impedance

1. Total opposition to flow. In electricity, when flow is steady, i. is simply the resistance, e.g., the driving pressure per unit flow; when flow is changing, i. also includes the factors that oppose changes in flow. Thus, deviations of i., from simple ohmic resistance because of the effects of capacitance and inductance, become more important in alternating current as the frequency of oscillations increases. In fluid analogies ( e.g., pulsatile flow of blood, to-and-fro flow of respiratory gas), i. depends not only on viscous resistance but also on compressibility, compliance, inertance, and the frequency of imposed oscillations. 2. Resistance of an acoustic system to being set in motion.

- acoustic i. the resistance that a material offers to the passage of a sound wave (colloquial); a property of a medium computed as the product of density and sound propagation speed (characteristic acoustic i.). Discontinuities in acoustic i. are responsible for the echoes on which ultrasound imaging is based. Unit : the rayl.

* * *

im·ped·ance im-'pēd-ən(t)s n

1) the apparent opposition in an electrical circuit to the flow of an alternating current that is analogous to the actual electrical resistance to a direct current and that is the ratio of effective electromotive force to the effective current

2) the ratio of the pressure to the volume displacement at a given surface in a sound-transmitting medium

3) opposition to blood flow in the circulatory system

* * *

im·ped·ance (im-pēdґəns) 1. opposition to the flow of an alternating current, which is the vector sum of ohmic resistance plus any additional resistance due to induction, to capacity, or to both. Symbol Z. The resistance due to the inductive and condenser characteristics of a circuit is called reactance. 2. In mechanics, the resistance to an applied force.