We have two Beta 58 mics and one SM58. The Betas are really hot and feedback before we reach proper gain. One Beta mic is in front of the drummer for the lead singer. Will a "banner" behind the drummer help reduce feedback-if so, what material and thickness?
First, search our Knowledge Base for the word "Feedback" by clicking on the following: Feedback Search. This will bring up many bulletins on this subject. These bulletins have multiple suggestions on controlling feedback.
Second, feedback is a very complex phenomenon with many variables. The loudspeakers you are using are just as much "at fault" as the mics, the mic locations, and the room acoustics.
Third, an acoustically absorbent banner behind the drummer might help IF the problem is caused by very high frequencies from the loudspeakers bouncing off the wall behind the drummer and hitting the front of the mic. But as you cannot see sound, how do you know this is the source of the problem? (The size, shape, and thickness of a banner is completely dependent on the frequency of the feedback, the size of the reflecting surface, and many other variables. The answer to this question must be calculated and the equation requires exact measurements of the variables.)
Fourth, if the feedback path is between the monitor loudspeakers and the mics, consider using an in-ear monitor system. As this places the monitor loudspeakers directly in your ears, it eliminates the acoustic feedback path that exists between regular monitor loudspeakers and microphones of any type.
Fifth, consider the use of a feedback reducer like the Shure DFR22. This device calculates the feedback frequencies in your PA system and automatically inserts very narrow filters in the signal path to reduce feedback problems. Note we said "reduce", not "eliminate".
We wish there was a simple answer to your question, but there is not. We hope these hints will lead you to the answer that works for your unique feedback situation.