Where does impedance collide?

The complex-valued opposition that a circuit presents to an alternating current: it combines resistance, which dissipates energy, with reactance, which stores it and shifts current out of step with voltage, written as Z = R + jX and measured in ohms. Kennelly's 1893 paper Impedance set out the complex-number way of writing this combined quantity, so that a source delivers most power to a load when their impedances are matched. The referent is a number describing how voltage relates to current in a wire, not the opposition a medium gives to sound, a mapping difficulty in software, or a ratio of force to motion in a vibrating part.[1]

Unit

The ratio of voltage to current in an AC circuit, a complex number of ohms

Recovery means

Not applicable

Measured by

Measured with an LCR meter or impedance analyzer as a magnitude and phase that vary with frequency

The opposition a medium presents to the flow of sound through it: for a travelling wave the characteristic acoustic impedance is the product of the medium's density and its speed of sound, and a large jump in this quantity between two media, such as air and water, makes most of the sound reflect at the boundary. Peake and Rosowski's 1991 Impedance matching, optimum velocity, and ideal middle ears treated the middle ear as a device that matches the low impedance of air to the high impedance of inner-ear fluid so that sound is transmitted rather than reflected. The referent is a physical property of a medium carrying sound, not the opposition a wire gives to current, a software mapping problem, or a mechanical force-to-motion ratio.[2]

Unit

Pressure divided by particle or volume velocity, in acoustic ohms (rayls)

Recovery means

Not applicable

Measured by

Computed from a medium's density and sound speed, or measured at the eardrum by tympanometry and reflectance

The set of conceptual difficulties that arise when an object-oriented program has to store and retrieve its data in a relational database, because objects with identity, inheritance, and nested structure do not line up cleanly with flat tables of rows and columns. Ireland, Bowers, Newton, and Waugh's 2009 A classification of object-relational impedance mismatch catalogued these conflicts and argued the label, borrowed by analogy from electrical engineering, names a real and recurring engineering problem. The referent is a mismatch between two ways of modelling data, a metaphor rather than a measured ratio, and it shares nothing with the opposition a wire gives to current, a medium gives to sound, or a part gives to a vibrating force beyond the borrowed word.[3]

Unit

A catalogued class of conceptual conflict between an object model and a relational schema

Recovery means

Not applicable

Measured by

Described qualitatively by the kinds of mapping conflict it produces, not read off a numerical scale

The complex ratio of an applied force to the resulting velocity at a point in a vibrating structure: it says how hard a part resists being moved at a given frequency, and it lets engineers treat springs, masses, and dampers with the same algebra used for electrical circuits. Firestone's 1933 A new analogy between mechanical and electrical systems set out this force-to-velocity quantity and the analogy that maps mechanical parts onto electrical ones. The referent is a property of a moving mechanical system, timed against its vibration, not the opposition a wire gives to current, a medium gives to sound, or the mapping problem that the word names in software.[4]

Unit

The complex ratio of applied force to resulting velocity, in newton-seconds per metre

Recovery means

Not applicable

Measured by

Measured on a shaker with a force gauge and an accelerometer as a frequency response function