This is an unfortunately true story. Not long ago, there was a jazz concert to be played, and they were having plenty of trouble with hum coming from the piano and the bass amp bleeding into the piano microphone. We were getting "hum" or "warble" or "wash" or whatever you'd like to call it. Either way, every time one of us would hit something around a Bb, ANY Bb, things would start to vibrate like crazy in the PA system. The hum was only coming out of the speakers.

And when we walked in to take a look at the problem before the concert, we found in front of ourselves an expensive Mackie PA system with two powered speakers (a very nice setup). Sadly, the owner had not invested in speaker stands and had sat the speakers on metal chairs.

I wish I was kidding.

Ladies and Gentlemen, something as simple as what the speakers are sitting on, be it a wood floor, linoleum, tile, carpet, hollow stage, or plastic tupperware, can DRASTICALLY affect your sound. We replaced the chairs with sturdy, painted, plywood cubes, each about the size of a small coffee table, and the problem was fixed!

The speakers themselves weren't causing the disturbance, and it actually had very little to do with the bass amp placement or the mic placement on the piano. The problem was what is called an object's "resonant frequency." There is a certain frequency range that can actually cause an object to vibrate in accordance with the molecules that make it up, effectively amplifying the vibrations that are already going on inside of it. This is why crystal shatters when singers sing very high notes. The chairs were vibrating at around a Bb and causing continued vibrations in the speakers.

Just a little tip for the next time you're setting up sound for your band or event. Wooden objects that have a full bottom (not LEGS, but a full base) are best. So are the speaker stands that you can buy. Carpet will send a great deal of your signal (especially the bass) through the floor. Tile and linoleum will likely cause a rattle, since there are very few points of contact between the floor and speaker. So, sit them on something solidthat does not rock or vibrate (like wood), or put them up on speaker stands if you can.

Oh, this brings back memories of when almost all bands used Shure columns for PA and put them up on chairs. Then we started using JBL 4560 cubes and putting then on tables. Then we started using two cubes and stacking them like Legos. Gitta admit i don't miss the days of dragging our own PA around.