This question is bass-/DB-related: Last night I was sound guy for a big band who played a 3 set job, corporate gig. The bass player had both an upright bass and a 5 string electric bass, played through a small combo with Direct Out into the mixer. He mostly plays the upright, which has a piezo mic placed close to the "bass leg" of the bridge. We had a feed coming from the bass channel, only hearable through the subs of the FOH system. I asked if he could switch polarity on his pickup, which he did, and it helped a bit. I also gained the signal down. The feedback continued, however, and he eventually switched to his electric bass. This removed the problem, at least for the FOH, but band members complained about still hearing the feedback in on-stage monitors. Here are some of my thoughts on why this could happen, feel free to suggest and chime in: 1. The sub speaker was standing on the stage, which is raised about one meter up from the floor, old, and made of wood. I am thinking it could be some self resonance thing going on, the sub making the stage vibrate (at least around where the sub was sitting). If so, it would have been wise to put the sub down on the floor instead of having it onstage? 2. The piezo pickup for the upright was picking up vibrations from the floor (in addition to what it's supposed to pick up) I know it's hard to help without more details, but feel free to share any thoughts on why this feedback occured, it would be a great help since I'm fairly certain to do the sound for this band again, and I'd like to avoid this problem next time.