Feedback buster FBR. Feedback buster FBR helps stop reduce and someties eliminate acoustic guitar feedback on stage. Feedback buster. The feedback buster is terrific for using in the sound hole of your acoustic guitar to eliminate feedback when using the inside microphone - pickup. Very popular! There is only the one size available.Soundhole plug fits acoustic guitar soundholes from 3 7/8" to 4" diameter (standard dreadnought size). Will accomodate fretboard extension into soundhole.