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Impedance Acoustic Impedance - The total reaction of a medium to the transmission of sound through it, expressed as the ratio of sound pressure to particle velocity at a given point in the medium. The acoustic impedance Z is useful in describing the acoustic radiation from sources such as surfaces and ducts.
Mechanical Impedance - Mechanical impedance is the ratio of a force-like quantity to a velocity like quantity when the arguments of the real (or imaginary) parts of the quantities increase linearly with time. Examples of force-like quantities are: force, sound pressure, voltage, temperature. Examples of velocity-like quantities are: velocity, volume velocity, current, heat flow. Impedance is the reciprocal of mobility. If force-like and velocity-like quantities are measured at the same point impedance is referred as driving point impedance, otherwise transfer impedance.
- Resistance - the real part of an impedance.
- Reactance - the imaginary part of an impedance.
Electrical Impedance - Impedance of a linear circuit element with two terminals is the ratio of the complex sinusoidally varying voltage applied across the terminals to the complex current that flows in response.
See also: Acoustic Impedance, Characteristic Impedance, Electrical Impedance, Force, Mechanical Impedance, Mismatch Loss.
  
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