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Most of the diagrams for them I've seen show that. Is it for the purpose of amplification, or some other reason?

enter image description here

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  • \$\begingroup\$ I don't know whether this is the case, but any mechanical engineer I know would not be happy about a drawing like that. My guess is that this is not two discs, but one ring, which you see here, cut down in the middle. \$\endgroup\$ Dec 18, 2021 at 20:34
  • \$\begingroup\$ Looks like an electrostatic speaker \$\endgroup\$ Dec 18, 2021 at 21:16

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One reason is that piezoelectric elements must be polarized by a DC voltage after being fabricated. That voltage is proportional to the distance between the electrodes. Thus thinner elements can be polarized with lower voltages which is easier and safer. Also, when driving the elements, two of them stacked together and wired in parallel can be driven with a lower voltage then one element equal in thickness. This makes it easier to design the driving amplifier. Another reason is that the capacitance of the single element is much smaller than that of stacked elements in parallel. This lowers the impedance of the transducer again making it easier to drive and less effected by the capacitance of the connecting cable. In sonar transducers, the number of staked elements is much larger because of the higher power requirements.

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It's quite normal to stack piezoelectric elements to get more motion for same drive voltage. They are mechanically in series and electrically in parallel. For example, this bare element:

enter image description here

From Kemet/Yageo has something up to 40 elements. Even with 40 elements and a drive voltage of 150V you only get a nominal length change of 42um (0.042mm).

The fact they've chosen two as the optimal number for your particular application (which operates at mechanical resonance) is related to the exact requirements.

Note that the preload on the elements is provided by the compression bolt shown in your diagram. When more motion is involved, springs might be used.

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