SBT pickup: Dummy experiment
 

Hi everyone,

I am experimenting with piezo discs.



I was wondering why the TA amulet, Dazzo or HFN sound different.

So I made a dummy experiment. I record my old Yamaha FG with a piezo disc.

* I played with position
* Sometimes I added a Lego piece between the pickup and the top. I tried different shapes and a red triange which is 3-D print to the Dazzo pickup dimensions.

Here are my 2 cents:
* The closer to the saddle the thinner the tone (listen to the high strings)
* The Lego pieces have their own resonances that color the sound.

Note that pickups (Amulet, Dazzo) would probably have the piezo inside the enclosure making a mix between the direc sound from the top and the resonances from the enclosure.

This is not a scientific experiment... so please don't fire at me. I am curious if you have the same interpretation.

I will chime in here and say that I don't know what's inside the pickups you mention, so I can't comment on how any of these experiments apply.

I will state a few things that might not be common knowledge:

1. The piezo elements used by K&K, JJB, and my Ultra Tonic, as well as others, are commonly called piezo benders. That's because when you apply a voltage, they bend. They either get more concave or more convex, depending on polarity.
   
2. Conversely, when they bend, the generate a voltage. This is the primary way they function as a pickup. The vibrations on the instrument are causing the top to become microscopically more convex or concave, alternating proportionally with the sound wave.
   
3. They do also function as a pressure transducer, producing a voltage in response to squeezing pressure. But because there is no mass (to speak of) on the opposite side of the disc, this accelerometer voltage is an order of magnitude or two smaller than the bending mode, and is therefore negligible.

Having them firmly glued results in all the bending action getting transferred. Having some other material in between alters the response if the material has a different modulus of elasticity, as you can imagine. The timbre changes with or without glue, and with or without different materials in between. Lot's of variables there, enough to make my head swim:)

Thanks James,

it totally makes sens to me now. I don't know anything about piezo and I am learning with that little DIY project.

I also just read
Peterman's pickup

http://www.peterman.com.au/music/ima...ckup-movie.gif

It explains also why the lego bridge is always louder... (I also did experiments with a plastic plate and an exciter and this was obvious)

As far the material over it, there are some really interesting experiments:

Different materials over piezo discs

It's fascinating!

Very interesting experiment. Unless I’m wrong it appears that there are no definitive conclusions.

You are right. I am not claiming anything. I try to understand things.

Here is a barcus berry pickup. The piezo element seems suspended.

https://pbs.twimg.com/media/CzGtYhzXUAEP3rQ.jpg

The HFN might be a variation of the same idea, suspend the piezo to excite a certain bending mode.

Some of the best pickup research is being done right here on AGF. Thanks Cuki and James and those willing to glue instead of tape. We're all learning from your work.

I do know from Talking to Teddy Randazzo of Dazzo (also formerly FRAP) fame, he says that Piezo crystal is not he same. Every piezo source is a bit different sounding. Teddy spent years trying various sources of piezo crystal until he found the one that sounds the best to him. I am sure this was a consideration, or should be a consideration for every piezo pickups maker. Again, piezo material differs in the resulting tone.

Yes, some pickup makers attach their piezo crystal to a different substance. Some are on brass plates, some are on wood or plastic, and some are in enclosures. They all have an effect on tone. The variables are endless. If Dieter, Teddy or any other creator/designer of pickups did their homework of time, testing and consideration of all these variables, they most likely came up with what sounds best to them.

I am sure of that. I was just wondering about the effect of geometry. Again I don't think I'll end up making anything half-decent. As James and you said, there are a lots of parameters...

However It's funny to explore some of the solutions by myself. I

I made some FEM simulations.

I compute the resonant modes of a brass plate with a disc shape. I supposed that the border were fixed (think Peterman's design)

With the size of the disc I've used in the video I get a resonance around 7.6 Khz
https://img.audiofanzine.com/image.p...hoto_id=316117

I did a similar simulation with a triangle of the size of a Dazzo pickup. (note again that I suppose the brass plate to be hold at the sides)

https://img.audiofanzine.com/image.p...hoto_id=316118

I get 14 KHz!!! The bandwidth of a Bose L1c!!

As a reminder from the wikipedia page for piezo sensor, we learn that the pickup works for the frequencies below the main resonance.

https://www.researchgate.net/publica...ge-applied.png

Let's remember that the resonance you are calculating is only valid when the edges are held fixed, and the field is free to vibrate. In practical applications as pickups, that is never the case. In the kind of practical pickups I know about using piezo benders, the resonances are far above audio.

None the less, it is impressive that you were able to come up with a resonance figure via Finite Element Analysis that is very similar to what is published by the piezo vendors.

James is right. 95% SBT don’t hang the piezo by the sides... I investigated that because I have seen Peterman videos advising to place the putty in specific points... so I run the simulation and it made sens since it would favour specific resonances that I did not display here but found readily.

I also thought about side hanging because of the ehrlund pickup (European patent for the triangular shape pickup)

http://www.doublebassguide.com/wp/wp...08/ehrlund.jpg

Not unlike green m&ms I found the green
lego most exciting.;).
Seriously though if the freedom to move
is important would not crazy glue restrict a piezos
movement more than tape more than putty?

James should answer that question.

For me as soon as you glue/tape the whole sensor: you mechanically load it and change its mechanical properties and resonance. It becomes a whole different system. I honestly have no idea how the piezo behave once it is fully glued to the top.

I guess the pickup has negligible effect on the top vibration, and probably the top fully drive the pickup so it’s own resonance disappears from the equation... I have no idea honestly. James would know better.

I like the idea of a suspended oscillator. (Barcus Berry? Peterman...?)

Somehow the Baggs Lyric has the same idea, the condenser mic is a lightweight suspended membrane that senses both air and top vibrations.

Piezo disc vs PVDF film experiment
 

Hi everyone,

I tried a dummy experiment. Again, here the sensors are not glued so the properties of the sensor count. Once mechanically loaded by glueing... I can't say.

First I try the piezo disc then the PVDF film. The PVDF film needs much more gain and is very EMI sensitive therefore the 50 Hz hum (I am in Europe).

Challenge for a company like PUTW is to shield those things without damping them too much I guess.

The very low mass of the PVDF film gives an extended high end. That's my interpretation but again... I could be wrong.



Note that depending how I hold the PVDF film against the guitar, I could definitely experience distortion and weird sound as previously reported by Doug Young with PUTW #54.

Thanks, Cuki! Is what we're hearing a direct-to-computer recording or a mic picking up the amplified tone from an amp? I prefer the tone of the PVDF film somewhat over the piezo disk.