Piezo sensor - mounting woes

[peskywinnets]

Ok, so I'm using a 1" (25mm) piezo sensor...I want it to go in my humbucker cavity...to pickup guitar body vibrations.

I'm using a preamp .....I've used both a charge amp and a basic fet preamp...each one was pretty much the same & revealed that the piezo is stupidly quirky wrt try to get maximum amplitude/fullest sound.

I've tried...

1. Gluing the sensor to the guitar body
2. drilling a hole in the centre of the piezo an putting a screw through to affix it firmly to the body
3. Attaching the sensor to a bit of fr4 then then screwing the fr4 into the guitar body (so the piezo sensor is wedged in between the fr4 & guitar body)....

all these ways have laughed in my face ...with poor fidelity being the outcome (with No2 being the most pesky - sometimes, if I screw the screw in just a little bit extra ithe piezo wouldn't even give anything out!

So I then experimented by randomly applying pressure directly to the piezo sensor with finger/plastic pens etc, whilst resting the sensor on my guitar surface & strumming...every now & then I get a sweet spot of sound (that I seek) when pressure is a applied in a certain way (Note: not a sweet spot of position...ie moving around the guitar body....a sweet s[pot of pressure), so this proves a decent signal can be had...but I'm damned if I can replicate it, with the non finger pressure method!

So that led me onto trying to establish the theory behind what the goal is....but short of lots & lots of duplication on the net about how the crystals expand & contract when voltage is a applied (or how voltage is output from the sensor when the piezo material picks up vibrations)...I'm struggling to work out the best way to get the largest & fullest sound.

I know most implementations involve mounting under the bridge...but I can't on my guitar...so I want to use the humbucker cavity...but like I say the piezo seems to be the most random thing I've ever dealt with.

I've read on a couple of sites that many seem to have success with the piezo disc half attached under the bridge & half hanging in free air...this ought to give me a clue as to what the piezo 'expects' (or rather how to give it what it expects)...but I can't grasp it! In particular, I'm wondering what part the brass disc plays when acting as a sensor (vs acting as output device) & therefore the best way to approach the mounting with that in mind

Any top tips towards the best implementation?

 

[indianajo]

Piezo's like to be squeezed. Try gluing a heavy weight to the back of the crystal (lead or iron) and attaching the front to a live (vibrating) part of the guitar with something that doesn't touch the weight on the back. The front and back are the faces with wires and/or a metal membrane on them.

 

[peskywinnets]

Ok...that sounds reasonable...I'm still wondering what role the brass disc plays when using a piezo disc as a pickup/sensor (vs. using as a sound output device when presumably the brass disc's role is to move more air...but we aren't dealling with airbourne vibrations when mounting onto a guitar body?).

Also when you say "the front"....do you mean the piezo side, upward facing in this picture...

[IMGDEAD]http://cigarguitarbox.com/wp-content/uploads/2011/03/-19301304925896892.jpg[/IMGDEAD]

or the brass disc side?

edit: Actually... two of the tests I've already done was to effectively squeeze the piezo against the guitar body ...firstly directly with a screw (I drilled through the centre of the disc & screwed the disc down onto the guitar body) & then by gluing the disc ontosome FR4 pcb board & sandwiching the sensor between the fr4 & the guitar body (I screwed down the fr4 ...which pressed the sensor into the guitar body......both results were poor.....so I'm left wondering about if these things needs less 'squeeze'?! As mentioned in my opening post...I managed to get the full sound I seek by randomly pressing about with my finger on the piezo...these things are quite whacky ...it's almost as if you get a pseudo-filtered result depending where & how hard you press

 

[JMFahey]

Best is between bridge and body but that's practically impossible in a standard bridge.

Second best would be to route 2 piezo diameter round cavities from the back of the guitar, one each below one mounting/height adjustment screw, so you can glue them real close, leaving only, say, 1/4" wood thickness in between ... but I guess that will be too drastic for a regular user.

Or buy a Parker Fly type bridge, with embedded piezos.

 

[jcx]

I believe most of the brass and piezo disc piezo transducers are "benders" - not primarily thickness mode

so supporting with one mechanical connection near the center, the other around/surrounding the edge would flex the piezo in its more sensitive direction

I would think it would be safer to glue to the brass disc

 

[indianajo]

The reason the screw mount doesn't work IMHO is that the screw into a live point vibrates at the same amplitude as the wood it is screwed too. So the front and back are vibrating at the same frequency in the same direction, so no squeeze, (or bend if the torque theory is more correct). A weight glued to the back but otherwise unattached would tend to stay at rest, with the wood point vibrating, so the element is stretched and squeezed.
I don't know much about electric guitars, I'm a physicist with electronics hobby and work experience. But I imagine the fretboard doesn't vibrate much, whereas the bridge does wiggle, pumping the wood shell with vibrations. So mounting one side of a piezo crystal to the fretboard, and the other side to the bridge, would get a lot of squeezing or torquing. Going across the length would maximize torque.
Whether the brass is the front or the back, doesn't matter much, IMHO.

 

[JMFahey]

FWIW my "shop Bass", the one customers use when they don't bring theirs, has a nice piezo brass disk sandwiched between the bridge and the body.
Works like a charm.
Fat, full, clean, even, my cheap bass sounds like a double bass, same kind of attack and harmonics.
Customers freak out when after testing the amp I tell them "flip the little switch" and enable it.
I used a FET unity gain follower with 1M gate resistor and 10K source to ground.