If you play an amplified acoustic you probably have feedback problems. This dense rubber device kills feedback. I got one after seeing a famous Australian fingerstyle acoustic guitarist using one on a very loud stage. Only 4 stars because I had to cut a little off the top to get it to fit, which is not easy to do cleanly because this this is so dense. My "made in china" guitar has a 100mm sound hole, about 1/10th inch smaller than the 4 inch sound blocker.