words, photo and audio by: Telegraphy
The first artificial audio effect to produced a reverberation sound is known as the echo chamber. A sound source and microphone are placed in a large caverness room which allows persistence of sound to be heard. This persistence of sound is created by bouncing the sound source audio off highly reflective surfaces of the chamber, this is heard as a reverberation. This is by far the best reverb effect one could produce and the most expensive.
A cheaper way to produce reverberation is to use what is known as a plate reverb. This device is best described as having similar physical characteristics to an audio speaker. While working on the same principal as a speaker, the plate reverb can produce reverberations better in quality then it's digital counterparts. Historically, this device has been used in studios since the early 1960's.
The concept of a plate reverb is quite simple. An electromagnet, like the one found on a audio speaker, is directly or indirectly coupled to the center of a piece of sheet of metal. Audio from a sound source is fed into this electromagnet (voice coil) which will physically vibrate the piece of sheet metal (plate). These audio vibrations are echoed many times, echos which are in fact persistence of audio. The amount of persistence (reverberation) is determined by the physical characteristics of the plate. These characteristics many include length, height, and stiffness of the piece of sheet metal. Once reverberation has been set off in the plate itself, it then needs to be detected. This is accomplish by directly or indirectly coupling microphones to the plate. The micophones pickup the reverberations and sends them back to to be mixed with the original "dry" audio.
The following article is a step by step description of a home built plate reverb
The cost of building this plate reverb was around $150. All materials needed (excluding electronics) were obtained at my local Lowes home hardware store. Electronics on the other hand, were from RadioShack.
Step 1: You can't have a plate reverb just hanging around without a proper home. The first order of business is to construct a frame from which the plate hangs from. I used 8 foot sections of galvanized slotted angle iron which has many holes in it. These holes are perfect for a quick and painless assembly. Two of these sections is good enough for this project. Cutting the proper lengths was a breeze with a simple hack saw. Now that all four sections are to length, it was time to make four corner bracket plates. These brackets are used to make the frame ridged. 1/32 aluminum sheet metal cut with a band saw into a triangle shape. Three 9/32 holes drilled like so are all that is needed. Attaching the corners brackets to the four lengths of angle iron was made by using 1/4-20 nuts and bolts. Now that the frame is ridged length wise, it was time to construct cross members to make the frame ridged radially. 1/2 aluminum angle stock is strong enough to do the job. Arranging them in a "V" patten prohibits the frame from twisting.
Oh yea.....I forgot...
You can't have a frame just laying around all day doing nothing - Thats what men are for. This frame is going to need some feet to stand on. Using the last of the galvanized stock, I cut to length small sections and then I made two of the same corner brackets I made earlier. Connecting the feet to the frame in the same fashion as the
Step 2: What's all of this hanging around non-sense. Speakers don't hang around - That's what teenagers are for. Using the last of the two 8 foot sections of aluminum angle stock, I cut to length two vertical mounting brackets. This will hold the driver in place.
Step 3: You didn't think I would forget about the most important piece of them all. Without it, we would have to name it the "Air-verb". Anyhow - Now I know that there are many different opinions out there on what type of plate material to use and how thick it should be to produce a certain sound quality. For that, I will leave it up to you to do your own research and decide based on what you want.
Just in case your wondering what I used:
At Lowes home hardware, they had in stock a 3 ft x 2 ft plated steel sheet metal. The thickness of this sheet metal is 26 AWG which is roughly .016 inch. This is quite thin in a reverb point of view. But I wanted something that would easily reverberate at low frequencies. So my theory is that the thinner the plate is, the more easily it will vibrate at lower frequencies.
Having about 2 inches of space between the frame and the plate, gave ample lee way to use medium strength springs to suspend the plate from the frame. I used four 3/4 inch heavy gauge springs for this project - it's enough tension to ensure good persistence. Before the plate can be installed, the corners must be reinforced. If the corners are not strengthened, the immense tension exerted by the springs will eventually ware out the corners. Using four more 1/32 aluminum triangles (yes, that's the last of the dreaded triangles.) I drilled four small holes on each triangle to accompany four revives, plus another 9/32 hole for the mounting nut and bolt. Duplicating the holes onto the plate was a breeze with a sheet metal hole puncher.
This next step takes a bit of elbow grease - tensioning the springs. This requires patience, strength and a third hand to keep the frame from moving (unless your like me who uses his knee, head, foot or any other available extremity instead). Use your knee folks, it's simpler. The first spring is the easiest - no tension. The second spring (of coarse) is much more difficult. I tensioned the spring on the opposite corners from the starting one. Using a pry bar of some sort to pull the spring to the mounting hardware, I bolted them down and took a breather.
