Piezo Disc Elements
I chose to use two Piezo discs in a recent guitar build project to blend with one Humbucker pickup. The sound will be unique because of that blend, but very often I mention these elements and I don’t fully explain them, so I thought that I would do that. The first thing I will do is give a basic explanation of what a Piezo disc is and what it does.
The Piezo element consists of a thin metal disc bonded to a thin ceramic disc. Wire leads are attached to the separate discs and when current is passed through the discs it stimulates them in one of two ways, either to produce sound or to transmit sound dependent upon the way you wire the disc. You may not realize it but there are Piezos discs in everyday items you have contact with. Smoke Detectors use them as a speaker to transmit that blaring beeping sound. Your Cell Phone probably has one in it to use as a speaker or microphone or both, and there are plans for practical application of the Piezo as a charging unit using the vibrations of the Piezo and converting it to current. The Piezo is a versatile device.
The Piezo element can do an equally good job at picking up vibrations in the soundboard of your guitar and turning those vibrations into sound. Preserving the Acoustic quality of those sounds as well. You have probably had experience or at least heard of under saddle Piezo elements that pick up vibration through the Acoustic Saddle and change that into sound. The Piezo disc works in exactly the same way except that the placement can be whatever you choose.
Placement of the Piezo Disc
Most of the string energy transfer will come directly through the bridge saddle on an acoustic guitar and radiate out into the soundboard from that point. There will also be energy transfer at the nut on the headstock and at the string mounting points on the bridge, whether that be pegs or a pegless attachment at the bridge or elsewhere, but that will be a very small amount compared to what will occur at the saddle. The saddle is where the majority of the string energy transfer occurs. That energy is transferred into the soundboard and then causes the soundboard to vibrate. That vibration produces sound which is then reflected off the back and sides of the guitar and out the sound hole. Acoustics in its most simple direct form. It’s the same thing that happens when you hold a glass to a wall and the press your ear to the glass. You will clearly hear the sounds from the room on the opposite side of that wall. That is sound energy transferred into the wall and then traveling through that wall via vibration and once again becoming sound as you place the glass against the wall. So, the closer to the source the better the vibration will be, the more your Piezo will pick up and convert to sound.
For that reason the best placement is usually the bridge. There are other placement options directly on the soundboard, either on top via a two sided tape or underneath via a bonding agent of some sort. Behind the bridge, sandwiched into the bridge if you are building a custom application, or attached directly under the bridge and cushioned in some way to defect the harsher sounds of the traditional Piezo. There are other ways as well, probably as varied as any builder can make them.
I have not seen a great many Piezo discs used in the way I am going to use them unless they are in a cigar box application. But I wanted the ability to have that acoustic sound and since I have converted the guitar they will be installed in to an adjustable bridge my options were limited.
A word on the harshness of the Piezo disc or even the Under Saddle Piezo. It is part of the design of the Piezo, they pick up sound very well. Fingers tapped against the top of your guitar, or accidentally tapping the soundboard as you play and so when Piezo discs are mounted directly to the wood they are much harsher because they are picking up all of that string energy transfer. The under saddle Piezo has a coating in most cases that reduces the effectiveness of the Piezo and so cuts down on that hardness/harshness in the sound produced. The disc has no coating at all and so a direct to wood mounting would pick up all the sounds in that top/soundboard, string zings, finger taps, maybe even slight buzzes from string changes. Because of that it is usually recommended to coat or shield the Piezo disc from the wood or mounting point. Recommendations include rubber, leather, plastic, silicone or epoxy. The harder the product the more energy transfer, the harder/harsher the sound will be.
Most of the discs you can buy are about 1 1/2” in diameter and you can cut them down to suit your tastes. As long as you make sure the areas that are wire attachment points are not disturbed they will still act as a pickup. Because of that you can fit them nearly anywhere at all. Still, the close to the bridge they are the more the energy transfer will be and the clearer the notes will be.
I used the silicone method of mounting the Piezo discs. I used two discs, one directly under the bridge the other one back a few inches where the vibrato mounts. Very close to the screw mounts for the vibrato in fact. I drilled about a 1/4” deep 1 1/2” hole in both spots and then over drilled the holes so that the Piezos could fit in them loosely. Next I filled each hole with pure silicone to the tops, making sure I left no voids within the silicone. I then pressed the discs into the hole with finger pressure and set them so they were about 1/8” deep, so there was the recommended 1/8” of silicone between the Piezo and the wood to act as a cushion and reduce the harshness of the Piezo discs. The one under the bridge will receive the most string energy the one further back will pick up the sustained energy through the tremolo mounting points. In the past that has produced a prolonged ghosting sound that seems to seep into sustains, almost seeming like a second wind in the sustain. Subtle but I really like it. I did this on one of the first guitars I built from scratch and the sound it produced has always stayed with me. I liked it that much. It may not be for everyone though, so you might consider a few other things as far as mounting goes.
First, you may want to consider doing the actual mounting of the Piezo discs at the very end of your build, that is if you have access to the area you wish to mount them in and you are in no danger of damaging the guitar as you mount them. This way you can use tape to temporarily mount them and check the sounds as you change materials and positions until you find something you like. I have done this a few times and I know what sound I am after so I don’t have as much of a concern in that area. The way I mount them, with the hole only slightly over drilled, the edges of the Piezo discs are bound to contact the sides of the wood. They will pick up sound in theses contact areas more easily, even so this is not a large or thick area, but the very thin profile of the disc itself. The rest of the sound will be cushioned by that silicon and it will produce a mellower sound, without the sharpness or harshness, the two should blend well and sound good as they have in the past. Blended with the Humbucker and the on-board EQ I should get just about any kind of repeatable sound I want. This is about the fourth or fifth build where I have done them this way and the sound reproduction has been the same to my ear each time.
Wiring the Discs
I wire these discs parallel; in other words one disc per circuit. You can wire these in series the same as any other electrical project, lights, speakers, etc. But wiring in a series changes or lowers the over all impedance of the circuit. You would end up getting less sound from the paired units. If you were wiring the Piezo as speakers that might be desirable to achieve a certain impedance coming out of an amp/driver. But as a sound pickup it isn’t all that desirable.
Here is the nice part for me. You can wire the Piezo directly into a 500 k or 250 k Volume potentiometer control or Pot as we call them. In other words the standard pot that came with the Strat donor wiring harness I am going to use. Humbucker into one, Piezo, Piezo. That output goes to an EQ/Preamp I could run it directly to the amp as a matter of fact. It doesn’t need the EQ/Preamp, but I want that to be able to tailor the sound to what I want so it will run into the EQ/Preamp and from there to the amp.