Good speaker coverage is one of the most important aspects of setting up a successful live sound system, yet it’s one of the most nebulous concepts. A discussion about proper PA coverage with three seasoned live sound engineers is likely to yield three different approaches. The goal is to adequately and evenly cover your entire audience with the PA system. Although there are formulas that can help you calculate proper coverage given controlled circumstances, conditions created by room shapes, ceiling heights, the presence or absence of walls, and many other factors combine to render the process of providing adequate speaker coverage as much an art as a science.
Striving for Consistent Coverage
The overarching goal of consistent coverage is to have no more than a 6dB variation in sound pressure level (SPL) from the loudest to quietest seat in the house. Before we get into the methods of achieving this, it’s important to note that coverage doesn’t have to extend to the entire venue, but only the part of the venue you want to cover. This is particularly important if you’re setting up a system in a bar or coffee shop, where there’s a performance space and seating elsewhere throughout the building. Also, consistent coverage is an ideal that’s seldom completely attainable in anything but a professional venue designed specifically for live sound. However, even if perfectly consistent coverage isn’t possible, the closer you can get to consistent coverage, the better the listening experience for the entire audience.
Horizontal and Vertical Coverage
There are two axes you need to consider when planning speaker coverage: horizontal (width) and vertical (depth).
The main consideration for planning horizontal coverage is covering both sides of the audience, without overlapping so heavily that you create a hot spot (spots) in the middle. However, this is less difficult to do than you may think, since even with a standard stereo speaker setup, the area in the middle where the speaker overlap is the greatest is only 3dB louder than the areas covered by only one speaker. Adding a center speaker cluster can spread out the SPL even better and ensure more consistent horizontal coverage.
Stereo or Mono?
If you consider your PA system from a home-stereo system perspective, then stereo sound may strike you as a given, but in live sound, stereo isn’t always best. There are two major reasons to consider running your live PA in mono. First, the stereo experience tends to vary dramatically depending on where you are in the venue. Pan an instrument hard left (fully counterclockwise), and an audience member sitting on the far right experiences a mix with none of that instrument in it rather than the richer sonic image that panning would produce over headphones. Also, the more stereo separation you create, the greater the risk of introducing phase problems throughout the venue. This can lead to a phenomenon called comb filtering, in which different frequency bands are noticeably boosted or cut depending on where you are in the venue. So unless stereo is a vital part of your sound, consider running most or all of your mix in mono.
Perhaps even more important than horizontal coverage is vertical coverage. This is where elevation comes in, as well as speaker distance. A large part of speaker coverage is finding a balance between speaker distance and speaker volume. To a certain extent, having your speakers farther from your audience makes it easier to achieve consistent coverage, assuming you have enough space and powerful enough speakers to enable it. The math is a bit tricky, but here’s a layman’s explanation of the inverse square law that should shed some light on the subject.
The Inverse Square Law
As applied to audio, the inverse square law states that in a controlled environment, direct sound will decrease in strength by 50% (6dB) every time it doubles in distance from the source. Between atmospheric factors and sonic reflections, the drop in SPL isn’t quite as dramatic in a live sound environment as the inverse square law suggests, but it’s a useful concept to keep in mind as you set up your speakers. If the goal of consistent coverage is to ensure that there’s no more than a 6dB difference between the loudest and quietest point in the audience, then you can ensure this by angling your speakers so that the closest part of the audience covered by each speaker is less than half as far from the speaker as the farthest part (or, the farthest part is less than twice as far away as the closest part). By elevating and angling your main speakers, this becomes considerably easier.
Down-fill Speakers and Line Arrays
One way to get even more consistent vertical coverage is through the use of down-fill speakers or line arrays. A down-fill speaker is a second main speaker (usually located in the center cluster) that points at a slightly different angle, covering a deeper section of the main seating. This allows you to cover a considerably greater area consistently. Line arrays, which are often placed on either side of the stage, accomplish the same effect but with even greater consistency and directionality by employing several small loudspeakers, each with a narrower focus than typical main speakers.
Even with down-fill speakers or line arrays, your mains can only cover so much area, and if you’re planning an installation in a large venue, dealing with balconies, or running sound in a large outdoor space, you’re going to need additional speakers to cover the audience. These fill-in speakers can assume a variety of forms, including full-sized side-fill speakers along the edges of the audience and small balcony speakers.
Any time you place speakers at different distances from the audience, you need to delay the speakers that are closer to the audience so that their arrival time at the listener is consistent with the sound from the mains. There’s a simple formula you can use to calculate this delay time:
Ds = X/C*1000
Ds is the delay in milliseconds X is the distance from the main speakers in feet C is the speed of sound in feet/second, which is dependent on altitude and humidity
As scientific as this appears, there’s still some art to setting delay times. For instance, fill speakers typically don’t need full low frequencies, and you’ll probably want to delay them by a few extra milliseconds, using the Haas effect to create the illusion that all of the sound is coming from the mains. For more information about this and speaker delays in general, check out our full Speaker Delay guide.
Venue-specific Coverage Challenges
Every venue has its own specific set of coverage challenges. For instance, a club with low ceilings will never provide enough vertical space to fly a line array, and the absence of walls makes outdoor sound reinforcement interesting. Here are a few venue-specific challenges to consider.
Indoor venues can be divided into two categories: those that were built specifically for sound reinforcement (e.g., theaters and auditoriums) and everywhere else. Even in professional live sound venues, your biggest challenge is achieving the best possible direct-to-reverberation ratio, without deafening your audience. Sound absorption is key for cutting down reverb to a manageable level, whether it’s in the form of specifically formulated acoustic absorption materials or plush furniture and thick curtains. Spreading out speakers and using fills and balcony speakers where necessary will take care of the rest. In smaller venues with low ceilings, you can use a combination of the direct sound onstage and a small central speaker onstage to boost vocals for close audience members, and use stands to project the mains deeper into the room (and not blow your biggest fans away).
Although too much reverb can make sound muddy and unintelligible indoors, outdoor venues suffer from a lack of natural reverberation. While introducing reverb effects may fill out vocals and instruments that may otherwise sound anemic, it won’t actually make your system louder or offer better coverage. Rather than throw a ton of extra amplification at the mains (remember, doubling the amplification only adds a 3dB or 50% boost at the source), you’ll want to rely on delayed side fills for greater coverage.
Use Your Ears
The bottom line is to remember that, regardless of the venue, speaker coverage is as much of an art as it is a science, particularly if you’re working with limited resources. Considering the concepts we’ve outlined can get you close to perfect coverage, but in most cases getting the best possible PA speaker coverage is a matter of putting in the time to position speakers, walk the venue, and use your ears to make critical sound-reinforcement decisions.