Acoustic Expertise: Worship Facility Structure
Oct 10, 2011 10:06 AM,
By Bob McCarthy
Understanding worship shape to fit your sound design.
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 2×90-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 3×90-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 4×90-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.
Acoustic Expertise: Worship Facility Structure
Oct 10, 2011 10:06 AM,
By Bob McCarthy
Understanding worship shape to fit your sound design.
An example of HOW with a very wide rectangle room shape and
stage. This was covered with 5 systems.
The long rectangle
This is the place where the song “Give Me That Old-Time Religion” comes from. This shape is found in the large-scale grand cathedrals down to the Elvis wedding chapel. Narrow, deep, and often very tall is the order of the day. There is no shape more favorable to a single center cluster from an acoustical point of view. Often the optimal central position is a non-starter visually, and the cluster is either pushed up into the rafters (a particularly bad idea in tall reverberant halls) or split into side elements. We can look at such rooms as an endurance contest: How far can we get before we have to call in for delays to carry things forward?
A downstage central location with enough height to reduce feedback risk and yet low enough to minimize the distance to the audience is the best starting point. The narrower and more reverberant the hall, the sooner we will need delays.
Halls of moderate width can do fine with a two-element (left/right) array and can have the highest percentage of seats experiencing stereo. If the sidewalls are fairly dead, we can allow for some extra horizontal pattern width to cross over into the middle for stereo and still comfortably reach the sides. However, if the length were long and the walls high and reverberant, we will need to narrow the coverage and aim the speakers inward in order to skim the walls. There still be overlap for stereo, but a balancing act must be maintained to keep things intelligible.
The short rectangle
If you start with a 180-degree-fan and square out the radius into corners, you have the short rectangle. A “thrust” stage is in the center and has nearly the same distance to reach the front and sides. The corners are the longest shot. This shape brings people in close together, creating an intimate environment. There is a twist, however. Two seats maybe equidistant, but there is quite a difference in experience between listening to someone facing you and someone turned sideways. This shape is the easily suitable to multi-element mono, with speakers wrapping around the stage to cover each area locally. The corners can get some delay help, and you are good to go.
The number of elements required here will depend again on stage depth relative to house depth. Figure on an absolute minimum of four (front left, right, and side left/right). If the stage goes deep, then you may find a five-element (add a center) or seven-element (add corners) system.
If you have stereo or multichannel ambitions, you have stepped into the ultimate challenge. If it takes four elements to get everybody one source, then how many does it take to create the double coverage needed for stereo? Either eight clusters or we have to greatly widen up the coverage of the existing four. If you have a seven-element system, you are looking at too many sources to count to go stereo. One strategy is to split clusters into left/right segments so that people hear both channels by either getting two halves of one cluster or two halves from different clusters. This strategy is fine for a parade route where all off the action is on a horizontal line in front of you. The short rectangle house of worship is not like this and throws into question why you would attempt stereo in such a space.
Consider the quandary that results from two actors on stage left and right respectively: From the front-of-house point of view, actors are spread horizontally, so are the speakers; visual and audio match. From the side-of-house point of view, the actors are spread front to back, and yet the speakers are spread horizontally. The visual image (two centered people at different depths) does not match the audio image (nobody in the center and two mystery people on the sides).
Multichannel strategy
We have seen mono and stereo systems. If we subdivide the system into vocal and music system, we can get the best of both configurations. The mono system can handle the vocals, providing all the intelligibility benefits of a single source. The center system must provide full coverage of the hall, not just a center fill. All too often I see left and right systems which each cover a two-thirds of the hall and the same system for the center. That is fine for left and right, which cover the room and overlap in the center, but it does not work to have vocals in two-thirds of the room. In most cases, a very wide center cluster is required as are some low infill speakers around the stage to bring the image down and keep the cluster off the stage.
A stereo system can then be employed for music only. This system can have overlapping coverage and pick up a lot more of the room. The music system is much more reverberation-friendly, and this allows us lots of room to work with overlap. The operation of L/C/R systems requires careful management of the separate channels. We have to keep voice out of the L and R and vice versa in order to prevent doubling up of these channels in the space.
Conclusion
There are lots of ways to divide these rooms for coverage. Hopefully this article will provide some guidance in the big picture of HOW speaker system design.
