I really do not know where this old rule came from, but many old sound persons used to budget a “number” of watts per persons in the audience as a way to size a sound system. Now this could be useful if the rig offered is fixed in size and dispersion so that multiple rigs splayed together could get close to a “watts per person” criteria. But imagine a 2-watt per head system in an outdoor festival gig with ground stack speakers. With 5,000 attendees, 2-watts per head means 10,000 watts of power amps driving speakers that may handle that power. In the late 1970s and 1980s, I could envision racks of Crown DC-300s or Peavey CS-800s driving full range speaker stacks for the mass of attendees. Yep, horn-loaded Perkins bins and Altec multi-cell horns galore in super efficient but low fidelity music, blasting to the masses.
But today I run nearly 8,000 watts into four speakers for 300-person clubs, and that watts-per-person equation just does not make sense. Granted, about 5,000 watts of the 8,000 watts might be going into a pair of subwoofers, and that just was not considered 30 years ago. So how does one allocate how much “PA” for an event these days?
The Pressure Of Sound
The current rule of thumb is that concert loudness is in the 90dB to 120dB SPL levels in a C-weighting. So how do you get back to gear in the trailer? I look at the venue and first get an idea how far back do I have to get +90dB SPL of rockin’ sound. If the venue is the infamous outdoor festival and you got a stage to beyond 200 feet to reinforce from that stage. Then all that is left is coverage angles and the math.
To convert back the math, we know that each doubling or halving of distance requires a 6dB SPL change in loudness. So first I go metric and get about 62 meters of distance. If I round up to 64 meters, the doubling math gets convenient; so that 64m is 90dB, 32m is 96dB, 16m is 102dB, 8m is 108dB, 4m is 114dB, 2m is 120db, and 1meter is 126dB. Simple enough? Here is where the fun begins.
Most smaller speakers cannot continuously provide 126dB SPL at one meter, let alone handle the amplifier power. And that perfect amplifier power is based on the efficiency of the speaker cabinets. If you have subwoofers with a 98dB SPL rating at 1-watt and 1-meter, you need 28dB watts or about 630 watts of un-obstructed subwoofer power needed for that speaker.
Thankfully, today we have speakers and amplifiers capable of easily handling this problem. And of course, the top boxes need to solve the same problem based on similar efficiency ratings (see Figure 1).
Coverage
Notice that I did not even mention the number of persons between the speakers and 200 feet from the stage for 90dB SPL. But if you have a lot of people, they are not going to conveniently congregate in a single speaker stack’s coverage angle. So you may double up on speakers to widen out the coverage from, say, 60 degrees wide to 120 degrees wide. Oh, did I mention a “stereo” mix? Now double that number of speakers and place them on each side of the stage. So what was just 2,000 watts evenly split into subs and tops just went to 8,000 watts and four times the speakers.
Mo’ Snort
If we were to complicate things and double the coverage distance from 64 meters to 128 meters, then add on 6dB more SPL from the stage. Now if you could keep the audience away from the stacks up front, that126dB SPL just went to 132dB SPL. Given the same old 98dB efficient speakers, then the 36dB difference means about 2500 watts. But many subs and tops do not handle this kind of power. So doubling the subs and running them at a modest 1250 watts gets you close enough if they are coupled together. Stacked tops might have to be four-up with close coupling and 625 watts power each. Or maybe it is time to break out the line array for the mids and highs. So what started as four subs and four tops just went to eight subs and 16 tops when the venue gets deeper.
Now if the promoter said that 90dB SPL at 200 feet is not loud enough, and wants 102dB at 200 feet, you got a worse problem. That 12dB more at 64 meters is 12dB more at 1 meter, or 136dB SPL. With the same 98dB subs, you have 38dB more power or 6,300 watts per coverage angle or 25,000 watts of dual zone stereo sub-woofer nirvana. But 6,300 watts can couple up into four subs or eight subs per stage side for stereo and 120 degree coverage. And now either track down 40 low-efficiency ground stack tops, or beef up your line array with less curvature and more boxes on the hangs. Back in the 1980s we had nice 105dB efficient top boxes ( like KF850s), so only eight of these cabs per side would work wonders.
Welcome to the world of racks and stacks…
