Piezo disc transducer spec question

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

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.

Replies

Kirk October 25, 2017 at 9:24am

A preamp is cheap, and easy to install, and can really bring down the harshness of a piezo (and John Nickel would be happy, because you don't need giant blobs of hot glue).

https://www.ebay.com/itm/3V-6V-TDA1308-Headphone-Amplifier-Board-AM...

https://www.ebay.com/itm/NE5532-OP-AMP-Audio-HIFI-Preamplifier-Dual...
- Gwil Colman > Kirk October 26, 2017 at 6:02pm
  
  A preamp is an excellent idea as long as it has a high input impedance - at least 1 Megohm and preferably higher. Can't help noticing that neither of these ebay listings quote an input impedance. Have they been tried in a CBG? They might be OK but personally I'd go for a preamp specifically designed for buffering a piezo pickup.
  - Paul Craig > Gwil Colman October 28, 2017 at 4:26pm
    
    C.B.Gitty has 2 for 20bucks or less that work well with rod piezos. Don't know how well they work with disc piezos or the input impedance, but I'm sure they'll handle it just fine.
RTZGUITARS October 24, 2017 at 4:00pm

The easiest way to control any piezo is to put a cap. acrossed the pickup leads. Simple no pots not perfect but cheap easy way to control it. try Gittys .068 caps.
- Paul Craig > RTZGUITARS October 25, 2017 at 3:56am
  
  That's basically a tone cap passing highs to ground.
  
  Haven't tried that without a volume pot. I'd rather have a volume pot so I can keep my amp settings at the sweet spot and also don't have to go back and forth to the amp that way. With a 500k volume pot, a .047 cap would probably work fine.
Bryan Pieper October 23, 2017 at 2:17pm

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.
- Paul Craig > Bryan Pieper October 23, 2017 at 6:02pm
  
  I've been wondering about using a high pass filter on one of these round piezo's myself. In a guitar situation, a .047 cap inline with the neck pickup's signal wire to the switch/volume pot can reduce the muddy tone of a neck pickup due to high bass. So I was thinking that a .022 or lower value cap would tame the high shrillness associated with the round piezo.
- Bryan Pieper > Bryan Pieper October 23, 2017 at 3:23pm
  
  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?
  - Joseph J. Rogowski > Bryan Pieper October 23, 2017 at 6:31pm
    
    High impedance transducers like piezo devices have a very high impedance and will get loaded down by using volume controls in the 500K ohm to 1 Meg ohm range. This passive volume control is also in parallel with the input impedance of a typical guitar amp which is about 1 Meg ohms. If you combine these parallel loads it Looks like a 500 K ohm load to the piezo device.
    
    Here is an experiment to try. Obtain a Scatten Design mini pre which has a 10 Meg ohm input impedance. Connect your piezo device directly to this preamp/buffer and plug your long guitar cable into the output of the mini pre and you will isolate the piezo device from the amp input loading and the effect of a 10 foot guitar cable having about 350 pico faraday (pf) of capacitance whose reactants (like resistance but at a specified AC frequency). The 350 pf cable capacitance looks like this ar various frequencies. 100 HZ 4,457,285 ohms, 1000 HZ 454,728 ohms, 5000 HZ 90,946 ohms and at 10000 HZ 45,473 ohms. So, as you can see higher frequencies are subject to more high frequency load but the Schatten mini pre isolates the piezo device from that loading because the output impedance of the mini pre is 3,500 ohms and even at the highest frequency the cable capacitance is over 10 Times higher than the output impedance and it looks like a bridging load on the preamp. Look up these words on the web “bridging impedance” and “matching impedance”. This will give you an appreciation for how cable types, lengths and source impedance values all interact to affect the sound you hear.
    
    I hope this helps?
    
    Joseph J. Rogowski
- Joseph J. Rogowski > Bryan Pieper October 23, 2017 at 2:29pm
  
  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

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