is handed to us and we are told to design the sound system into it. Again, we have to sort out those same sonic priorities before we can decide whether to go with a mono center cluster, stereo stacks, or a more complex multi-source design. The macro shape will have a very substantial impact on how easily we can meet the priorities. Certain shapes lend themselves strongly to voice transmission while others favor the immersive. The most traditional of all church shapes, the grand cathedral with its deep, high-walled rectangular seating areas, is the worst for voice and the best for immersion. It is the ultimate challenge to deliver intelligible voice in such a place, and yet it can make an elementary school choir sound good (unless they are off key). By contrast, the original classical theatrical shape, the Greek theater fan, is highly favorable to voice clarity over immersion. Special efforts are needed to return the sound back into this type of room to create a spatialized soundscape.

So think of this like a stream. We can paddle upstream or downstream. We can get there either way, but upstream takes a lot more effort. Some room shapes are downstream for the voice and upstream for musical immersion, others the reverse. Some have strong currents; some are barely detectible. If you have the wrong type of room shape for what you want to do, there are very real consequences. It can be disappointing, cost more money, or both. This article is intended to explore the impact of macro shape on your sound design ambitions in houses of worship.

Priorities

Before we consider the room in a physical sense, we have to know what will go on in the space. The possibilities range anywhere from voice with minimal music to full bands, vocalists, and choirs. If we look at this as a continuum, we see the needs of voice intelligibility on the left and musical envelopment on the right. Some important sound design decisions will move us toward one side or the other. A simple example is the question of center mono or stereo. Center mono favors voice but makes musical content spatially invariant, which contrasts to how we experience the mix of direct sound from musical instruments coming off a stage. Stereo is better, at least for the central area, at creating a spatially variant musical experience, but it costs vocal intelligibility. If we must have 100 percent of the best in both categories we may find ourselves buying two speaker systems, the multichannel configuration known as left, center, right (L/C/R). This configuration, however, is an extreme operational challenge and should not be entered into lightly. More on that later.

So we will proceed by looking at different shapes and see how we can fit mono, stereo, and multichannel configurations. We will explore three main shapes: the fan, the long/narrow rectangle, and finally the short/wide rectangle.

The fan shape

With a 7m stage platform we can use 2x90-degree systems at 45 splay.

With a 7m stage platform we can use 2x90-degree systems at 45 splay.

This is a very popular shape for the modern HOW. The congregation can be kept close to the platform and maintain a visual intimacy with the clergy. This shape also has good locations for the all-important video screens. The first question about the fan is one of degree(s), as in: "How wide is the angular spread of the fan?" The typical range is 60 degrees to 120 degrees, but it may approach 180 degrees at the most. You might think for a moment that a 60-degree-speaker would be a good fit for a 60-degree-room shape, but this would only be true if the speaker was placed at the rear of the stage. Instead, the speaker location(s) are found near the front of the stage, which is typically also a fan shape.

A fan shape for a house of worship is difficult to cover from a single central location for two reasons: First, there is always a lot of open mic activity at downstage center. Covering the whole room from there presents a high risk of feedback. Second, reaching the near seats on the sides requires aiming downfill speakers wide—another feedback risk.

With a 10m stage and 3x90-degree systems at 30-degree splay.

With a 10m stage and 3x90-degree systems at 30-degree splay.

So then the question becomes "How many speaker/cluster locations are needed?" Start with two and go up from there. The quantity will rise as stage depth increases and fan angle increases. The further pushed out we find our starting positions, the more we need to subdivide to keep the slices manageable. Two or three positions are typical for 60 to 90 degrees. Three or four positions should get things done for a 90 to 120-degree-fan and then up from there for the oversize fan. The fan shape favors a spokes-on-a-wheel approach to speaker aiming, which will subdivide the space into slices.

A 14m stage extension with 4x90-degree systems at 22.5 degrees.

A 14m stage extension with 4x90-degree systems at 22.5 degrees.

The first casualty of the fan shape is stereo, especially with wide fans. Stereo requires that the overlap between the left and right sources be fairly close in level and time. The radial approach to the speaker placement disfavors overlap since each element is aimed to its own section. If we covered a narrow fan, such as 60 degrees with two clusters, there will still be plenty of angular overlap and relatively close timing in the center so we can get away with stereo. Such a narrow slice is not much different than a rectangle, the most stereo-friendly shape. But when the fan gets wide, the speakers must be spread out to their respective zones as soloists. Attempts to crossfire sources over a wide fan will result in a substantial loss of intelligibility because the arrivals cannot be kept anywhere near close in time or level at more than a few seats. The negative side effects of this approach double and redouble if we attempt it with the three and four speaker configurations. Designers of these configurations have been known to play games such as alternating left and right or splitting clusters into left and right sections. The result is such a mess that one person hears the keyboard on their left and another hears it on their right, and if they look at the keyboard, it might be centered in front of them. What is that? One person's stereo is another's "oerets". We would all agree it's not mono, but it sure ain't stereo. Let's not forget that in a wide fan everyone sees the event centered in front of them. Therefore it is no crime if the sound comes from the same place.

The approach is fairly straightforward then: radially subdivide the space into sections. Sometimes the seating chart will do the design for you: three seating sections favors three clusters, while four sections favors four clusters. If the space is large, then delay speakers can be added that follow the same radial pattern of the mains.



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