Piezo sensors... "Pressure, pushing down on me"

[peskywinnets]

So I've been dabbling with piezo sensors...and they've been messing with my head.

Re mounting piezo sensors, if you read everything that abounds on the net there's much conflicting info (no surprise) ....many settle on squeezing a sensor somewhere under their bridge - not an option for me.

I actually wanted to mount a piezo sensor in my bridge humbucker pickup cavity against the body of my solid body electric ....but I can't get the mounting arrangements right, what I've tried...

Double sided tape - weak signal
Drill a hole in the middle of the sensor & then screwing down so that the piezo presses firmly against the guitar body - weak signal
Attaching the sensor to some FR4 & mounting the fr4 to the body - weak signal.
Physically holding the sensor whilst randomly pressing its brass disc against the body - occasionally I hit a sweet spot (not on the body, but more angle/pressure of the sensor) & the sound level/signal leaps up & is "fuller" than a fat bloke who's had 10 courses of high calorie fat stuff, but hasn't farted yet.

This last test/result proves my fet preamp is fit for purpose & that a nice signal can be coaxed out of the pickup cavity.

I'd rather not deal in randomness...indeed, it'd be impossible to replicate the mount position/pressure when I hit that whimsical sweet spot of pressure/angle ...so rather than just hit & hope, I'd rather try & fathom out the best approach.

Ok - Piezo sensors....so the white piezo material reacts to pressure/vibrations.....what is the brass disc's purpose here? Would it be safe to say that we ideally want the brass disc to stay as static as possible with only the piezo material getting the vibrations?

Top tips warmly received.

[Steve Conner]

Those piezo disc beepers are designed to bend. In use, they are clamped around the outside circumference of the brass disc, the way a drumskin is attached to a drum. The piezo disc expands and contracts in the radial direction, causing the brass disc it's bonded to to bow in and out, again like a tiny drumskin, with a resonant frequency of a few kHz, as required for making beep sounds.

I found I got the best results by twanging the disc like a diving board. I fixed one edge to the guitar body and attached a small weight (a "seismic mass") to the other edge 180 degrees opposite. This gave by far the biggest output of any configuration I tried. The different bending mode and added weight lowers the resonant frequency, giving a fuller sound with more bass.

[J M Fahey]

Agree and add: they are *also* sensitive to straight pressure.
FWIF I have successfully:
a) sandwiched the disk between my cheap nass bridge (which basically is an "L" bent piece of 1/8" iron sheet) and bass bosy: strong humless signal, both because it gets it in full amd the grounded bridge shields it
b) on my nylon string Spanish guitar I Krazy glued it on the inside under the bridge, after I found a sweet spot.
Got good signal, yet as an experiment I krazy glued a coin to the free surface, it significantly raised output and gave me a lot warmer sound.

I used "Gel" type krazy glue, not the watery usual type.

[peskywinnets]

I found I got the best results by twanging the disc like a diving board. I fixed one edge to the guitar body and attached a small weight (a "seismic mass") to the other edge 180 degrees opposite. This gave by far the biggest output of any configuration I tried. The different bending mode and added weight lowers the resonant frequency, giving a fuller sound with more bass.

That ties in with my own tests...the best sound I got was actually with one edge of the brass disc pressed against the body & the opposite end angled up off the body.

When you say they expand & contract in the radial direction (forgive my lack of grasp here!)....you mean the piezo material expand/contracts along its diameter line?

JM...they may well react to straight pressure...but I couldn't get any meaningful level out of the sensor with all the direct pressure permutations I tried!

[eschertron]

what I've tried...

Double sided tape - weak signal
Drill a hole in the middle of the sensor & then screwing down so that the piezo presses firmly against the guitar body - weak signal
Attaching the sensor to some FR4 & mounting the fr4 to the body - weak signal.
Physically holding the sensor whilst randomly pressing its brass disc against the body - occasionally I hit a sweet spot (not on the body, but more angle/pressure of the sensor) & the sound level/signal leaps up & is "fuller" than a fat bloke who's had 10 courses of high calorie fat stuff, but hasn't farted yet.

This last test/result proves my fet preamp is fit for purpose & that a nice signal can be coaxed out of the pickup cavity.

I'd rather not deal in randomness...indeed, it'd be impossible to replicate the mount position/pressure when I hit that whimsical sweet spot of pressure/angle ...so rather than just hit & hope, I'd rather try & fathom out the best approach.

Ok - Piezo sensors....so the white piezo material reacts to pressure/vibrations.....what is the brass disc's purpose here? Would it be safe to say that we ideally want the brass disc to stay as static as possible with only the piezo material getting the vibrations?

Top tips warmly received.

From the other posts (and the experiments I'vre tried) I'd say that any weakly-coupled arrangement, like the double sided tape, will be a poor conductor of pressure waves. And my intuition suggests that, from the edge-only and edge-like-a-diving-board experiments, that the piezo reacts more strongly to vibrations laterally across its surface which is why the center-mounted experiment was also weak. When the piezo is part of the pressure wave transmission from the strings to the acoustic space (glued under the bridge, or as part of the sounding board) it seems to work the best. It's not a point sensor, but rather a surface sensor. Get as much of the piezo surface vibrating as possible.

