The requirements for a good loudspeaker are not unlike those for good stage lighting – but in reverse. Because light is more tangible than sound, let’s use it as an example.
A stage lighting fixture is selected and placed to illuminate a certain area of the stage. The two variables that determine its coverage are distance and coverage angle (which we will call directivity). Higher directivity means more confined coverage.
The directivity of a loudspeaker is related to its physical size; therefore, physically large loudspeakers will have more directivity than smaller ones. This general principle is independent of brand name and price tag – and it’s very important.
In general, the farther a loudspeaker is from the audience, the more directivity it must have.
Observe The Trade-Offs
For a visual example of directivity, point a flashlight at a wall. Observe what happens as you move it closer and further away. This is how loudspeakers behave with regard to sound.
Better yet, if the flashlight has a focus option, observe the trade-offs between a sharply focused light (high directivity) and a broader coverage (low directivity).
If you want to confine the light to a given area on the wall, and you move it farther from the wall, you must increase the directivity of the light. This is a physical law. Photographers must live by it, lighting designers must live by it, and it even affects how you water your lawn.
A lighting designer makes the appropriate trade-offs between distance and coverage angle to assure that all of the necessary parts of the stage are illuminated while minimizing spill onto the audience.
A loudspeaker must do exactly the same thing, only the goal here is to “illuminate” the audience and not the stage. Sound system designers work with the same two variables as lighting designers – directivity and distance.
The farther the loudspeaker is placed from the audience, the more directional it must be to avoid illumination of areas with no listeners.
It’s safe to say that in most churches, the loudspeakers are always a considerable distance from the audience, so directional loudspeakers are a must. This also means that they must be physically large, because loudspeaker directivity is generally proportional to physical size.
When an architect or church committee decrees that loudspeaker(s) must be located at a certain place, they are indirectly establishing the required physical characteristics of the loudspeaker. For instance, if the loudspeaker must be mounted very near the ceiling in a large room, it will have to be physically large to have enough directivity to confine the sound to the audience area from such a great distance.
If the loudspeaker can be placed closer, its physical size can generally be reduced, since the required directivity is lower from a closer vantage point. An architect that wants a small loudspeaker placed near the ceiling in a large church is condemning the space to permanent poor sound reproduction. This is engineering in reverse!
It’s the job of the sound system designer to evaluate the possible loudspeaker placements that will produce even and confined audience coverage. This, in turn, determines the required directivity of the loudspeaker(s), which in turn determines their required physical size.