I have read several discussions and articles about resonant frequency, many of them going pretty deep into the science/engineering of piezoelectrics. Much of it was over my head as a beginner, and I've had a hard time finding a rough rule of thumb answer to a more fundamental question about resonant frequency.

So I have a very basic, bottom line type of question:

Q: In a very general sense, what is the optimal resonant frequency for a single disc piezo pickup to be mounted inside the box of a very simple cigar box guitar?

When shopping for Piezo discs, I have seen them of various size discs and ranging from a resonant frequency of 1.9±.5kHz to 9.2±1kHz, with most being between 4.0±.5kHz and 4.6±.5kHz. I don't know which resonant frequency rating is preferable for my purposes.

Please dumb the answers down for me. I know there are a million directions and a million level to which this type of conversation can go and that a thorough and precise answer probably has to begin with, "Well, it depends. . ." I know that's all important stuff, but I'm not there yet. I'll get there. Right now I'm really looking for beginner-level rough and general guidance for the fifth grader, so assume the following:

1. I'm using the simplest single disc to jack pickup with no preamp, pots, external power source or anything else.  Single piezo disc --> wires --> jack. (Again, I'll get there, but not this round)

2. I will mount it inside the cigar box, probably on the inside of the box top, just under the bridge, favoring the lower/bass string side.

3. I'm not looking for the highest quality sound or how to finesse it. I'm just wondering if there is any sort of general guidance for a quick build that sounds kind of cool when I plug it in in my living room to show a friend or two.

Final note: I have built one CBG with a simple single disc inside-the-box pickup and installed pickups in an acoustic guitar and in a ukulele. I also put a rod piezo in another acoustic guitar. I am very pleased with how they sound. However, I don't recall what the specs were on those first piezo discs I bought, and I may have just made a lucky guess when I bought them. If I'm going to buy a bunch more and start building more of these as a hobby (which I most certainly am, because I love it so far), I want to make a more educated guess when I buy the next batch.

In the following round of builds, I'll start thinking about preamps, pots, wiring a rod and disc in parallel, multiple disc locations, perhaps even active pickups and magnets, and all the other stuff I've been reading about.  

On a related but totally unrelated note, I also built an ES-335 style semi-hollowbody guitar from a kit, but that involved humbucker pickups. This is by far the coolest hobby I've ever stumbled upon, and since I build them with my 16 year old daughter (the family artist), it's doubly cool.

Thanks.

Bryan

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Good question. I don't really know if it's apples to apples, but I remember reading somewhere that magnetic pickups have their resonant frequency somewhere in the 3-5khz range, so the 4-4.6khz that you're finding would fall right in there.  I'd be tempted to experiment with going lower than that to see if it helps tame the piezo harshness any.  

Yeah, the armchair pseudoscientist in me wants to buy one from each extreme end of the range, just to see what different results I get with all other factors remaining constant (and with my compulsive curiosity, I’m sure I’ll do that at some point), but before I buy a batch and build my next few instruments, I prefer to know I’m buying a batch that are a safe bet. Fortunately, worst case scenario, they’re pretty cheap even if I buy a batch of losers, but I’d rather skip that step in the learning process if I can tap into the knowledge of the more experienced.

Nothing scientific to offer here, just a couple of observances, during the induction pick up thread, it was found that transformers in the 20- 200 khz range worked best in that system, and in tests done by others a while back, that 7 discs, in parallel, seemed to smooth out piezo's to best effect, i've not tried the 7 discs, as i reckon a pre amp saves a lot of messy wiring, my understanding was there was improvement each added disc up to 7, then it started losing advantage

http://www.americanpiezo.com/knowledge-center/piezo-theory/determin... This may help answer your question.

I've used the round piezo's in a few guitars, usually 2 wired together in parallel with 500k pots and sometimes 1 meg pots to help cut down the impedance. There's always a lot of handling noise and mounting them with noise reduction methods(bed of hot glue, double sided tape or bottle caps) also tend to work against the sound. 

I recently used a rod piezo with a 500k volume for a passive circuit and it worked great. I have good volume, good string response and the handling noise is very low. I'll probably never use a round piezo again, these rod piezo's are fantastic and even better with the preamp circuits.

