impedance: A measure of the complex resistive and reactive attributes of a component in an alternating-current (AC) circuit. Impedance is what restricts current flow in an AC electrical circuit; impedance is not relevant to DC circuits. In DC circuits, resistors limit current flow (because of their resistance). In AC circuits, inductors and capacitors similarly limit the AC current flow, but this is now because of their inductive or capacitive reactance. Impedance is like resistance but it is more. Impedance is the sum of a circuit, or device's resistance AND reactance. Reactance is measured in ohms (like resistance and impedance) but is frequency-dependent. Think of impedance as the complete or total current limiting ohms of the circuit the whole banana. Since AC circuits involve phase shift -- i.e., the voltage and current are rarely in phase due to the storage effects (think time; it takes time to charge and discharge) of capacitors and inductors, the reactance is termed "complex," that is there is a "real" part (resistive) and an "imaginary" part (bad terminology, but it means the phase shifting resistance part). To summarize: resistance has no phase shift; reactance (capacitors & inductors in AC circuits) includes phase shift; and impedance, is the sum of resistance and reactance. Just that simple.
Making the output driving impedance and the next stage input impedance equal, often requiring the insertion of a special
impedance matching network. For why impedance matching is not necessary (and, in fact, hurtful) in pro audio applications, see
William B. Snow, "Impedance Matched or Optimum" [written in 1957!],
Sound Reinforcement: An Anthology, edited by David L. Klepper (Audio Engineering Society, NY, 1978, pp. G-9 - G-13),
and the RaneNote Unity Gain and Impedance Matching: Strange Bedfellows.
In audio circuits and components, many different impedances are encountered. A loudspeaker, for instance, is a low-impedance device, usually about 8 ohms. This means that a given voltage across it will result in relatively large amounts of current in it. The power accepted by the speaker is equal to the voltage multiplied by the current. A condenser microphone, on the other hand, is a very high-impedance device, generally several billions of ohms. The voltage generated by a condenser microphone results in vanishingly small amounts of current because of the high impedance. In general, impedances are relatively low where large amounts of power are being transferred, and are relatively high when power levels are low. An exception to this is found in low-impedance microphones, such as dynamic microphones, where power levels are also very low.
Low-impedance circuits are less susceptible to electrical interferences such as 60-hertz hum than are high-impedance circuits, and they are used to transmit audio signals over cables. Most audio transmission lines used in the broadcast industry are of 600 ohms impedance, except for speaker lines, which are much lower in impedance. It is interesting that 600 ohms would be chosen as a working impedance for commercial sound and broadcast work.
Four reasons can be stated:
In radio frequency and video transmission, impedances are matched to avoid signal reflections from the ends of the lines. Such reflections cause double images, or "ghosts." This effect is due to the relatively short wave- lengths of the signals involved. Such reflections could be a problem at audio frequencies if the transmission lines were hundreds of miles long. They were sometimes heard in analog long-distance telephone circuits as an echo, but the effect is of no consequence to normal audio circuits. Today long distance telephone lines are almost exclusively fiber optics, and they are not susceptible to this type of echoing.