Way back in early July (see inSync 7/8/99) we did a Tech Tip on acoustic guitar feedback and, while many valid tips for help were listed, we did omit one of the most effective and simple remedies. Naturally, several inSync readers informed us, but we haven’t been able to get back to this thread until now.
There is a device called a Feedback Buster, which is basically a thick piece of rubber that fits snugly in the sound hole of your guitar and really works wonders. It works by decoupling the air inside the guitar from the outside, where the strings are vibrating. While the guitar still has resonances, the main body of air inside can’t vibrate with the strings as easily. Only the vibrations of the top of the guitar body resonate with the strings, which greatly reduces the volume of the guitar acoustically as well as making it much less prone to feedback. These work great for round sound holes, but if you have a guitar with unusually shaped sound hole(s) you may have to improvise. You will also find that guitars with sound holes that are not directly behind the strings are much less prone to feedback anyway.