I've built a few plate reverbs and learned a lot from the experience. The first was a very crude 2 x 4 frame with a cheap piece of thin galvanized steel from the hardware store, just to test the concept. The driver was a speaker with the cone ripped out, and it was coupled to the plate with a paper-towel cardboard tube glued on both ends. It sounded amazing!
The second was a more sturdy double 2 x 6 frame with a 5 x 3 steel plate suspended under enormous tension -- basically a wooden version of an EMT. It sounded okay, but the plate was too thick and it didn't have the magic of the first, crappy crude version.
I kept wondering why plate reverbs were rectangular instead of round, like a gong. After all, there's a reason why resonant objects like musical instruments have rounded edges and not square corners, right?
I knew a guy in town who owned a small metal fabricating shop, so I convinced him to let me use his hydraulic bender and bent two half-moon shapes out of 2" steel pipe, then welded them together. Then I cut a circular plate with a plasma cutter and assembled a round plate reverb.
It wasn't long until I realized why plate reverbs are not round! With a rectangular plate there is a great degree of randomization which causes the reverb to decay somewhat naturally, similar to how reverberation decays in a real room. A circular plate resonates so much, like a cymbal, that the reverberation builds up into a giant mess of mush, like a room with a million standing waves. The detail is totally lost and the effect is basically undesirable.
So, now I have a nice sculpture in my shop that doubles as a somewhat dangerous, spinning playground object when my nephews visit:
Some things I learned along the way:
1) Use the thinnest plate possible that will still be strong enough to hold up to tension. (Desiring an ever-thinner plate is what prompted the original inventor to design the EMT 240 with the thin gold-foil sheet.)
2) Use as much tension as possible. I've heard that the proper way to tension an EMT or Echoplate is to tighten the tensioning bolts, while counting the turns, until the bolt breaks. Then, replace the bolt and tighten it one less turn.
3) Stick with rectangles!
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