Though people here have repeatedly expressed interest in making a spring reverb, there has been surprisingly, and perhaps dismayingly, very little interest over the years in building a plate reverb. Here's a place to start:http://www.google.ca/search?client=firefox-a&rls=org.mozilla%3Aen-US%3Aofficial_s&hl=en&q=Building+a+plate+reverb&meta=&btnG=Google+Search
The thing to remember about reverberation is that it depends almost entirely on the physics of momentum. In other words, energy is applied to some body that transmits the energy in a reasonably conserved or efficient manner, such that the energy and the momentum lasts for some period of time.
In fact, that is EXACTLY what happens in a physical space. Sound is transmitted through the air, and if the walls and other surfaces are highly reflective, the air will provide very little impediment to that acoustic energy continuing its momentum. That is why walking outside in the falling snow always sounds to intimate and quiet - the snow reduces that momentum and the reverberation comes to a halt.
Plate reverb relies on the omnidirectional (at least in 2-dimensional space) distribution of acoustic energy. A driver is attached to a flexible, but rigid enough, suspended metal plate (like springs, the suspension is to make sure the vibrations are not damped, so that all momentum is conserved and maximized). Ideally, or rather traditionally, it is mounted dead center in the plate so that vibrations move outwards from the most flexible part of the plate - a bit like getting more bandwidth from a cymbal by hitting it in just the right spot.
Unlike a cymbal, where the "rings" are deliberately built into the structure to introduce ring modulation-like textures, the plate is perfectly smooth so that no interference will occur. Because the plate is a 4-sided geometry (either square or rectangular), the driver is attached at a point which can never really be equidistant from ALL points along the edge (THAT would be the case if the plate was circular, rather than rectangle or square). This works in the plate's favour because, like a rectangular room, you get different reflections occurring *because* of that different distance-to-the-edge.
One or more contact mics of some sort (piezo, etc.) are placed in the corner/s of the plate, and what it/they sense is the reverberant energy transmitted by the driver through the plate.
Springs vs plates:
- While a single 4-sided plate produces omnidirectional radiation of sound, a spring transmits in one direction only. In a sense, a plate mimics the use of many sporings of different lengths and tensions.
- Because a spring is coiled, rather than flat, the actual transmission distance covered in a spring is much farther than that of even a fairly large plate. The "space economy" is the primary reason why springs have been the default option for so many years; decent plate reverbs are like huge gongs, and expensive as a result of their bulk.
- Because springs are "softer", and the transduction process results in some absorption by the spring's softness, bandwidth in a sopring unit tends to be smaller than that of a plate unit. People often talk about how bright plate units are. Of course, whether you *want* brightness is another thing.
- There is only one place to stick the transducer on the spring - at the end. Plates allow for the use of many different concurrent locations of transducers, which can then be mixed in varying proportions to get different sounds. Just keep in mind that the attachmentment of any transducer is a bit like filling the air with more snowflakes - it tends to dampen the plate and reduce the momentum.
One of the things I have never seen, largely because I haven't looked very hard, is the use of nontraditional plate shapes. For instance, a pie-shaped plate, with the driver transducer at the pointy end should have an action a bit like a wave pool, where the acoustic energy spreads out, but disappears at the wide end, never to bounce back. In theory, anyways, that could produe a clearer, less cluttered sound, though clearly working out an appropriate suspension system that would let the plate hang freely (to reduce damping and maximize momentum), without placing too much pressure on any single corner or edge would be a tricky calculation (MUCH easier to do with a square or rectangle!).
In any event, if you have the space and curiosity, don't discount plates. Plenty of places will sell you sheets of thin guage metal for a reasonable price. Although things like those high-end EMT units use gold foil, I imagine that one could wring something useful out of a 2' x 4' sheet of flat 24 guage galvanized steel or aluminum. As for drivers, the traditional driver for homebrew plate units has been those "make-your-wall-into-a-speaker" units that screw in directly to the drywall and use it like a cone. Sensors can be anything from piezo discs to those suction cup things people use to put on telephone handsets to record conversations.