Question about "Thickness Factor" in Single Element Focused Ultrasound Simulation

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Hi, Do you know how to set the parameter "Thickness Factor" in single-element focused ultrasound simulations? I couldn't find any information about it in the manual or tutorials.

The default value in Sim4Life is 10, but I'm not sure what it actually represents. I understand that it's related to the thickness of the ultrasound transducer, but I don’t know its precise meaning. Is there a formula for calculating it?

Thanks!