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For example, if piezo disc has lowest resonant frequency of 4kHz. At that frequency, would the electrical impedance be at its maximum or minimum?

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It's both. Just like regular crystals, piezo exhibits resonance and anti-resonance within a small part of the spectrum. See this for example. At one resonant point it will act like a series resonant circuit and appear like a short circuit and at the other point it will be a parallel (high impedance) circuit: -

enter image description here

See also this tutorial from Maxim - it contains the basic electrical model of a pizeo and note its similarity with a regular crystal: -

enter image description here

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  • \$\begingroup\$ I see this graph alot , and I've never really understood it. Thanks , I'll have a good dig through the references you've given me. Thanks a lot. The reference you provided paper is fantastic. \$\endgroup\$ – Saif Nov 11 '15 at 13:05
  • \$\begingroup\$ Is it possible to conclude from the paper referenced above that electrical impedance is in fact at minimum, when the piezo disc is vibrating at its fundamental resonant frequency (or any frequency of its resonance series for this matter). Anti-resonance = opposite of resonant frequency, where piezo disc vibrates least, therefore at its maximum electrical impedance. Have I gotten this wrong? this is what I got from your answer. \$\endgroup\$ – Saif Nov 11 '15 at 22:29
  • \$\begingroup\$ both mechanically resonate. \$\endgroup\$ – Andy aka Nov 11 '15 at 22:42
  • \$\begingroup\$ @Saif - have you put any music in the public domain yet? \$\endgroup\$ – Andy aka Nov 12 '15 at 18:56

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