Piezo question

Discussion in 'The Projects Forum' started by TSOMB, Apr 26, 2015.

  1. TSOMB

    Thread Starter New Member

    Jul 31, 2014
    So I am looking to make an audio device to control raccoons and rats passing through my property. I would like to experiment with high freq sound, and have picked up some piezos.

    I have a few questions if anybody has knowledge of this. Firstly, is the resonant frequency which a piezo operates determined by the size of the brass disk diameter, thickness etc? Could altering the diameter or the like 'tune' for efficiency at a specific freq? How much heat can be applied while soldering before damage occurs?

    Most importantly, how do you figure safe operating limits at a given freq? I picked up a few of these > https://www.fasttech.com/products/1453503 but there is no mention of specs which seems to be common for these devices.
  2. pwdixon


    Oct 11, 2012
    I would think that a piezo has a fundamental resonant frequency and all that changing the physical mounting will do is damp the output to varying amounts, normally these types of device have the frequency specified and it's often in the low kHz range as these things are destined to be sounders of some kind for humans to hear.

    I would think if you try to attack these with a tool (eg. a file) then there's a go chance you'll just kill them. I've tried soldering to these type of device before and they can be pretty intolerant of solder abuse. Perhaps these particular devices were meant to have some sort of clamping type connection?

    Even when I have seen specs on devices like these it's been hard to determine what their drive capability/ limits are, sounds like you might need to experiment, do you have a captured racoon handy to torture, whoops experiment on?

    As a final note I was getting mice in my loft so I bought a mains powered sonic device that is silent to humans but obviously scares mice away very well, I think it cost me £12ish.
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
    piezos have their own resonant frequency depending on shape,size, housing, you need to choose the best sweep frequency for the pests you want to get rid of,then use a piezoelectric speaker with that resonance, use a 555 oscillator or other type, check out the hearing range of the pests...

    Last edited: Apr 27, 2015
  4. TSOMB

    Thread Starter New Member

    Jul 31, 2014
    Hmm, didnt get notification for replies to this thread. Thanks for the responses. I was thinking of mounting the piezo with a low temp alloy to a brass rod which I could then alter in my lathe.

    I was curious if a smaller disk diameter would be favorable to higher frequencies? Also for propagating the
    sound over various ranges, I have had some good results with AL tubing but again cannot find any formulas to design a
    setup as one would with say a subwoofer and its cabinet.

    Is there a way to translate a given freq to the diameter and length of tube that would be the most efficient? Kind of like a waveguide
    to microwaves?

    Thanks again gents : )
  5. Søren

    Senior Member

    Sep 2, 2006
    Read this: http://www.cresttech.com.au/pdf/PiezoOperation.pdf

    Don't ever add heat to those discs, they don't take it well.

    Molex made some Helmholtz chambers for loose piezo discs back around 1980. I have no idea if they still do, as the one I bought back then lasted me a long time. It was for a specific disc of course - I got it as a set, but it's easy to find the node of a disc - let it rip and sprinkle a bit of graphite powder or similar on it (I've used fine ground black pepper at times) and the powder will assemble at the node - with graphite, you can just put a piece of paper over it and rub a bit, then you have it drawn on lightly.

    The smaller the disc and the lower the mass, the higher the resonance (obviously).
    Read the pdf, it's only 4 pages, but each is brimful with good info - it's even got the formula to calculate a resonance chamber.

    That would be a tube where the animals fit in - add sound in one end ;)
    The Helmholtz chamber is how you get the highest amplitude AFAIK

    What you really want is piezo tweeters. Check that they work at the frequency you have in mind - some stop above 4kHz, some can go all the way to 80kHz and they can handle quite high SPLs.
  6. TSOMB

    Thread Starter New Member

    Jul 31, 2014

    Thanks for the link to that pdf. So the heat issue, I use a really low temp <50c and it flows like solder. So it seems that the larger brass disc that the element is bonded to is larger to support lower freq that is audible to the human ear. I think the idea of turning this down a few mm may be worth it.

    Mounting I plan on using various length tubes to cover an long target area. What is the purpose of the small hole in the calculation? Should the inside diameter of the tubes be this size? or should each tube have an I.D the size of the node with a cap and exit hole to the calculated size? Im a bit confused on that.