# Dictionary:Impedance

(im pēd’ ∂ns)

**1**. The apparent resistance to the flow of alternating current, analogous to resistance in a dc circuit. Impedance is (in general) complex, of magnitude *Z* with a phase angle . These can be expressed in terms of resistance *R* (in ohms), inductive reactance , and capacitive reactance :

*Z* is in ohms when frequency *f* is in hertz, *L* is inductance in henrys, and *C* is capacitance in farads.

**2**. Acoustic impedance, the product of density and velocity, the ratio of the pressure to the volume displacement at a given surface in a sound-transmitting medium. Because reflection coefficients change with angle, **elastic impedance** is sometimes used for non-normal incidence.

**3**. In magnetotelluric exploration, if *E*_{x} is the electric-field component in an arbitrary horizontal direction and *H*_{y} is the associated magnetic-field (in the orthogonal horizontal direction), the **wave impedance** (**Cagniard impedance, field impedance**, or **surface impedance**) *Z*_{S} is:

The surface impedance is a complex number because of phase differences between *E*_{x} and *H*_{y}. Measurements of surface impedance versus frequency can be interpreted in terms of the electrical properties of the subsurface. The surface impedance is a tensor if the conductivity structure is nonlayered. The reciprocal of impedance is called ** admittance**.