Take a loudspeaker and put it in a box. Connect the speaker to the output terminals of an audio amplifier. Now plug a microphone into the amp and turn the volume knob clockwise. Enjoy your new PA system.
If only it was that simple. Well, actually, it sort of was, once. The introduction of loudspeaker systems into houses of worship happened in fits and starts, over the course of decades, and they weren't a particularly good fit at first. Churches came in two basic flavors for most of the past 200 years: the cruciform basilica and the stage/theater designs. Both long predate amplified sound and neither are inherently architecturally compatible with it. The former was designed with liturgical music in mind, and its highly reflective stone and tile walls and high ceilings add welcome reverberance around music. Plus, when the Catholic Mass was still in Latin, speech intelligibility wasn't high on anyone's list. The latter come from the transition to larger sanctuaries as evangelical Protestant denominations began to adopt auditorium-style church designs with theatrical features in the late nineteenth century. It's not that acoustics weren't understood well until recently; it's that they just weren't emphasized until the end of the last century, when intelligibility became a significant criterion in audio design, along with volume and reach.
The history lesson is a useful point of departure for a discussion of sound system design for houses of worship, because, according to several knowledgeable sources, some churches can go through at least two sound systems before they get one that really works. It can be an expensive lesson. Knowing the fundamentals about system components—and, loudspeakers are the basic building blocks of PA systems—can save a lot of grief later.
Points to Consider
There are two basic loudspeaker system configurations: point source and line array, and subgroups within them. Point source, also known as point-and-shoot, is largely self-explanatory: a speaker enclosure is aimed a specific part of a space and sound is directed towards it. Multiple enclosures facing various parts of the space would theoretically cover that space. You'll see the point source approach used extensively in larger space where the sound has to be thrown a considerable distance, such as in sports venues. This design approach is simple and cost effective, using as many or as few speaker enclosures as necessary to fill the space, and is generally simpler to rig and install.
But the point source approach has its implicit limitations. While the sound emitted from a speaker propagates outward, it expands as it does so, in the process losing some of its forward energy as the circular sound wave expands. As a result of this continuously expanding spherical wave, sound pressure decreases by 6 dB for every doubling of distance, a formula also known as the inverse square law. Also, depending upon the dispersion characteristics of the enclosure, horizontal and vertical coverage can vary considerably, and this type of system can require a large number of fill speakers—loudspeakers specifically aimed at areas in a space, such as a balcony, that the forward propagation of sound from the source cannot reach—or at least can't reach coherently, meaning that loss of certain frequencies would negatively impact speech intelligibility or music fidelity. But for large, architecturally simple spaces, the point source approach is highly useful.
There are two basic loudspeaker system configurations: point source and line array, and subgroups within them….
If a point source is easy to explain, a line-source—the basis for the line array speaker configuration—is a bit more complex. A line source is a series of equidistantly spaced drivers arrayed in a straight line, and its primary benefit is that it creates a very narrow vertical dispersion—instead of the circular propagating wave issued by the point source system, the line array develops as a cylindrical wave. It has some clear advantages: a cylindrical wave's energy decreases by 3 dB every time the distance from the source is doubled, half that of a spherical wave's energy attenuation. However, to function properly, the line array has to be at least four times as tall as the wavelength it is radiating, and the sources on the array have to be less than one-half wavelength apart. There's plenty of math behind these formulas, but the bottom line is that the spatial relationships between loudspeakers in the array are critical, and that only very long line arrays can function as a line source at low frequencies, while only very short modules can couple at high frequencies.
Where line arrays really shine is in precise vertical coverage of a space, which is useful for keeping “sound on seats” and away from walls and other reflective surfaces. This is especially helpful in environments like churches, where speech intelligibility and musical clarity are vital. Line arrays are also defined by their curvature, which lets their geometry vary to expand coverage to areas directly below the arrays, thereby reducing the need for fill speakers. Line arrays are available from virtually every loudspeaker system manufacturer.
