What is Impedance and What Does it Mean For Microphone Cables?

Impedance is similar to electrical resistance. Both forces oppose the flow of electricity through a wire, but resistance occurs in direct current or DC cables, while impedance occurs in alternating current or AC cables. Unlike resistance, impedance is affected by the frequency of the electrical signal that is being carried. Impedance is also known as Z.

Microphones can be either high or low impedance. High impedance or high Z microphones can produce outputs with higher voltages that may require less amplification. Low impedance or low Z microphones usually require a transformer in front of the pre-amp, in order to strengthen the signal. Most consumer microphones are high Z in order to avoid the need for this additional transformer. Low Z microphones have commonly been used by professionals, however, because the loss of high frequency audio signals is reduced and there is usually less interference.

High Z microphone cables are similar to the cables used with electric guitars. They are usually unbalanced coaxial instrument cables that contain a live wire. The live wire is insulated and surrounded by an earth wire that protects it from some electrical interference. Coaxial cables are also usually shielded against electrostatic effects using layers of conductive PVC or semiconductive tape wrap. This drains away the static electricity that is generated when the shield rubs against the insulation around the inner live wire. When impedance is very high, it is possible to hear these parts of the cable rubbing together. It produces a crackling sound when the cable is moved and flexed.

Low Z microphones use balanced cables. These contain a shielded pair of wires, the live and return wires. Balanced cables are not affected by electrostatic interference and therefore do not need to be shielded against it.

Although low Z cables do not need to be shielded against electrostatic effects, both high and low Z cables are shielded against radio frequency interference. This is caused by electrical equipment in the surrounding environment. Braided, spiral or foil shielding, often made of copper, is wrapped around the insulated live wire or live and return wires.

Star quad cables are an alternative to normal balanced cables. They have four small wires instead of two larger ones, but they work in the same way as other balanced cables. Star quad cables are better at preventing interference from environmental electromagnetism than balanced cables with two wires. Star quad cables may therefore be used when audio quality is of the utmost importance and there is a lot of electrical equipment nearby that could cause interference, for example in a television studio or on a film set.

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