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Strange foam behavior

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  • Strange foam behavior

    I have just received some foam samples so that i can do some tests and see what I like etc, and I made a little makeshift drum - table, piezo and the foam block. I attached the output to a oscilloscope and had a look at the different foam types. This is where the bizarre bit comes in. I set the oscilloscope up so that it was triggering from a light hit from my finger (with no foam on top). I then put the foam on top. It wouldn't trigger, so I watched the live wave and discovered that the phase of the piezo had inverted so I was getting a negative spike. I swapped the wires and all was well.

    So my question is:

    Why does adding a block of foam on top of the piezo invert its phase?

    Any ideas would be most welcome! It has completely confused me!

    Cheers

  • #2
    Ya know, I can't answer your question but I definitely like your style and your quest for knowledge. Do not give up. I'm being a bit selfish here but I think the way you think and explore will ultimately benefit forum members here.
    TD9 Frankenkit

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    • #3
      Originally posted by tomrbland View Post
      ...I set the oscilloscope up so that it was triggering from a light hit from my finger (with no foam on top). I then put the foam on top. It wouldn't trigger, so I watched the live wave and discovered that the phase of the piezo had inverted so I was getting a negative spike. I swapped the wires and all was well.

      So my question is:

      Why does adding a block of foam on top of the piezo invert its phase?
      The polarity is a function of which way the piezo flexes -- convex versus concave. So, I am assuming that the foam distributed the force away from the center and towards the circumference of the piezo. That would change the direction of flex and also the polarity.

      Here is a picture that should help to explain what I mean:


      The above picture is actually using a piezo in a different application -- a buzzer. So, as a buzzer, the piezo is made to flex concave vs convex depending on the polarity of the forcing function. But, in a drum trigger application, the piezo is used to generate a voltage when it is mechanically flexed. In this case, the polarity of the voltage generated is a function of the direction that the piezo flexes.

      -SD-
      Attached Files
      Last edited by SiliconDrummer; 11-05-08, 06:29 PM. Reason: added picture

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      • #4
        Position Detection

        Hey I just read your piezo research doc -- good work. I thought you might also benefit from the following post that contains an excerpt from a Roland patent describing how they measure PS:
        http://www.vdrums.com/forum/showpost...9&postcount=10

        -SD-

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        • #5
          Originally posted by chiphead View Post
          Ya know, I can't answer your question but I definitely like your style and your quest for knowledge. Do not give up. I'm being a bit selfish here but I think the way you think and explore will ultimately benefit forum members here.
          Thank you chip head, I just feel if I am intrested in something then somebody else out there will probably be! and my results could help theirs etc

          Originally posted by SiliconDrummer View Post
          The polarity is a function of which way the piezo flexes -- convex versus concave. So, I am assuming that the foam distributed the force away from the center.
          Interesting idea, possibly the most plausable I have heard yet. I asked my physics teacher today he suggested that the foam (perhaps) compresses slowley but rebounds very fast which would produce the spike in the negative.

          I didnt know that roland were triggering off the end of the 1st peak. This gives a small lag. the way I was going to do it was trigger just after the peak. So that I can tell voltage etc. But that is fascinating. Its given me lots to think about.

          When I say what I was going to do I mean that I am planning a module which converts pads to midi. Mainly for sample/lighting triggers.

          Cheers

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