Impedance

Impedance is defined for mechanical systems as force divided by velocity, while the inverse (velocity/force) is called an admittance. For dynamic systems, the impedance of a ``driving point'' is defined for each frequency , so that the ``force'' in the definition of impedance is best thought of as the peak amplitude of a sinusoidal applied force, and similarly for the velocity. Thus, if denotes the Fourier transform of the applied force at a driving point, and is the Fourier transform of the resulting velocity of the driving point, then the driving point impedance is given by

In the lossless case (no dashpots, only masses and springs), all driving-point impedances are purely imaginary, and a purely imaginary impedance is called a reactance. A purely imaginary admittance is called a susceptance. The term immittance refers to either an impedance or an admittance [34]. In mechanics, force is typically in units of Newtons (kilograms times meters per second squared) and velocity is in meters per second.

In acoustics [325,326], force takes the form of pressure (e.g., in physical units of Newtons per meter squared), and velocity may be either particle velocity in open air (meters per second) or volume velocity in acoustic tubes (meters cubed per second) (see §