Impedance
Overview
The term "impedance" is used in electronics to describe a form of resistance that varies with frequency. Two common examples of electronics components that have impedance are capacitors and inductors.
Basics
Another example is a dynamic speaker driver. Typically; the impedance is near zero at DC, and rises rapidly to a peak at the driver's resonant frequency. It then drops back down to a low level; and most speaker's rated impedance is measured at this "minimum" point. The impedance then rises steadily as the frequency of the input signal increases.
This plays a large part in why dynamic speakers are considered to be a "difficult" load to drive; and why factors such as speaker cable impedance and amplifier output characteristics can have a significant effect on the audible performance of the speaker system.
In analog interconnects used for audio; the impedance effects of the cable are minimized by the practice of employing near "zero-Ohm" output amplifiers to drive the cable and medium-high impedance inputs.
As the frequency of the signal increases to very high frequencies found in digital audio; the impedance of the cable used in digital interconnects becomes more critical. Typically, the characteristic impedance of the cable is chosen to match the source and receiver impedance. For twisted-pair AES digital audio connections; the impedance is 110 Ohms. Coaxial S-PDIF, AES3 and Word Clock connections are typically 75 Ohm impedance.