However, the physical—i.e., height—requirements of line arrays can limit where they can be used. There are solutions available for smaller spaces with lower ceiling heights, although these are often simply two-way speaker cabinets that have been reoriented on their sides and connected to form a vertical line of speakers. They may take a classic line array form factor, but they cannot produce the cylindrical wave needed to achieve coherence between speakers. They will be less expensive, though, more so than the typical true line array, which can cost substantially more than point source designs.
Pillar of Speakers
Another development descended from the line-array principle is the columnar array—a series of equidistant drivers arrayed in a straight line, but in this case they are arrayed inside a fixed cabinet. The concept goes back 50 years, most famously manifest in Shure's legendary Vocal Master, which lined up four eight-inch and two 10-inch speakers in a straight line. They were designed for use in small clubs but the Beatles reportedly used several Vocal Master columns at their 1965 Shea Stadium concert. (Not that anyone heard anything there anyway over the screaming.) Modern columnar arrays have found excellent applications in houses of worship, both in the typical configuration of left and right PA speakers, and as a perfect fit on the support columns of highly reverberant environments like cathedrals, blending in architecturally and esthetically as part of distributed sound systems that can deliver more intelligible audio in those kinds of spaces.
An array of smaller speakers, all driven in phase, is the equivalent of several larger speakers but with additional inherent directivity. Coupled with DSP that can enhance this natural effect, steerable arrays are of particular use to houses of worship because of their high directionality in the 1-kHz range, the sweet spot for speech intelligibility. The “beam” of sound produced by these highly directional arrays can be focused on specific areas of a space. This is especially useful in keeping sound away from reflective surfaces such as hard walls and floors, where reverberation can smear the original source sound.
Another approach to loudspeaker configuration is the distributed system, in which a large number of small speakers are installed above the seating area. This approach is often used in conjunction with some type of speaker array in the front of the room, whose audio is delayed so that the sound from the stage and from the furthest distributed speakers arrives at the same time. Distributed systems have very high intelligibility but can only be used in environments with relatively low ceiling height, and they're not ideal for music reproduction.
All About That Bass
There are two overarching themes that have emerged in church sound system design in recent years: steerability, which we touched on a moment ago and which is used to keep sound away from reflective surfaces; and low-frequency accommodation. The latter continues to grow in importance for music because, for better or for worse, contemporary worship music continues to incorporate secular sonic trends, and the biggest one of those this century is bass. No sound system intended to support contemporary worship can be considered complete without the inclusion of subwoofers. How many, and where they are crossed over (i.e., at which frequency do they become active) is case-dependent and should be determined by a qualified system designer. But the subwoofer has become necessary because of the increased importance of low frequencies in music.
Subs, which generally operate in the 20-Hz to 100-Hz spectrum range, have an additional benefit: they can add fullness to sound without necessarily increasing overall volume in the room. Adding subs also takes some of the pressure off of the rest of the system, by not asking it to cover very low frequencies. Just because a loudspeaker is rated for 20 Hz to 20 kHz doesn't necessarily mean that it will handle the bass as well as it handles upper frequencies.
The issues of low-frequency management and the steerability of sound do ultimately converge. Steerability control will decrease as you move down the frequency spectrum, and an increasingly common strategy in steerable-array sound is the cardioid subwoofer. These can take two forms: a single product or multiple subwoofers configured in a cardioid array. In both cases, they come down to three elements, one of which is facing 180 degrees away from the other two, creating a phase inversion that cancels out reflected energy. To cap this basic configuration off, according to Bennett Prescott's excellent analysis of cardioid subwoofer physics on SoundForums.net, “Delay is then applied to the rear-facing subwoofer to time align all loudspeakers towards the rear. Inverting the polarity of the rear-facing subwoofer then creates cancellation as the energy from the front-facing subwoofers arrives only to be met by the inverse energy radiated by the rear-facing subwoofer, which nullifies it.”
The information here isn't intended to make you into a loudspeaker or PA system designer, but it will give you some insight into what modern houses of worship need to get their message across.
