In the past several years my city has been cracking down pretty hard on the festivals to keep noise pollution down. The magic number they came to was 80db a weighted at mix position. Being a festival you are usually running snakes to foh and 100' is pretty standard. So by the time you run the snake out and around, you're about 50-75' away from the stage. And having done many years of these festivals from 10-10 every day for a week you begin to appreciate that low level.
The municipal officials' concern isn't necessarily the health & hearing of the attendees, but to keep peace in the neighborhood. If you're at 80 dB 75' from the stage, that's roughly 74 dB at 150', 68 dB at 300', 62 dB at 600', and somewhere around 55 dB a quarter mile away. If your festival is in a commercial/industrial area, you might not annoy the neighbors if you're hitting 100dB at FOH. But if you're in a residential area, 80 dB at FOH could mean SPL in the mid 60's in the neighborhood.
For those attending the event, they WANT to hear the music. For those not attending the event, they didn't want to hear the music. 68 dB might not seem loud at all, but the sanctum of their own property will be violated because sound does not respect property lines. Once in a while might be tolerable, but if you have to listen to music you didn't ask for every. single. weekend. you'll get tired of it pretty fast. Given an SPL of 80 dB at FOH, for a house 600 feet away, 62 dB is reasonably low enough that a house built to modern energy efficiency codes will be able to block most of that sound from entering the house.
Note that a nearby source and a faraway source will act differently on a house. For example, if there is a noise source 600 feet away and I measure it at 62 db, and I compare it to a noise source ten feet away and I measure it at 62 db, (both sources and measurements outside in free air,) it will seem to have the same loudness to my ears. But inside the house, the faraway source will seem louder because that sound is acting approximately equally (62dB) over the entire wall surface of the house, where the nearby source will only be 62dB over a small area. More total energy is reaching the house from the faraway source than the nearby source, even though both measure the same -- remember, your dB meter is more or less a point sink where the wall is not.
Not all of the energy will be reflected or absorbed by the wall. Some will be transmitted; the wall acts like a planar transducer reproducing sound over a large area rather than a point source. Even though the SPL is low, because the plane is large, the total energy can still be high. While hearing damage is unlikely to result, it can be mentally fatiguing when your mind must filter it out.
I believe this is one of the reasons why, if you're sitting out in your yard, you won't notice a truck driving by, but you do when you're sitting in your living room. Outside, it's a point source and your mind is able to easily determine its location and filter it out from other sounds. Inside, it's a planar source and your mind cannot determine its location, so cannot easily filter it out from desired sounds.
EDIT: And if you're at 100dB at FOH, that could be 75dB a quarter mile away, which is still pretty loud when all you want to do is enjoy a beverage on your deck after a hard day's work.
P.S. -- This is somewhat theory; I haven't actually proven it in practice. I may be misunderstanding something, and I'm counting on you all to call me out on it.
