Acoustic Expertise: Worship Facility Structure
Oct 10, 2011 10:06 AM, By Bob McCarthy
Understanding worship shape to fit your sound design.
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).
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.
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.
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