When it comes to room acoustics, definitions of “correct” or “appropriate” can be largely dependent upon the musical tastes of the listeners, often making it a sensitive subject. The decisions made are significant in terms of cost as well as the listening experience.

Fortunately, not all of the aspects of auditorium sound are so subjective. Large rooms require a sound system, and the criteria for performance are more universally accepted. All successful sound systems must:

1) Provide even sound coverage of all audience areas
2) Provide adequate loudness before distortion
3) Provide adequate loudness before acoustic feedback
4) Be easy to understand
5) Reproduce musical sources with adequate clarity and fidelity

I call these the “Big 5.” While there are numerous other criteria that can be used to judge a sound system, these are the ones that aren’t negotiable. Regardless of the proposed design of the system, if the system doesn’t fulfill the Big 5, there will be ongoing problems with sound reproduction.

For an existing system, the best instrument for determining if the sound system meets these criteria is the audience. Listen to them. Complaints such as: “I can’t hear when I sit over there” or “Why does the system squeal so much?” or “I can’t understand the words” often mean that a system is in violation of one or more of the Big 5.

Most of these problems can ultimately be traced back to the system’s transducers. A transducer is a device that converts energy from one form to another. The two types of transducers in a sound system are microphones and loudspeakers.

The microphone converts acoustical energy into electrical energy. The electrical signal flows through the system’s electronics on its way to the loudspeaker, which converts the electrical signal back into an acoustical signal. The acoustical signal flows through the air to the listeners’ ears.

Both electrical and acoustical signals are vital to the operation of the system. It should be pretty obvious that deficiencies in one will bring out the worst in the other. The transducers are always its weakest links. Never forget that.

Between the two, the loudspeakers get priority with regard to investment dollars. They are second in importance only to room acoustics in determining the sonic performance of the space, and therefore, it’s vital to understand some basic principles regarding the selection and placement of loudspeakers in a room.

The goal is to help you sort through the sundry opinions that are often expressed by those around you. The principles are general, but physically defensible and time-proven.

Limits Of Performance
There are two critical factors regarding loudspeaker deployment into an auditorium - selection and placement. They are of equal importance – when loudspeakers are selected and placed, the limits of performance have been established for the sound system in terms of the Big 5.

Further, the majority of common sound system ailments can be traced back to loudspeaker issues. Unfortunately there is relatively little that can be done with electronics to remedy the problems caused by the improper selection and placement of loudspeakers.

If someone walks into your church and asks “Where should I sit for the best sound?” the answer should ideally be “It doesn’t matter, sit anywhere you like.” In a permanently installed sound system, the sound coverage should be as uniform as possible over all seating areas – no overpowering the audience members in the front to achieve adequate level for those in the back.

Coverage is best checked by listening to speech over the system. Don’t waste time wandering around with a sound level meter in an evaluation of this type. Sound level meters display the total sound field level present at a given position, but the information needed by the listener is the early sound energy, with special instrumentation required to measure it.

Therefore, the best way to check coverage is to play speech tracks (usually from a CD) that is unfamiliar with and simply walk around and listen.

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.

A good system designer will look at achieving coverage from smaller loudspeakers placed closer to the audience, versus larger loudspeakers placed at a greater distance.

The role of the client is to decide which approach will work best with regard to cost, aesthetics, and a host of other issues. Once this decision is made, it is the architect’s job to integrate the loudspeakers aesthetically in a way that does not impair the operation of the loudspeaker (no loudspeakers in cavities, please!).

Complicating The Process
It all seems pretty simple, right? It’s not. Two things complicate the selection and placement of loudspeakers with regard to coverage:

1) Their directivity characteristics tend to differ significantly with frequency. We say that the directivity is “frequency dependent.” So, a placement that works well for the woofer (low frequencies) may not for the tweeter (mid and high frequencies).

2) When two loudspeakers are placed in close proximity, it produces an entirely different directivity that almost never follows the intuitive expectation. For example, it’s common practice to place two loudspeakers side-by-side in an attempt to achieve wider coverage.

In fact, this will actually narrow the coverage for some ranges of the loudspeakers response. It may also cause interference effects that produce “drop outs” in areas of the audience.

The poor performance of many sound systems can be traced directly to loudspeakers placed by intuition with no regard for the physics of the acoustic interaction between multiple sound sources. When loudspeakers are placed in close proximity, they form an array - like it or not. Arrays are very complex in behavior and require careful design to produce an intended result.

As with other engineering problems, loudspeaker selection and placement is a matter of trade-offs. Improvements made in one aspect of performance come at the expense of some other performance benchmark. For instance, adding loudspeakers to cover additional seating areas will likely reduce the sound quality elsewhere in the room.

Using Powerful Tools
The good news is that loudspeaker selection and placement need not be left to chance or trusted to intuition. There are tools available for evaluating performance in advance.

Very powerful computer modeling programs can map the sound coverage onto the audience, allowing the system designer to evaluate the trade-offs. These same programs can model the interaction between loudspeakers to allow for the optimal design of arrays.

The bad news is that you probably don’t own one of these programs. They are rightfully expensive, and once acquired have very long, steep learning curves - their effectiveness is limited by the knowledge of the user. In short, loudspeaker selection and placement is a task for someone who is tooled up to do it AND has a lot of experience in doing it.

The audio system consultant is second in importance only to the acoustical consultant. Successful professionals have made a life-long study of their trade, and the value of what both bring to a project means that they always pay for themselves.

Some sound system projects of a smaller nature can benefit from the services of a design-build contractor. However, make sure that they have the necessary measurement and prediction tools to fully assess your room’s acoustics and provide predictions regarding the response of the proposed sound system.

Further, take the time to check out other church projects that they’ve done in both acoustically live and dead architectural spaces. Listen to the sound systems, and talk to the technical staff. Attend a service or two and evaluate the sound quality , especially with regard to speech intelligibility. 

Some churches sound great from day one, while others never sound good. The difference is in the planning, and that should be done under the direction of qualified audio consultant and contractor to determine the appropriate selection and placement of loudspeakers.

After that, it’s a matter of providing them with the latitude and budget that needed to satisfy the “Big 5.”

Pat & Brenda Brown lead Syn-Aud-Con, conducting audio seminars and workshops around the world. Synergetic Audio Concepts (Syn-Aud-Con) has been a leader in audio education since 1973. With nearly 15,000 “graduates” worldwide, Syn-Aud-Can is dedicated to teaching the basics of audio and acoustics. For more information, go to http://www.synaudcon.com

More articles by Pat Brown on PSW:
Proper Loudspeaker Placement: How To Avoid Lobes and Nulls