Nasty resonance - from an unusual suspect - solved
I finally got around to redoing my studio space, mostly just moving things around, among other things, to make everything fit better with people in the room (always a nice thing!), as well as acoustically (of course!). After the inevitable tweaking around with the new placement of the desk and monitors, I started noticing a weird ringing that I hadn't heard before. Now understand, I have bass traps in 7 out of 8 corners, plus dampening on the walls, so it's not the usual romm treatment sort of thing. The ringing was also "purer" than what you get from an untreated room... it was, shall we say, "metallic", like a plate reverb? The ringing was hard to locate while sitting at the sweet spot, so finally I just stood up and started clapping my hands, turning my head and listening for where the ringing was coming from. After a few attempts from different spots in the room, I managed to locate it.
It turned out to be the metal stand for my digital piano (Roland FP-5c). The stand is constructed of long tubes, and they were acting sort of like tubular bells. Tapping on the stand revealed strong vibrational modes (roughly around 500 Hz up) along the main horizontal and vertical tubes. The ringing was so bad that it would add a noticeable reverb tail to a sung note, if you belted it out even a little. So something had to be done.
I first thought I could stuff the tubes full of something dense, like how they fill speaker stands with sand. But all the trouble of taking things back apart, not to mention the mess of the sand, made me think twice! After some digging, I came across these acoustic damping sheets:
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=268-030 I picked them because 1) they were cheap and 2) they got high ratings. I also liked that they were made of vinyl and didn't have asphalt in them (seemed cleaner to me). I took a sheet and cut it into strips wide enough to fit along the tubes, pulled off the backing, and stuck them on. I placed them in the middle of the span of the tubes, and also at the ends of the longest horizontal tube. *Result: The ringing is gone!* Granted, if you tap the tubes hard you can still hear a little of the ringing, but it doesn't happen at all now with clapping or loud sounds in the room. And you can bet that it makes a difference sitting at the sweet spot!
So folks, if you hear funny things when you clap your hands in your studio, you may want to check for metal structures that are acting like reverb plates (or tubes!) and damp them down. Check that metal rack stand, for instance, or maybe that rogue piece of furniture.
I mean, when you clap your hands, the sound of clapping is all that you should hear, right? (Next I'm going after the metal grating on an air return - suddenly I can notice it... and I don't think it's my ears ringing...)