I think you're over thinking it...  I buy 27mm Piezos... they work well... 20's aren't hot enough...

Probably am, but surely resonant frequency matters. I’m just wondering if there’s a quick answer to what level of resonant frequency is better.

If you hung the piezo disk up by a string and struck it like a gong, and measured the note it produced, that's the resonant frequency,  the note it would naturally vibrate at on its own.

its more important in speaker design than when using it as a mic or pickup, sizing the cabinet or horn to take advantage of it.  As a pickup, it makes a fractional difference in how much juice it sends to the amp based on the git playing a note very near or at the resonant frequency.

which is trivial since in a git that frequency range is in nth harmonic. 

for a pickup, bigger ceramic makes bigger signals for the amp.

Thanks.
Most piezo disks are high impedance and are affected by the typical capacitance in the connecting guitar cable of about 35 pf per foot. This cable capacitance acts like a treble roll off capacitor places across the piezo device. The key to getting the fill sound out of a piezo device is to buffer it by placing a very high impedance buffering device near the piezo device. Search on “Tillman buffer” to find his Jfet buffer that will isolate the piezo device from coax cable capacitance loading.

I hope this helps.

Joseph J. Rogowski
Without getting into all the details about impedance, high pass filtering, and the impact of adding a preamp, I think I’ve figured out the simplified general answer from continued reading. Tell me if I’m close: A piezo disc with a 1.0 kHz resonant frequency peak is going to have an easier time transmitting (i.e., will transmit more of) the frequencies closer to 1 kHz (for guitar, that’s midrange), with the strength of the frequencies dropping off as they move farther away from 1 kHz. It will transmit other frequencies, just not quite as well as frequencies near it’s resonant frequency peak. A piezo disc with a resonant frequency peak of 4.0 kHz is going to be best at transmitting frequencies near 4 kHz (upper midrange or presence), again, with it dropping off as the frequencies move farther away from 4 kHz. Similar story with the 9.0 kHz piezo disc, which will have the easiest time transmitting frequencies in the brilliance/harmonics/air range, while still transmitting others. Note that none of the discs I’ve seen have their strongest point (resonant frequency peak) in the lows or bass range, so while they will transmit low frequencies, they won’t do so quite as well as they transmit the frequencies near their respective resonant peaks. Maybe this is why piezo disc pickups often act as a high-pass filter and/or have a “tinny” or higher, thinner sound than the acoustic instrument itself does. Am I at least getting warm there?

I know what I’m going to do. I’m going to buy a disc from each extreme end and one from the middle, then test them each separately on the same instrument to see what difference, if any, I can hear. I’m too curious not to do that kind of experiment. Assuming my theory is borne out by the experiment, I may wire three disks (one with lower resonant peak, one middle, one high) in parallel to the same jack, mounting each disc inside the instrument where those holographic interferometry images say the instrument vibrates most at each respective disc’s “favorite” frequencies. I wonder if that will provide a fuller sound profile that more closely approximates the acoustic sound being produced by the instrument? If that seems to work and I’m still feeling curious, I might even add a rod piezo under the saddle, wire it in parallel with the three discs, and see how that affects the combined result.

I realize a simpler solution might be to just add a buffer or preamp to one piezo transducer and improve the sound that way, but it will be fun to see what kind of result this gets.
If you load the piezo with from 3 meg ohms to 10 meg ohms you will boost the lower end of the response. Piezo devices are capacitive driven and have a higher impedance to lower frequencies just the opposite of wire wound inductive guitar pickups. Just connect your piezo device to a small amp with a 1 foot cable and listen to the difference compared to using a typical 10 foot cable. Eliminating cable capacitance and raising the impedance very near the piezo device is the simplist solution.

Joseph J. Rogowski
I am just now learning the jargon, so I’m not sure what you mean by the phrase, “If you load the piezo with from 3 meg ohms to 10 meg ohms. . .” I have read about piezos being high impedance, and that therefore you may need a preamp for them to work with a guitar amp, but I’m not sure if that is what the above line means exactly. Can you elaborate on that for me?

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