Silent disco was something I never expected. I never expected to see a club-floor quiet except for the shuffling of feet, and the couples chit-chatting in corners. It looks like performance art by very imaginative mimes. Sometimes they sing along or clap, but if you don't have a radio and headphones you cant follow the program. In reality everyone in the room is dancing to the same beat, but it's broadcast over a micro-powered transmitter operating under Part 15 (in theory). This was brought to you by the power of radio. Here's a random one from Youtube.
This wasn't exactly what the FCC had in mind when they drafted the Part 15 language, but it covers it quite nicely. The part 15 regulations permit broadcasts originating from unlicensed transmitters to an effective service range of approximately 200 feet. That's from the FCC's own webpage. It does not cite HAAT, wattage, or ER. Without a doubt there are extremes they'd find objectionable but the guideline stands as the sole legal goal post on that page. More Here.
If you actually read the whole part 15 rules.. it's more detailed but really still boils down the the 200 feet. More here. If you want to get picky that works out to 250 uV/meter a measured at 3 meters (~ 10 feet) and a radius of 200 feet. This is actually somewhat difficult outdoors, but not indoors. Indoors we have walls. It over simplifies the situation to say so, but.. walls are lossy. So as a result, the radio/club DJ is relatively well contained to the room they're performing in.