Sound System Engineering: It’s Not Guesswork

Sound System Engineering: It’s Not Guesswork

Depending on your needs, a line-array loudspeaker system may suit your worship space best, but in some cases, so too could a point-source speaker system.

Designing a sound system is a daunting task for many people, and understandably so. There are many decisions to make, and a lot of them require a good amount of experience to get right.

One of the most fundamental considerations in modern design is whether or not to use a "line array" speaker system.

Most importantly, you can't just guess at a design, because it "looks about right." In fact, that phrase is jokingly referred to as the "LAR approach," among design professionals. That’s because so many systems are installed without any sort of scientific approach or real expert experience.

The results of an “LAR” system, as you may guess, are usually pretty terrible.  So, when in doubt, get the help of an acoustical consultant or trusted design-build firm, i.e., an integrator.

One of the most fundamental considerations in modern design is whether or not to use a "line array" speaker system.

Line arrays, the often J-shaped vertical sets of loudspeakers you see at large venues, came about as a way to solve the problem of getting even coverage from the front all the way to the back of a deep room. The operating principle of line arrays is that the acoustic output from adjacent boxes couples together to provide more energy to a destination further away.

You may be thinking that you could also just turn up a conventional loudspeaker loud enough to get enough energy to the back. There are some problems, though, with that approach. First, it would take an absurdly loud product to reach the distances needed in concert production. Second, people in the front of the room would be practically knocked over.

So how does a line array do it so well?

The interaction of the array’s individual boxes actually causes less energy to go where it doesn't belong (vertically, anyway). The more boxes you have, the better control you have of where the energy goes and where it doesn't. This interaction I'm talking about is known as constructive and destructive interference. All that means is that the boxes sum together in front, where you want them to. In addition, the acoustic outputs from the various boxes actually nullify each other in the areas (vertically) where you don't want their sonic contribution.

Why are line arrays so often shaped like a "J"? Remember that we want more energy directed toward the back of the room. That’s because we have a longer distance to get there. Getting toward the front of the room, we need less energy because the array is closer to the people. Therefore, we start angling the boxes more aggressively to reduce the level going to those seats.

If the number of boxes and the angles between them are exactly right for the room, and the array height and overall tilt is exactly right, you can enjoy smooth, even coverage from front to back.

The key here is that it all has to be exactly right. You cannot guess the proper deployment of a line array. Conveniently, most manufacturers offer software to help you make good decisions about the installations of their array products. Just remember that it's a precise science and you must pursue the process very diligently.

Even though the original intent for line arrays was to cover deep rooms, they also work in different room configurations.

The strength of line arrays, though, is in accurately controlling their output over long distances. Another important consideration is that you only get the benefits of this "control" if the array is sufficiently long. For the sake of this article, let's say that 6 feet is the minimum. Why does that matter? Consider just one line array box by itself. A lone "line array" box is almost just a traditional loudspeaker that happens to have very narrow high-frequency coverage (vertically).

You might read a line array’s specifications and see that the vertical coverage of each box is listed as 10 degrees. In this case, though, that is only for high frequency content. Midrange will be somewhat omnidirectional and bass will be entirely omnidirectional. That's because the ability to control where sound goes is primarily a function of size.

For high frequencies, which have short wavelengths, it's very easy to control their direction. For low frequencies, which have long wavelengths, it's very difficult to control.

Recall that constructive and destructive interference are the intentional interactions that happen in a line array to control where we want sound to go. Since lower frequencies have longer wavelengths, we need larger (longer) products to be able to control those frequencies. A line array that's not long enough will not actually control midrange content very well. The result then makes for a muddy sounding room.

Therefore, an array of only a few boxes, in my opinion, isn't really a line array at all. It's just an overly complicated loudspeaker.

You may gain additional SPL capability, because there are more drivers involved, but that's often about it. You don't have real pattern control (determining where sound goes or doesn’t go), until you have a long enough array.

Thankfully, the manufacturers' software will often help make a good design decision. I encourage you to therefore download one of the free applications from a number of the large manufacturers. Then experiment to see how changing the number of boxes and their angles impacts the desired coverage area.

Now that we've talked a lot about line arrays, let's consider the alternative: point-source boxes. These are the traditional loudspeakers you're used to, and they shine in applications where the coverage distance isn't very large.

In many cases, point-source boxes sound better than line arrays, because they are simpler. And in sound, simpler is often better.

While the selection, placement, and aim of loudspeakers is beyond the scope of this article, just remember that there are plenty of situations where point-source boxes will serve your needs very well, and quite possibly better than a line array.

Then there is the topic of subwoofer location, which is a frequent topic of debate. In many settings, and especially in churches, I often prefer flown subs. This gives more even coverage throughout the room. Ground subs provide the additional free acoustic gain from being up against a rigid boundary (the floor). They also tend to knock over the front row, though, without providing much impact to the back.

You might lose that free acoustic gain in the air, but the goal of even coverage is sometimes more important.

When I mention free acoustic gain', I'm referring to the fact that subwoofers are naturally omnidirectional products (with the exception of a few specially engineered boxes). When you place one against a rigid boundary (such as the floor or a stiff wall) the energy that would have gone in that direction is forced to go back in the opposite direction.

As a result, there is twice as much energy available to the room. All of that energy would have been in the room anyway, but now it's more concentrated'. So, you’ll lose that effect in the air, but with sufficient products, you can still have plenty of energy available. And the reason having them flown makes coverage more even is basically just geometry.

With ground subs, the distance between the subs and a person in the front is very small. The distance to someone in the back, though, is very large. With flown subs, the difference in distance between those two locations and the subwoofers is much smaller, and therefore the SPL variation is less. Even with all that said, you still may like the free acoustic gain or sound from ground subs. That of course, is a personal choice.

Here are a few other things that must be considered in determining a good system design, but for which we don't have time to address in depth here:

Have a structural engineer review your intended structural load and rigging methods and sign off on their safety.

Ensure that you have a qualified rigger do the actual rigging work and that the devices are specifically designed to be rigged in the first place.

Plan for proper and adequate electrical system capacity and topology (Hint: you want the sound system to have its own electrical panel board ("breaker box") exclusively for sound, and you want properly-wired isolated technical ground outlets for all circuits).

Make sure there is a lot of large conduit running to each location that might need cable pulled in the future, and have pull strings installed ready to make the job easier.

Whenever a cable is pulled, always pull extra (it's so much cheaper to do it at the same time rather than later).

Use either flexible-architecture DSP or amplifiers with built-in DSP, or both, to do the loudspeaker tuning (never do any of the loudspeaker tuning in your console, as that makes it easier for people to inadvertently re-tune your system, and it means that if you change out your console you have to start over on the tuning process).

And speaking of tuning, have an experienced consultant tune your PA if you really want it to sound great.

Sound system engineering, as you can tell, is a complex process and not for the faint of heart (or the inexperienced).

However, when you get it right, the results are astounding: people can hear well; sound is consistent everywhere; and people are simply free to worship in the midst of excellent audio.

TAGS: Audio
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