So, I know nothing about sound.(Reference: http://www.controlbooth.com/forums/showthread.php?t=4602) Well, I know where the on button is on my board and the gain knob... a couple weekends ago I was running a followspot for an event at my school. Early on in the show I heard the most god awful feedback I'd ever heard. Strike that. I ever felt. The theatre was shaking. At first I dismissed it as a passing train, but quickly came to my senses, thanks to the audience's groans. I nearly laughed afterwards, for my mistaking the tremors for a train. So this event made me quite curious, and for my edification, I'd like to know: Is this within the "normal" range of feedback? Any thoughts / comments greatly appreciated.