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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 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 §