Crowd Size and Physics of Pi(e):Yet another sizing post
Posted: Fri Feb 06, 2026 2:02 pm
Good day all,
I wanted to bounce my experiences off you and see if you have guidance before I launch into my next build. I'm also searching for confirmation, since I'm reading and learning. We find our gigs, where we have to provide sound, are a mixture of indoor/outdoor from 100-500 people. My intention is to build a minimum of four (4) BFM design mains so that I have a basic rig. I may be wrong, but I find that two mains just can't do it, no matter how much spin the sales department puts on their product.
The problem is covering a 500 person audience outside. Given our catalog and style, our audience likes about 96-100dB SPL (c-weighted slow) average maximum, and 90db SPL average minimum. SPL peaks are 6-10dB above that, so I use 106dB peak as the maximum for the nearest crowd. Since SPL decreases by 6dB every time the distance doubles, it appears that I basically need an area that starts 25 feet from the mains extending to 75 feet. I know that falls off too much, approximately 10dB instead of 6dB, but even outdoors we're limited by venue "size" since they're usually blocking off streets, want people to get into the bar easily, etc. So, would you please let me know if that 25-75 foot distance makes sense to you? Or are there other ways, power shading, of setting up mains that would bring the crowd in closer and still provide the 6-10dB fall off.
I'll provide a concrete example. I'm sorry to use subs in the example, but I don't have experience with BFM mains yet. My 2 Titans hit 106dB at 40 Hz at 25 feet using about 55V (800W - 4 Ohm drivers); I need more Titans. Run that out to 75 feet, they are down to 96dB, about 10dB drop and reasonable coverage. So a 90 degree, quarter pie, wedge in-front of the stage gives me about 4000 sqft, or enough space for 500 people, assuming 6 sqft per person.
(3.14*75^2-3.14*25^2)/4/6 = 650 people
I guess what I'm saying is that sizing a PA comes down to the physics of packing people into a space with desired gradient of SPL. Is that how you size mains? I guessed at 6 sqft per person, thoughts? I'm also assuming 1/4 pie based on 45 degrees of axis, and of course that's venue specific even outdoors, but I'm also open to suggestions on the slice width. Obviously I'm simplifying the mains to a single point source with no distance between them. I can cover more people in less depth, by spreading the mains, but it's squishing a balloon of SPL that only produces so much: ie., I can only smooth out humps, not add more.
I have read many other sizing posts, and I apologize for duplicating what's in some of them, but I didn't see a direct discussion of "depth" of the crowd. It appears to me, that the depth, SPL fall off, is what actually determines the mains sizing. Pick an audience size, then some slice of a pie to fit that many people. Finally use those distances to size the mains: not too much for the close people, and enough for the far people. Or in other words, plan for the farthest experience using the amount of people that can reasonably fit into a space and plan on some complaints from the close people.
I hate to complicate it a little, but I also think some additional headroom should be built in to the system. If I expect a 106dB peak, then I plan for a mains that are capable of 112dB peak. Is that reasonable?
Best, and thank you!
I wanted to bounce my experiences off you and see if you have guidance before I launch into my next build. I'm also searching for confirmation, since I'm reading and learning. We find our gigs, where we have to provide sound, are a mixture of indoor/outdoor from 100-500 people. My intention is to build a minimum of four (4) BFM design mains so that I have a basic rig. I may be wrong, but I find that two mains just can't do it, no matter how much spin the sales department puts on their product.
The problem is covering a 500 person audience outside. Given our catalog and style, our audience likes about 96-100dB SPL (c-weighted slow) average maximum, and 90db SPL average minimum. SPL peaks are 6-10dB above that, so I use 106dB peak as the maximum for the nearest crowd. Since SPL decreases by 6dB every time the distance doubles, it appears that I basically need an area that starts 25 feet from the mains extending to 75 feet. I know that falls off too much, approximately 10dB instead of 6dB, but even outdoors we're limited by venue "size" since they're usually blocking off streets, want people to get into the bar easily, etc. So, would you please let me know if that 25-75 foot distance makes sense to you? Or are there other ways, power shading, of setting up mains that would bring the crowd in closer and still provide the 6-10dB fall off.
I'll provide a concrete example. I'm sorry to use subs in the example, but I don't have experience with BFM mains yet. My 2 Titans hit 106dB at 40 Hz at 25 feet using about 55V (800W - 4 Ohm drivers); I need more Titans. Run that out to 75 feet, they are down to 96dB, about 10dB drop and reasonable coverage. So a 90 degree, quarter pie, wedge in-front of the stage gives me about 4000 sqft, or enough space for 500 people, assuming 6 sqft per person.
(3.14*75^2-3.14*25^2)/4/6 = 650 people
I guess what I'm saying is that sizing a PA comes down to the physics of packing people into a space with desired gradient of SPL. Is that how you size mains? I guessed at 6 sqft per person, thoughts? I'm also assuming 1/4 pie based on 45 degrees of axis, and of course that's venue specific even outdoors, but I'm also open to suggestions on the slice width. Obviously I'm simplifying the mains to a single point source with no distance between them. I can cover more people in less depth, by spreading the mains, but it's squishing a balloon of SPL that only produces so much: ie., I can only smooth out humps, not add more.
I have read many other sizing posts, and I apologize for duplicating what's in some of them, but I didn't see a direct discussion of "depth" of the crowd. It appears to me, that the depth, SPL fall off, is what actually determines the mains sizing. Pick an audience size, then some slice of a pie to fit that many people. Finally use those distances to size the mains: not too much for the close people, and enough for the far people. Or in other words, plan for the farthest experience using the amount of people that can reasonably fit into a space and plan on some complaints from the close people.
I hate to complicate it a little, but I also think some additional headroom should be built in to the system. If I expect a 106dB peak, then I plan for a mains that are capable of 112dB peak. Is that reasonable?
Best, and thank you!