This past weekend, I had the opportunity to teach a class on the above topic at NorthWest MinCon in the Seattle area. We had a a great group show up, and it was a lot of fun talking about the process of designing PAs. While I can’t condense the entire class into a single blog post, I thought it would be good to hit some of the highlights. Here are four design principles I employ when looking at a sound system design.
Even coverage throughout the seating area
We really want to see the same level and very similar frequency response in every seat in the house. Achieving that goal—perfectly—is nearly impossible. So we have to give ourselves a range. We typically want to shoot for a ±3 dB level in every seat in the house. That means the back of the house is likely to be no more than 6 dB quieter than the front row. And depending on your room and PA, it might be the sides, front corners or even center that is quieter. But the overall variation should not be more than 6 dB. With a good design and proper speaker selection, this is a very achievable goal. Often, we can get it down to ±2 dB.
Minimize overlapping sources
Another key design principle for me is to minimize overlapping sound sources. Again, this is easier with some designs than others, but as much as possible, I want one speaker to cover a seating area. The exception to this rule is a stereo PA design, but that’s a whole different kettle of fish, and I’m taking a pass on that one for now.
The issue with overlapping sources is that you will almost always get some kind of phase cancellation, which can really mess with our even coverage goals. Delay speakers are one situation where we will have some overlap, and it requires careful attention during the commissioning process to make sure they sound even, natural and are in time.
Keep sound in the seats and off the walls
This seem so simple, and yet, I’ve seen many, many designs where speakers are pointed at walls and not the seats. This is bad. In fact, in my last church, 8 of the 12 speakers were pointed at walls and not seats; as a result, 80% of the seating area was off-axis of the PA. The resultant sound was terrible. We had variations of 12-18 dB in our coverage, and the sound field in the seats was almost all reflected, not direct. Intelligibility was terrible and it simply sounded bad. No amount of EQ or DSP will fix that. Basic rule of thumb—point the speakers at seats, not walls.
More direct than reflected sound
A high direct to reflected sound ratio means the sound will be very clear. If you’ve ever been in a venue where it seemed like the speaker’s voice was right in front of your face, that system had a high direct to reflected ratio. The problem with high levels of reflected sound is that it diffuses the sound field, causes some nasty cancellations in the form of comb filtering and makes it really hard to hear what is said or sung. We can minimize reflected sound with a judicious use of treatment, and that should be part of the sound system design. But we also want to choose our speakers carefully, and position them properly to keep as much direct sound from hitting the walls—causing reflections—as possible.
There is of course more to the story, but we’ll let this serve as an introduction. I’ll touch on some additional speaker system design points in upcoming articles.