The first answer to this question is to persuade the venue's owner or manager to invest in some acoustic treatment. Music venues that sound poor acoustically are far too common, and this could be a whole topic in itself. So what can the sound engineer do, using only their own equipment, to improve the situation?
1. Point the loudspeakers at the audience, not at the walls or ceiling.
People are great acoustic absorbers. Not only do they pay to come into the venue, they bring free acoustic treatment in with them. We don't have any data on whether muscle or fat is better, but across a range of people I'm sure it averages out.
It makes sense also to point the sound where it is wanted. The saying 'walls have ears' may or may not be true, but they didn't pay to get in.
To summarize this point, the more you can point the loudspeakers away from reflecting surfaces and towards the crowd, the better off you will be acoustically.
2. Don't make things worse!
Clearly if the room is over-reverberant, you won't want to add any digital reverberation. Or if you have to, keep it down to a minimum.
3. EQ out any problem frequencies.
Please be clear that this isn't a solution to the problem. It's just a way of making it less bad. Once sound has left the speakers, there is nothing EQ can do to suck it back in again!
If you find however that certain bands of frequencies are reverberating more than others, it makes sense to put less energy into the room in those bands. Don't expect a cure; hope for a small improvement. But often even small improvements are very well worth having.
A final point to bear in mind is that the musicians might be having a tough time too. I remember once playing in an auditorium that had a large flat wall at the opposite end to the stage. Everything the band played came back loud and clear around two seconds later, which made it very difficult to perform as well as we would have liked. There's nothing the engineer could have done.
P.S. The pic... it's Echo and Narcissus. Oh, you knew already...
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