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I was at an event this evening in a restored ballroom - absolutely beautiful, but parquet flooring, high ceilings, solid walls etc all made for terrible acoustics, and in fact I could hardly make out the voice of the individuals on stage.
I think it is safe to say the venue will not be modifying the hall physically, so are there active solutions or mitigations for this scenario?
(I'm thinking something like noise cancellation, but not sure how that would work)
Noise cancellation never works on scales larger than the sound's wavelength, so it's only useful for headphones.
Modifying the hall physically is the only real solution to this problem.
Short of that, the first thing to try is indeed to bring the sound as directly to the audience as possible, by using a suitable PA arrangement. Big, single speakers located "out of sight" are generally worse than multiple speakers close to the audience, either at different levels (delay lines) or as a stack (approximate line array, which partially cancels the sound that goes to the ceiling).
If that's not enough, the only thing you can do is modify the sound signals itself. Obviously, a small acoustic ensemble will have less of a problem with reverb than a large, loud rock band. In particular, uncompressed drum accents cause reverb throughout the frequency spectrum that often overpowers everything else. So you want to make the drums themselves as quiet as possible. The monitor levels should also be kept down, because those signals arrive at the audience only indirectly.
Then, on the PA, you should aim at a strongly frequency-separated mix and rather agressive compression. This way, all instruments should come through reasonably well even when the time-domain is completely smeared out by the reverb.
If you can't treat the room, you can try optimizing using the sound system and the band.
I know that sometimes for concerts there will be "shells" around the ensemble, curving down and around the group. This will help reduce any noise escaping backwards. Not knowing the venue, this may not be part of the problem.
It also depends on whether or not the ensemble is acoustic or electric. With electric you could point the speakers in different ways to avoid bouncing the sound off of the walls or what-have-you. If it's acoustic, look into speakers and mics.
You could also pack the hall full of people! Or chairs. Or some other kind of buffer. Were there curtains on the windows? Make sure they're closed. You just want to avoid having "shiny" surfaces and want something your sound can grip onto.
You don't have to modify the room to add physical damping to it. Velvet drapes absorb a lot of sound, even when they don't actually cover any surface, and would be very sympathetic to the decor. A few loose rugs covering unused floor areas, where the audience isn't directly looking at them, would also help a lot. The drapes and rugs could be put in place right before the event and removed at the end: they're just another piece of equipment for the event, like the PA system and the seating.
Aside from this, music in acoustically poor spaces relies a lot on singing/playing technique. Performers have to be warned in advance that the space is "boomy", so they know to play appropriately. Don't forget that a ballroom is purpose-made for a small acoustic instrumental ensemble, and any other kind of performer will have to adapt to sound more like that.
My method of dealing with problem rooms was to bring along a real time analyzer and test for the room resonant peaks and equalize the room with my graphic eq. The bass and lower mid-range tones are heftier and will reflect much more uncontrollably than the higher tones which can cancel themselves just by reflecting once.
Placement of the speakers up high and aimed down at the audience can go a long way to improving the audience experience. Also using highly directional horn loaded cabinets especially for the bass tones can help keep those frequencies away from problem areas in the room.
Just like playing an instrument, experience is a factor in how successful a person might be in dealing with these problems. In my experience, it's a balancing act when dealing with the physics of sound and music and someone who can balance things well, will have more success than someone who doesn't seem to understand this.
I guess my overall point is that, if you can't change the acoustics of a room, you'll need to change the way you present yourself.
You can't. What you CAN do is get closer to the source, so that direct sound dominates reflected sound. Maybe a 'silent disco' approach, where no sound goes into the room at all, everyone uses radio headphones!
This is an ever-present problem in churches. The usual solution is to use a PA system with LOTS of small speakers, so that everyone has a speaker within THEIR 'critical distance', the range within which direct signal is stronger than the reverberation. Same principle as using (very) near field monitors when the room is imperfect. And close-micing when room sound isn't required.
Another thing you can use is a high quality digital delay for the high frequencies through the stage mains since they travel faster then the bass frequencies. Things will sound less muddled. You can also use a 3rd and 4th speaker in the back of the room with the live mix delayed on the stage mains and the rear or mid-room speakers non-delayed. Van Halen and Pink Floyd have had microphones measuring the volumes of many points in the stadium.
