Although I've never done this, a few possibilities come to mind:
•Might be a good time to use a figure 8 pattern, with the PA at 90 degrees to the mic.
•Playback the song as quietly as possible. (duh)
•Playback the song with with a low pass filter as low as possible; that way you could high-pass the recording in post and hopefully the two don't overlap too much, frequency-wise.
•If you could playback the song on video karaoke-style, you may not need to play it as loudly.
Ideally you would want a headphone amp and enough headphones. Remember you can record it in small groups, you can even do it individually then just mix it all together afterwards. The only benefits of doing it in one big group is time, you would get the bar "vibe" and presumably a lower signal to noise ratio.
You're going to find that it is nearly impossible to pull this off. Tricks like reversing phase work great on paper, but not so much in real life.
Your only hope is through the use of multiple mics and using their acoustic cancellation to isolate them from the PA.
As great as the null is in a Figure-8, I'd be careful because you'll be pulling in a lot of reflections through that rear lobe. I'd probably stick with hypercardiod and short shotgun mics. one trick I've done in the past is to position my HyperCard mics on the speakers themselves. Use a desk stand or something like that with the capsule just out in front or even with the front of the speaker grill. The null of the mic will block out the majority of the sound. I would also consider a couple other things- shotguns from the front to capture some direct sound and dead hang HyperCard mics over the crowd. Once again make sure the null of the mic is taking advantage of clear sound.
How do you record a "crowd" track for a prerecorded song without bleed from the PA?
I'm planning on recording a large group in a bar singing along to a song and adding it later as a crowd track.
How do I set it up to not have phasing and bleed?
How involved do you want to get?
If you have total control of the project, you can do it.
If you're "piggy-backing" on a pre-existing event, not so much.
What will make it work for you is to have the live sound operator and an MC/song leader working together. Said MC gets the crowd going and when they're good and strong the sound guy takes the playback down under the crowd, leaving the crowd to predominate. If you've got the sound system in the nulls, you'll get one or two good choruses this way.
That's the only way I see it happening with paid participants and rehearsals.
In doing crowd vocals for some movies here's what I have used:
First rehearse everyone with playback from speakers.
Then give as many headphones as you got to people sprinkled in the group - those with the best projection.
If your crowd isn't as big as the one on your video do some takes with the front row moved to the back and those who were in the back up front. Any really loud/noticeable people might want to take a break.
You might want to pull back the low end. low freq is more onmidirectional in nature unless you use cardiod woofers. Otherwise you get a very muddy soundtrack and normal/bright audience. You want them to be equally bright on the mics.
I think a lot depends on the space. I recently did two such things using speakers for playback. Same basic idea - a location overdub onto an existing track with a lot of singers.
one with a large group in a church and one with school kids in a classroom. The school kids was in a deadish room and the bleed was actually enhancing to the sound. Not bad at all.
the church thing OTOH, was quite problematic. Even with careful attention to placing the speakers in the nulls of the mics etc, there were enormous amounts of spill off the back wall. But turn it down and then it was too low for them to stay locked in. I ended up dropping their cue mix down to only guitars and let that be the guitar reverb for the song! .
I wanted to do what eoats suggested- distribute my limited amount (12) headphones to the critical members - the 'musical leaders' or 'first chairs', as I have seen that work well in the past. They can "pull" the others along. but the choir director nixed that idea because he felt it would create "bad feelings" with the ones who did not get the headsets!
A trick I read about, but have never tried involves using one of those small FM transmitters and asking everyone bring a portable radio with headphones. You need something with some decent range, not the kind you use in the car to go 2 feet from your iPod into the FM radio. You would want to test it out in the location well ahead of time. This probably worked better back in the 80's when everyone owned a Walkman and most of those had the FM radio built in. But if you hit the right garage sales, you might be able to collect quite a few of these cheaply.
Maybe you could stream it over the internet and everyone could log on with their Smart phones? Probably would not be in sync, but that would be funny.
Hire an FM transmitter and get everyone to bring earbuds and an FM receiver or install FM radio App on their smart-phones?
(Or hire a bunch of IEM systems, or headphone amps, extensions etc)
Perhaps rig a Video system and program a DVD/computer playback with karaoke style lyrics, (where the text colour changes at the appropriate timing for alignment).
Have the track run through a PA, so your audience can pitch off it, then drop the audio out at the appropriate time you wish to start tracking (the dropout could be pre produced when you create the DVD).
You could just record some Gang vocals in the studio and then get some generic crowd noise and ride the fader in time with the singing and it sounds like everyone is singing the words.
If you use white noise instead of crowd noise then it sounds like 80s rock