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I am building a ultrasonic transducer like this (with double-exponential (75%-25%) match and 2 ring PZT8 50x20x6.5 (for the moment I do not take account of pre-stress on bolt) :

enter image description here

When I simulate the 2 piezo ring I got 33kHz resonant frequency. So in my mind, to get a maximum of z-displacement on the matching of the transducer, I thought it was a good idea to tweak the length of the matching to get 33khz resonant frequency for the transducer (solid - without piezo effect being simulated). After doing this, I make a simulation with solid + piezo effect and I get no 33kHz resonant frequency :

enter image description here

I done some other simulation for other length and get same thing :

enter image description here

So it seems that if solid & piezo ring are both at 33kHz, I got effectively a maximum z-displacement but not at 33 kHz...

Here is how I polarized thr rings :

enter image description here

So my question is : Why I don't get a 33kHz resonant frequency with piezo effect when solid + piezzo ring are both at 33 kHz resonant frequency?

Here are some other simulated result for displacement & stress :

enter image description here

Is there something wrong with it or can someone explain me what happen?

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    $\begingroup$ Which software are you using for this simulation? $\endgroup$ Nov 23 '20 at 13:24
  • $\begingroup$ Hello I used comsol $\endgroup$
    – rdpdo
    Nov 23 '20 at 15:49
  • $\begingroup$ How many smaller resonant frequencies are there? Because finite element methods is only an approximation and usually only accurate for the handful of resonant frequencies. Another option might be their interaction due to better impedance matching. $\endgroup$
    – fibonatic
    Nov 24 '20 at 13:54

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