[Joe Gwinn]

Search Fishman's early patents.

More generally, there are lots of piezo guitar pickup patents, so one can get a good survey of the various approaches.

[rjb]

Search Fishman's early patents.

And Rick Turner interviews.

[Joe Gwinn]

And Rick Turner interviews.

Yes. Actually, he used to post here, so it may be possible to ask him. He may also have patents.

[Leo_Gnardo]

Yes. Actually, he used to post here, so it may be possible to ask him. He may also have patents.

And I hope Mr. Turner will return & enlighten us sometime.

In any case, a useful scrap of knowledge Rick passed along to me @ 1994 is that the typical piezo "matchstick" pickups, as well as his own "Highlander" pickups, required a fair bit of pressure to start with. The guitar signal could be interpreted as a delta function, or CHANGE in pressure. Which leads to locating them under the saddle where there's plenty of static pressure as well as changes in pressure due to string movement. A carefully cut flat-bottomed saddle groove is a good place to start, as well as having a saddle that isn't so wide the strings won't push it down into the pickup.

Early Barcus-Berry and FRAP pickups were simply stuck to the back of a guitar's soundboard with various glues or removable adhesives - and needed massive preamp gain & skilled (lucky) placement to sound like anything. (Some avant-garde musos stuck 'em anywhere & everywhere - I read of Frank Zappa having piezo's in his headstock but who knows if that ever made it to record - might have worked if squeezed under the nut.)

[big_teee]

If they were in the headstock, how would you cable that to the output jack?

[Leo_Gnardo]

If they were in the headstock, how would you cable that to the output jack?
I bet that never took off as a trend!

Wired through grooves behind the fretboard I'm guessin'. Then again some of that may have made it to record. There are lots of doinky guitar tones on FZ's "Shut up and play your guitar" and other recordings, that may or may not be piezos in weird places.

There have also been guitars with "light up" fret markers & others that use strings & frets as "crossbar switches" for electronic triggering. So far none of that has proved to be a trend, mostly they are one-off's or experiments. Thank goodness! I gots enough little wires to figure out what to do with. Including a prototype Pedulla bass that looks like an explosion in a pysychedlic spaghetti factory inside. Someday I'll get to that...

[peskywinnets]

There seems to be two strands of piezo sensor info on the net...

1. those who do it properly (& put piezo material in the bridge/bridgle saddle etc)

2. Those who want to do it either cheaply or can't locate in the bridge saddle.

For the second category, there's a lot of poor info about!

Just thinking this through a little...if the piezo disc sensor works as an output device by making piezo material expand/contract & then using this movement to make brass disc flex back & forth (which amplifies the original movement) .......then the reverse is probably going to have the best results if wishing to use the same thing as an input device.

Therefore I guess I need to think about fashioning a solution where the brass disc can flex in the same way as it's meant to...so perhaps an O ring around the brass disc perimeter - this would see the brass sensor flex in/out (albeit very little) in sympathy with the incoming sound pressure?

[Leo_Gnardo]

Not trying to Jack this, but on the LEDs I think John Paul Jones plays a bass with LED fret markers.
That would be pretty cool.
T

As if - a musician of JPJ's stature - and ability - would need such a gimmick. But late 70's Alembic & others offered this sort of thing. I wouldn't put it out of the realm of possibility he might have one, or more.

Late 90's late night TV infomercials had an affordable LED light-up neck guitar & what a salesman - no less than Peter Frampton. All sorts of ways to make a buck I s'pose...

Everybody sing along now, "You - - - light up my neck - - -"

[Leo_Gnardo]

There seems to be two strands of piezo sensor info on the net...

1. those who do it properly (& put piezo material in the bridge/bridgle saddle etc)

2. Those who want to do it either cheaply or can't locate in the bridge saddle.

For the second category, there's a lot of poor info about!

Not to knock your inquiring mind pesky, but lots of engineers have been through this one before and your 1. seems to be the solution that can be counted on. Without some background pressure on it, the piezo crystal seems to make a lousy transducer. Who knows maybe you'll be the one with the breakthrough.

[rjb]

Another goose to chase

There seems to be two strands of piezo sensor info on the net...

FWIW, the D-TAR (Duncan-Turner Acoustic Research) Perfect Timbre(TM) soundboard pickup is referred to as a "piezo accelerometer".
D-TAR Duncan-Turner Acoustic Research [www.d-tar.com] /SoundSpots

Also, some shmoe on some acoustic guitar board says the LR Baggs iBeam pickup (which attaches to an acoustic guitar's bridgeplate with double-sided tape) is also a piezo accelerometer design.

Maybe you can rig up something with a weight and spring, or two piezos mounted at either end of a mercury-filled tube...
Piezoelectric accelerometer - Wikipedia, the free encyclopedia