# Impedance

- Resistance - the real part of an impedance.
- Reactance - the imaginary part of an 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.

### 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.

The ohmic value of an antenna feedpoint, matching section or a transmission line. An impedance may contain a reactance as well as resistance component.

### 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.

Mechanical Impedance is a measure of how much a structure resists motion when subjected to a given force, and it is the reciprocal of mobility. The mechanical impedance of a structure varies in a complicated way as frequency is varied. At resonance frequencies, the impedance will be low, meaning very little force can be applied at those frequencies. Mechanical impedance measurements of machine foundations are sometimes made to ensure their suitability for the machine in question. For instance, it would not be good to have a foundation resonance near the turning speed of the machine.

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.

**See also: **Acoustic Impedance, Characteristic Impedance, Electrical Impedance, Force, Impedance Matching, Input Impedance, Linear Impedance, Mechanical Impedance, Mismatch Loss, Nonlinear Impedance, Output Impedance.

**Subjects: ** Electronics Noise & Vibration Physics