Impedance

Impedance is a concept associated with the transmission of waves and electrical signals. There are many kinds of waves, and impedance is different in each of them, hence this disambiguation page. However, impedance is also a unifying concept. It is related to the load that is imposed on the source that generates a wave. It governs the reflection and transmission of waves incident on a change of medium. If the impedances in the two media match, there will be no reflection. One simple approach identifies two parameters for a wave: the restoring force that tries to return to equilibrium and the inertia of the medium displaced. Then the impedance is

Z = the square root of (restoring force) times (inertia)

while the velocity of the wave is

v = the square root of (restoring force) divided by (inertia)

For example, for a sound wave the restoring force is the modulus of elasticity, while the inertia is just the density. For some waves (light waves, for example) it is not obvious how this simple picture for mechanical waves can be applied, but the same general picture holds.

Impedance may refer to:

*the ratio of the voltage phasor to the electric current phasor, as in
**Electrical impedance, a measure of opposition to time-varying electric current in an electric circuit.
**Characteristic impedance, a measure of opposition to electric current propagation in a transmission line.
*Impedance matching and Impedance mismatch.
*Vacuum impedance, a universal constant.
*Electromagnetic impedance, a constant related to electromagnetic wave propagation in a medium.
*Mechanical impedance, a measure of opposition to motion of a structure subjected to a force.
*Acoustic impedance, a constant related to the propagation of sound waves in an acoustic medium.
*Linear response function, a general way to represent the input-output characteristics of a system.
*Object-relational impedance mismatch, difficulties encountered when a relational database management system is in an object-oriented programming language.