General form of resistance. Impedance Z is a complex physical variable (actual and notional element), which takes the following form
R = resistive element of resistance
jωL = inductive element of resistance
Impedance is clearly a measure of the resistance (a product of forces) with which structures counter the excitation of oscillations. Because the inductive and capacitive elements of impedance are frequency-dependent, this results in a frequency dependency for impedance Z (Figure I 2).
Depending on the application involved, it is important to distinguish clearly between various forms of impedances, since these are associated with different units:
electrical impedance Z
mechanical impedance Z m
specific impedance Z S
Figure I 2: Elements of impedance