Actual “concert rigs” are capable of being really loud. They’re also really expensive.
The opinions expressed are mine only. These opinions do not necessarily reflect anybody else’s opinions. I do not own, operate, manage, or represent any band, venue, or company that I talk about, unless explicitly noted.
There’s a question in this business that’s rather like the quandary of what someone means when they say “twice as loud.” It’s the question of how a PA system “classes.”
To a certain degree, the query is unanswerable. What might be a perfectly acceptable rock-band PA for one group might not be adequate for a different band. Even so, if you ask the first group whether or not they play through a “rock-band” system, they will probably say yes. In the end, it all comes down to whether a rig satisfies people’s needs or not. The systems I work on are just fine for what I need them to do (most of the time). If you gave them to Dave Rat, however, they wouldn’t fit the bill.
Even if the question can’t be definitively put to rest, it can still be talked about. In my mind, it’s possible to classify FOH PA systems and monitor rigs by means of acoustical output.
Right away, I do have to acknowledge that acoustical output is a sloppy metric. It doesn’t tell you if a rig sounds nice, or is user-friendly, or if it’s likely to survive through the entire show. Reducing the measure of a system to one number involves a LOT of other assumptions being made, and being made “invisibly.” It’s sort of like the whole problem of simple, passive loudspeakers. The manufacturer suggests a certain, broadband wattage number to use, all while assuming that major “edge cases” will be avoided by the end user.
But one-number metrics sure do make things simple…
My Proposed “Rule Of Quarters”
So, as I present my personal taxonomy of audio rigs, let me also mention some of my other assumptions for a “pro” PA system:
1) I assume that a system can be tuned such that any particular half-octave range of frequencies will have an average level of no more than +/- 6 dB from an arbitrary reference point. Whether the system is actually tuned that way is a whole other matter. (My assumption might also be too lenient. I would certainly prefer for a rig’s third-octave averages to be no more than +/- 3 dB from the reference, to be perfectly frank. I’d also like a $10 million estate where I can hold concerts.)
2) I assume that the system can provide its stated output from 50 Hz to 15000 Hz. Yes, some shows require “very deep” low-frequency reproduction, but it seems that 50 Hz is low enough to cover the majority of shows being done, especially in a small-venue context. On the HF side, it seems to me that very few people can actually hear above 16 kHz, so there’s no point in putting superhuman effort into reproducing the last half-octave of theoretical audio bandwidth. Don’t get me wrong – it’s great if the rig can actually go all the way out to 20 kHz, but it’s not really a critical thing for me.
3) I assume that the system has only a 1:100 chance (or less) of developing a major problem during the show. To me, a major problem is one that is actually a PA equipment failure, is noticeable to over 50% of the audience, and requires the space of more than 5 minutes to get fixed.
If all the above is in the right place, then I personally class PA systems into four basic categories. The categories follow a “rule of quarters,” where each PA class is capable of four times the output of its predecessor. Please note that I merely said “capable.” I’m not saying that a PA system SHOULD be producing the stated output, I’m only saying that it should be ABLE to produce it.
Also, as a note about the math I’m using for these numbers, I do make it a point to use “worst case” models for things. That is, I knock 12 dB off the peak output of a loudspeaker just to start, and I also treat every doubling of distance from a box to result in a 6 dB loss of apparent SPL. I also neglect to account for the use of subwoofers, and assume that full-range boxes are doing all the work. I prefer to underestimate PA performance, because it’s better to have deployed a Full-Concert rig and wish you’d brought a Foreground Music system than to be in the opposite situation.
Minimum potential SPL at audience center, continuous: 97 dB
This isn’t too tough to achieve, especially in a small space. If the audience center is 25 feet (7.62 meters) from the PA, and they can hear two boxes firing together, then each box has to produce about 112 dB at one meter. A relatively inexpensive loudspeaker (like a Peavey PVx12) with an amp rated for 400 watts continuous power should be able to do that with a little bit of room left over – but not much room, to be brutally honest.
Also, it’s important to note that 97 dB SPL, continuous, is REALLY LOUD for speech. Something like 75 – 85 dB is much more natural.
Minimum potential SPL at audience center, continuous: 103 dB
This is rather more demanding. For a 25-foot audience centerpoint being covered by two boxes, each box has to produce about 118 dB continuous at close range. This means that you would already be in the territory of something like a JBL PRX425, powered by an amp rated for 1200 watts continuous output. (It’s a bit sobering to realize that what looks like a pretty beefy rig might only qualify as a “background” system.)
Minimum potential SPL at audience center, continuous: 109 dB
Doing this at 25 feet with two boxes requires something like a Peavey QW4F…and a lot of amplifier power.
Minimum potential SPL at audience center, continuous: 115 dB
If you want to know why live-sound is so expensive, especially at larger scales, this is an excellent example. With $4800 worth of loudspeakers (not to mention the cost of the amps, cabling, processing, subwoofer setup, and so on), it’s actually possible to, er, actually, NOT QUITE make the necessary output. Even in a small venue.
Also, there’s the whole issue that just building a big pile of PA doesn’t always sound so great. Boxes combining incoherently cause all kinds of coverage hotspots and comb filtering. It’s up to you to figure out what you can tolerate, of course.
And, of course, just because a system can make 115 dB continuous doesn’t mean that you actually have to hit that mark.
Don’t Be Depressed
Honest-to-goodness, varsity-level audio requires a lot of gear. It requires a lot of gear because varsity-level audio means having a ton of output available, even if you don’t use it.
In the small-venue world, the chances of us truly doing varsity-level audio are pretty small, and that’s okay. That doesn’t mean we can’t have a varsity-level attitude about what we’re doing, and that doesn’t mean that our shows have to be disappointing. We just have to realize where we stack up, and take pride in our work regardless.
As an example, at my regular gig, “full-throttle” for an FOH loudspeaker is 117 dB SPL at one meter. “Crowd center” is only about 12 feet from the boxes, so their worst-case output is 106 dB continuous individually, or 109 dB continuous as a pair. According to my own classification methods, the system just barely qualifies as a “foreground music” rig.
But I rarely run it at full tilt.
In fact, I often limit the PA to 10 dB below its full output capability.
“Full Concert” capability is nice, but it’s a difficult bar to reach – and you may not actually need it.