Objects of Respect; Mics, equipment and sound

By Sue Sawyer

Part three of a three-part series

Impedance: It's technical but no Viagra needed here!

An important characteristic of a mic is the output impedance. It's the measurement of the AC resistance to current flow that would be observed looking back into the mic.

The mic's impedance should never be matched to what it is plugged into because some of the signal will be lost. Ideally a mic should connect to a load whose input impedance is roughly 10 times the mic's output strength. If the mic has an impedance of 600 ohms, the ideal load would be 6,000 to 10,000.

Mics are usually divided into two basic groups; low impedance (50-1,000 ohms or "Low-Z") and high impedance 10,000+ ohms (or "High-Z"). Most mics designed for long cable runs or minimal noise are low impedance devices, since they are far less susceptable to extraneous noise pickup in the cable, and can be used with very long cord runs (up to 1,000 feet) with very little loss in sound quality. High-Z mics are limited to about 20 feet before signal degradation occurs.

A mic's impedance is not necessarily an indicator of quality or performance, it's a factor that should be considered when connecting a mic to other equipment in the sound reproduction chain. Also a two-wire audio connection is unbalanced. One wire carries the audio signal and the other is connected to the ground. A balanced connection uses three wires. Two separate wires carry the signal in equal, but opposite polarity, while the third is grounded. This three-wire balanced system is more immune to noise, and the preferred method of making connections, especially where long cable runs are used. Unbalanced connections work with high impedance mics.

Of course, there is more to choosing mics than electrical considerations. The physical shape can matter a great deal. Lavalier mics are the tiny varities that clip to the lapel, can be woven into the hair or taped to the body. Hand-helds are the most popular and most rugged. Boundary mics are table-top mics and mostly appear in court rooms and board rooms. Shotgun mics are long, tube-shaped mics that look like shotguns. They are used for on-location work to isolate sounds not needed and to zoom into whatever the technician wants to record, such as on a movie location.