I recently read an article from ‘Experimental Musical Instruments’ (www.windworld.com) and this is a quick summation.
Because of the electrical nature of Piezos, wiring in series yields poor results. Parallel wiring is the way to go. It also means that the maximum number of discs wired together in parallel is 5, without significant loss of signal strength.
Phase cancellation can be a problem and should be addressed. If this feature is taken into consideration at the initial setup much of “try and see approach” could be eliminated.
The way the piezo is positioned or wired could result in the piezos transmitting the opposite phase of the soundboard vibration. This could happen in the following circumstances (a) Piezos are positioned on either side of the soundboard. (b) One piezo is reverse wired in relation to the other one. (c) One piezo is upside down in relation to the others. If any of these occur there will be an out of phase deficit of 180 degrees and serious cancellation will occur.
If the 2 piezos are relatively far apart on the soundboard they will pick up different phases of sound waves as they travel through the soundboard. It will be less than 180 degrees so cancellation will be less under this circumstance.
Phase reversal is easily corrected. With the second, “trial and error” has a place in it corection and varying the location and distance apart will help.
Two or more piezos are said to give a warmer and more satisfying sound. A single piezo gives a brighter and a more edgy tone.
For better signal to noise ratio it is better to use shielded wires for the whole piezo setup.
I will follow this post, I can't help because I've been using piezos very recently. What I can say is that, I used my guitar with and without preamps, the difference is huge.
I have not used a preamp but it is next on my list. In my limited experienced both volume and tone pots cause a significant loss of volume.
The same article writes about Piezo cable. They found that it was much less sensitive to vibrations and had to be place under the bridge where vibration was at the maximum. I have found a lack of sensitivity with rod piezos. I must emphasize that I have very little direct experience in these matters as I have only had 4 builds
Experimental Musical Instruments is a goldmine of good information for instrument builders
I've built instruments with various different piezo installations (single disc, double discs, rods) and I still find it difficult to say for sure what is the best arrangement. I do think there's something in the suggestion that twin (or multiple) piezos can give a more balanced frequency response. But coming up with an exact formula or exact method for what type of piezos to use and where to place them is difficult.
I think one of the big lessons I've learned is that science can be a true helper and useful guide in instrument building but there will also always be an element of trial and error (which itself is a scientific process).
I have to say the best sound I've got from a piezo pickup came from a purpose-made pickup designed for bridge mounting. (see this pic http://www.cigarboxnation.com/photo/piezio-pickup) This is a budget pickup designed for standard guitars and cost me £8 (for our US members that's about 12 bucks at current rates), but I managed to separate the sections to get two pickups for that price.
Interestingly these aren't single pieces of piezo material - they seem to be comprised of individual piezo elements for each string (one section has 4 individual elemnts and another has 2). You can also get straight "6 string wide" pickups which have 6 individual elements. Unfortunately I can't tell whether the elements are connected within the pickup in series or in parallel - my guess would be parallel.
As regards piezo discs, I have to say my personal experience is that two sounded somehow better than one but with the following reservations:
Firstly, like most of the experiences you read on most guitar forums everywhere, this is not a proper "double blind" test. This is a really important point. You'll probably read all sorts of things about how product "X" or technique "Y" have made a difference to someone's sound. But science tells us that people - even experts (indeed especially experts) - fool themselves. So I make clear that all I'm doing is reporting what I think I heard.
Second, the difference between one disc and two doesn't seem so massive as to automatically be worth the additional complexity. Maybe it is, maybe it isn't. I think two sounded better but I'm content to build with one disc for several reasons...
It seems to me that how and where you mount your pickups makes a very big difference to the response and tonal characteristics. There's reams that could be written on this so I'll leave most of it for another time. The main thing for the moment is that this seems to be at least as important as the single-versus-multiple question, if not more so. It is also one of those aspects where trial-and-error seems very relevant.
Last, if you want to try to compensate for the deficiencies of piezo pickups then one obvious solution is to use a buffer or pre-amp circuit in the guitar. You can buy readymade preamps or there are plenty of circuits around on the internet if you want to build a simple battery-powered buffer circuit. The idea of buffers is to deal with the mismatch between the impedance of piezos and the impedance for which guitar amps are usually designed. They can help to overcome problems with excessive losses of certain frequencies.
I have to say that the simple arrangement of a single piezo disk on the underside of the soundboard has generally worked surprisingly well for me. Maybe it's not perfect in terms of how you expect a regular guitar to sound but we're not building regular guitars.
Here is a test I made for friends who want to hear the difference between using a preamp and not using any.
The CBG has a D bass string G B D acoustic strings. The piezo is a 20 mm disc with a simple output.
The preamp is a Joemeek with compression, enhencer, a bit of boost. NO EQ.
See the difference ? It's huge ! Clearly, when one doesn't use distortion, for exemple... a dry sound is not nice with a simple piezo.
Watched your recording. I don't find the same differences as you do using piezos. i think it depends on the guitar, perhaps even the box itself, although I don't know exactly how you wired it to say.
Mark surely one of he most interesting article I read on the subject! I began to compile all the information I can on piezo pickup capabilities (see my question in this group about Pinecone Fletcher double piezo pickup ! here is the secret I think ;0)
Wha? we know?
I agree that two will surely (I'm building my CBG at the moment) better than one. One on the bridge the other on the back of of the soundboard.
An internal preamp will surely help to "increase" the sound but "decrease" the feedback (Pinecone do not have any control on his guitar)
The quality of the wood + the quality of what is put around the piezo pick up (and of the piezo itself!)+ the quality of cable, its length and solders mais surely influence on final result.
There is surely a recipe that permit a 80% satisfying installation and the rest is pure magic touch and personal experiment. As you said science can help us to get it or to be near of it.
Good information, Redbelly!
I've been studying up on fiddles. Fascinating little string boxes!
I'm curious about the value of a cbg as resonant tonewood soundbox. Especially in many stick-on-box and stick-through-box designs where the actual construction of the cigar box may not necessarily resonate as one might hope for... well, in comparison to a fiddle! :-)
It almost seems to me the neck is the primary resonator, in many CBG's???
There has been a lot of study regarding the top plate of a violin and it's relationship to the bridge of a fiddle, which is a critical element of it's design.
Yet I'm uncertain of the validity of this information, as a fiddled is a bowed instrument, and not generally plucked, as a CBG would be.
My approach has been simple and reliable. I wire two piezo disks in parallel and right next to each other. The sound is just fine. Pre-amp or no doesn't seem to make much of a difference after all--some but not as monumental as I thought.
What I'd like to know is if the size of the piezo disk makes a difference at all.
Very good info Redbelly....I am going to print this out for my notebook....thank your for posting it.