This was the easiest of all my machines to make. It took about a week. It is basically a sheet of 0.5mm x 1m x 2m stainless steel stretched as tightly as possible, with 1 drive coil and 2 pickups for stereo.
The hardest part about making a plate reverb is finding a sheet of steel that is thin enough and big enough. I found mine in a scrap yard. I would have preferred it to be thinner than 0.5mm, but no one in Sheffield (so called steel city) was selling it.
The metal is stretched on a wooden frame using hooked bolts though holes in the corners of the plate. The sound is really nice especially on backing vocals. It is lo-fi and more metallic compared to a real EMT plate, but In some recordings it can have more emotional qualities than a real EMT, because of its wrongness. It sounds bitter-sweet rather than just sweet.
This is well worth a try it is quite easy and there is no need to build any electronics because it can be driven by a standard hi fi amp, and the output can be treated as if it were a microphone and plugged straight into a desk or sound card. Mine was a first attempt so you can probably do better.
A plate reverb is a 2 dimensional reverb, a room reverb being 3 dimensional, and a spring reverb 1 dimensional. The sound vibrations travel across the metal like ripples in a pond. The sound travels across the plate much faster than it would travel across an average sized room, yet the sound of the plate gives the illusion of an enormous space. This is because the sound is reflected off the edges of the metal with very little loss of energy, so it stays trapped, bouncing around for a long time, and to our ears this sounds like a big room, or a small room with very hard shiny walls.
Plate reverbs are nearly always rectangular. You might think that this would create problems with resonances caused by standing waves bouncing off the parallel sides. This is what happens in a real room, and it is why echo chambers were made in irregular shapes. The metal plate is under tension and the tension is not regular, so the sound travels at different speeds across different parts of the plate. This creates random resonances and might explain the way the sound seems to shatter into a sparkly haze. A plate that is small and thick has few resonances and has a harsh metallic ring. One that is big and thin has more resonances and has a wonderful luscious tone.
The classic EMT plate was 2.4m x 1.2m and 0.5mm thick, so this is a good reference for a home made one. Finding a similar sized piece of steel might be very hard. Classic plates were made from carbon steel, but the steel has to be hardened by cold rolling. I used stainless steel which is naturally hard and springy, and probably easier to find. Aluminium might be worth a try, it does seem to have a bright musical tone when struck. The frame needs to be very strong, some kind of metal scaffold would be better than my wooden frame. My frame was bending a lot, and it is pretty chunky.
I have often read about tuning a plate, but the term tuning suggests tightening to some precise frequency. However the resonances of the plate are so random and numerous that I think the best thing to do is to just stretch it as tight as possible. A tight plate will wobble less and have fewer problems with low end muddiness. I found that even when I stretched mine until the metal was tearing, it should have still been tighter, so I recommend reinforcing the corners.
The Drive and pickups
The Drive is a moving magnet type. I used a neodymium magnet because they have a very high force to weight ratio. It is important to keep the mass of the moving part as low as possible. A mass is the mechanical equivalent of an inductance and will suppress higher frequencies. The impedance of the drive coil is about 8 ohms this is so it can be driven from any standard Hi-Fi amp. I recommend supporting the drive coil securely, this is so it vibrates the plate and not itself. The pickup is like the driver but in reverse. I used the pick-up coil from a old spring reverb, and a magnet small enough to fit down the middle. The impedance is about 100 ohms, too low will produce a week signal, too high will make the signal vulnerable too high frequency loss down a long cable. Use balanced mic cable and XLR input to the amplifier.
The biggest problem to overcome is loss of treble. I don't like very bright reverbs where all treble is preserved however there is no danger of this happening here. You will need to boost the treble on the send and maybe on the return. Also, cutting the base is normally necessary, bass frequencies tend to shake the plate and mince up all the other parts of the signal. I have found the plate most effective on vocals especially backing vocals, and least useful on drums, explosive sounds seem to loose their punch and the plate turns the sound into a kind of noisy soup. However anything is worth a try. Your homemade effect will sound like no other, and by randomly experimenting you will discover how to make your recordings sound like no other.
Other similar effects worth a try
Think of the plate reverb more generally as a type of effect that electronically sends sound into a material and recovers it at the other end, so the sound takes on some of the qualities of that material. Effects like this could be really useful for adding real world qualities to electronic sounds. A thin piece of wood in place of the metal sheet might not sustain sound for long enough for a reverb effect but it could add interesting richness, warmth and spacial qualities to a sound. I remember reading about a very eccentric synthesizer that was built about 90 years ago, shortly after the Telharmonium. It filled an entire house and the basement was full of different materials like glass sheets, bed springs, metal and wood. All wired up to the synthesizer to add natural qualities to the sound.