Step 4: Now folks, I know what your thinking. You can't just connect the output leads of your mixer directly to the plate and expect it to perform. No, you need a driver of some sort. There are different methods for driving a plate reverb. The method I'm using in this project involves directly connecting a speaks voice coil to the plate itself. Again, there are many different opinions out there regarding how to drive and detect audio too and from a plate reverb. So as another disclaimer, I will let you do your own research.
Just in case your wondering what I used:
I wasn't about to spend another $150 for a brand new speaker, only to have it disemboweled for it's precious voice coil. One good thing about being a man is that you tend to hang on to things (like your ex who treated you badly...please come back baby!!). I had speakers laying around - not expensive ones but they had voice coils. These in particular were old Chrysler automotive 8 ohm speakers. I wanted to directly couple the speakers voice coil to the plate itself. This required some manly modification to the speaker.
How to disembowel a speaker:
(the ionosonde way)
1: Ripe out cone body. Use hobby knife to cut away cone right down to the dust cover. "Making first incision"
2: Cut away dust cover (being careful not to cut into the voice coil or the spider. DOING SO WILL RESULT IN COMPLETE DEATH. ) "nurse Mary, please hand me the forceps".
3: Using what ever tool at your disposable, cut away speaker basket taking care not to cut away the connection terminals.(I used sheet metal nippers) "this limb needs to be removed, cut off saw nurse Mary"
4: Remove terminals leaving the metal mounting tabs( I'll use these as the voice coil mounts) "O.K. sutcher up the patient nurse Mary - My bill will be in the mail"
Now that I had a working voice coil (driver) ready for mounting on the plate reverb unit, it is time to fabricate a coupling link that will directly connect the driver to the plate. But first, I need to do something with this mess I made while performing that gruesome surgery on our patient. A quick sweeping with my trusty broom and......wait a minute - broom - broom handle... Perfect! A broom handle is the right size for making a driver coupling link. Just cut off about 1 1/2 inches of the broom handle (the length depends on the distance your driver will be away from the plate) - file or sand one end to size for a slip fit into the voice coil. Glue the coupling to the voice coil. It is essential to be cautious while gluing. If glue seeps down into the gaps between the coil and the magnet, then you essentially made yourself a nifty refrigerator door magnet. In other words, the driver won't work if the coil and magnet are glued together. After the glue has dried, mount the driver to the frame. If the coupling link length is correct, it should lightly touch the plate. If the plate is bowed outward, then the coupling link is to long.
Drivers in place and ready to use. All that is left is the pickups. Again, there are many different opinions out there of what type of pickups to use and how to use them. Everybody has different needs to fulfill in their reverb project. It just isn't right for me to tell you what my opinion is(sorry folks). But........
Just in case your wondering what I used:
Piss...hey you....What if I told you that Radio Shack has had a secrete only known to musicians for many years. You would say to yourself, "Telegraphy your crazy!!", has you search through those pro-audio gear catalog's looking at $100+ acoustic pickups. Well just between you and me (and keep this on the down-low) Radio Shack sells pickups for $4.49 each. They don't label them as such but instead their called Piezo Elements 1500-3000 Hz model: 273 073 The sound quality of this devices are great for the price.
Taking those $4.49 acoustic pickups (Piezo Elements) out of their packages, I realized that these pickups didn't look like your average piezo element.Well for starters, the elements are buried inside a blast proof, geek proof, flame resistant, and radiation proof - black plastic case. To extract this rare element, you need a good sharp hobby knife, a skrew driver, and most of all - patience. All you have to do is pry open the top with a skrew driver (sounds easy enough. Boy are you in for a treat). Down inside of it's impregnable case is the piezo element. It's almost press fitted down in there, so your going to need a hobby knife to gently pry it loss.
Attaching them to the plate reverb is straight forward but keep in mind - Yes for the last time. Everybody, say it with me. "There are many different opinions out there of where to place these elements on the plate". So please, do your own research. Knowledge is power. The most complicated and scientific of these opinions, is to find out where on the plate the wave front anti-node of certain frequencies are. Oh, yea I forgot.....
Just in case your wondering where I placed them:
Attaching them at equal distances from the ends and having them placed on opposite faces of the plate works for me. The reason why I placed them on opposite faces is to "simulate stereo". Remember that we're only using one driver (mono). Now it would be cool if I had enough room, time and money to have two plate reverbs; one for each channel.
After wiring up the driver into an audio source and running the pickups into a low noise pre-amp, it was time to test this plate reverb out !
Here's an audio sample. Enjoy.