Q: “I play solo acoustic guitar/vocal gigs in small clubs. Although I’m not a very loud act, at some clubs I shake the mirrors behind the bar, or the ceiling tiles! What’s happening and how can I stop this?”
A: What’s happening is the output of your sound system is triggering resonant frequencies through sympathetic vibrations from different elements of the environment around you: the mirrors and parts of the ceiling assembly (we’ll bet the room in question has a suspended ceiling – more on this in a moment). It doesn’t require a lot of volume to generate sympathetic vibrations. Instead, they are caused by sound waves that match the resonant frequency of the vibrating element – the glass, the ceiling, whatever.
Major touring groups (and smart regional rock bands) check every arena or room for resonant frequencies by using a Real Time Analyzer, or by running a test tone through the system, sweeping from 20Hz to 20kHz at different volume levels. They can locate and mark problem frequencies and adjust them by cutting corresponding levels via EQ (although technically only a band-aid it does help significantly).
If your sound system doesn’t already have one, you need a good equalizer. A 31-band equalizer (because of its 1/3-octave precision in frequency adjustments) will give you the most control over offending resonances. The combined output of your voice and guitar are generating thousands of frequencies, and some of them are triggering the vibrations in the room. Once you locate the resonant frequency of the bar mirrors (which you can do fairly simply through experimentation) lower that frequency by a few dB to reduce the problem. You may have to lower the level of that frequency a lot, but be aware that too much adjustment can adversely effect your sound in other parts of the room that may not be experiencing the problem. If the problem persists after lowering the problem frequency you may want to investigate adding some damping to the resonating items.
A word about suspended ceilings: if you’ve ever looked above the tiles, you know that these systems are made up of hundreds of wires, tied to up to thousands of feet of metal framework. It’s the equivalent of a very poorly made musical instrument, and is likely to resonate at multiple frequencies, from low to high, though the tiles themselves mitigate this to a degree. After you’ve made some preliminary EQ adjustments, there’s not much you can do about this; you could spend days tracking down individual resonances. A less painful step would be to simply reduce your stage volume.