Replies to This Discussion

Permalink Reply by Alan Hodges on October 13, 2011 at 5:29am

Hi Graham, I am no expert, but you have raised a great point. Would a smaller piezo give a more focused pick up, so therefore would you put it under the bridge and does the larger one give a wider surface area to pick up more of the resonance from the surface of the box?The age old question, does size matter???? I have only used two of the same size, fitted either side of the bridge so no further help from me i'm afraid, but it would be interesting if anyone else can help

Alan.

Permalink Reply by Yellowbelly Flatt on October 13, 2011 at 7:38am

I've used two different sized pickups in a cigar box and also in a crude stomp box. I used the larger one near the bridge in an attempt to generate a bass sound and the smaller one nearer the neck for the opposite reason. I used separate volume and tone pots for each pick up in order to be able to mix the sounds from the piezos.

It worked fairly well but not really much better than a single (large) piezo with tone and volume pots..A similar results for the stomp box.

I'd be interested to find other peoples results with mixed piezos.  <======{0 OI }

.

Permalink Reply by John Baines on October 13, 2011 at 9:56am

I think piezos work via vibration. The bigger they are the better. I don't think smaller ones are more trebley or anything like that. Ideally you would want a piezo the same size as the soundboard to capture the full range of the sounds it can produce. I think.

Permalink Reply by David L. on October 13, 2011 at 10:42am

I use 2 - both under where the bridge is situated (one on each side).  I wire them in parallel (both black to earth, both red to live) as this halves the impedance and theroretically makes for a better sound.  I glue in place with "serious" glue and once fixed cover with more serious glue as it reduces feedback issues.

The more discs you add the more box noise you pick up if it rubs against you and comparing the first couple of CBGs I used 3 discs in of different sizes with the more recent ones with 2 of the same size there isn't any improvement in sound to my ears.

I also like the piezo rods which fit under the bridge saddle and pick up no box or guitar rubbing noise and produce so little feedback you can run them through an effects pedal easily.

Piezo discs will pick up sound from anywhere  and as they are cheap it is best to stick the discs around the box with a bit of tape until you find the spot or spots that give a sweet sound to your ears and then glue them in place. :O)

Permalink Reply by smojo on October 13, 2011 at 10:38pm

I only once fitted two thinking it would give me more variety of sound. One under the bridge one nearer the neck. The bridge was bright and clear and I only ever fit one here now. The neck one was duller and muddy. Don't think there's much to be gained by different sizes and yes they do work on vibration alone so in my humble opinion the optimum place is right under the bridge where the string vibrations connect with the soundboard. Also I am of the opinion now of keep it simple. Add a tone and vol pot to give sound colouration if you want it, you'll probably spend more time flicking from one pup to the next if you fit two than it's worth.

Permalink Reply by Graham Alibone on October 16, 2011 at 8:23am

Thanks, Alan, Yellowbelly Flat, John, David and Smojo, for your input. I've fitted a 30 mm under the bridge and it sounds o.k.

Take care, G.

Permalink Reply by ChickenboneJohn on October 17, 2011 at 6:46am

If you look at the spec of piezo discs, they have a rated frequency response..I'm guided by this rather than the size and choose lower frequency response, as un-preamped piezos always seem very trebley.

Permalink Reply by Mark on October 18, 2011 at 5:46am

As John says, the important thing about piezos (or any pickups for that matter) is their frequency response characteristics. The trouble is the specs that are usually quoted don't give the full picture. There are two frequencies of interest in figuring out whether a particular piezo will do a good job - the useful frequency range for the pickup will be the range between the two frequencies. Unfortunately the specs you generally see when you go to buy piezos tend to quote only one of those frequencies. So, as with many other aspects of CBG design, it comes down to a bit of guesswork and some trial-and-error learning.

My trials-and-errors lead me to think the following:

1) There isn't much risk that you'll find a piezo is lacking at the higher end of the frequency range. (As John says, un-preamped ones can often sound "trebley"). So the challenge is to find ones that give a good and even response at the lower end of a guitar's frequency range.

2) I make a wild guess that if the quoted resonant frequency is lower then other characteristic frequencies might also be lower. I have no scientific basis for this, it's just the guess I'd make if I had to make a complete guess.

3) The manufacturing tolerances on cheap piezo discs are quite loose - so you probably shouldn't worry too much about being ultra precise over these things.

Maplin offer discs with resonances of 1.8kHz, 2.8kHz and 4.2kHz. I tend to opt for 1.8 or 2.8. All the ones I've used so far have sounded fine. I also experimented with pairs of disks wired together in parallel, including a 1.8kHz with a 4.2kHz. Those sounded fine too.

Finally, I figure other things will possibly have a much greater effect on the sound characteristics of a piezo pickup. In particular, where you place the thing and what glue (or other medium) you use to mount it.

 

For reference, there was some discussion about a year ago on the topic of piezo characteristics (see following thread)

http://www.cigarboxnation.com/group/thebritsarecoming/forum/topics/...

 

 

Permalink Reply by smojo on October 18, 2011 at 7:04am

A note on the resonant frequency thing. These piezos are primarily intended as buzzers so I think the res. frequency is more likely to refer to the frequency at which it will sound (resonate) rather than it's ability to receive sound as a pick-up device.

Permalink Reply by Mark on October 19, 2011 at 2:54am

It's true that piezo disks are most often sold as buzzers/speakers, but I don't think it's as simple as the resonance frequency being the pitch they produce.

There are essentially two types of piezo elements: Those with two wires and those with three. For use in buzzer/speaker situations the two-wire ones work much like a normal speaker - they need to be driven by some sort of signal source with appropriate amplification. In this configuration they can output a range of frequencies. If you're using it as a speaker you might well want to avoid the resonance falling within your operating frequency range. On the other hand, if you just want to output a single tone then you might want to choose your piezo so the resonance matches that tone and gives a louder result.

The three-wire piezo disks can be used as part of a simple self-oscillating circuit, which produces a monotone buzz (with no need for a separater tone generator). In this latter configuration I guess the resonance frequency of the piezo will have a strong influence on the resonance frequency of the buzzer circuit.

The resonance is just an inherent characteristic of any piezo element. It seems to be relevant both to situtaions where a piezo is used as a sounder or situations where one is used as a detector/pickup.

 

 

Permalink Reply by jabes on October 21, 2011 at 4:30am

i've cut them in half and they still work..http://www.youtube.com/watch?v=Xuw9frP1GNo  this is interesting too..;)

 

john