Basic Training for Sound System Operators
Articles from Pro Comm's Quarterly Newsletter
Microphones: High vs. Low Impedance Reprinted from Sound Advice
Impedance, or "Z" is the name for the resistance of flow of alternating current.  It is a force that opposes the flow of current at a specific frequency.  This opposition force is measured in ohms and is written using the Greek letter "W".

High Impedance

A high impedance microphone has an impedance from 10,000 to 100,000 ohms.  The most common values range between 10,000 and 50,000 ohms.  About the only advantage of high impedance microphones is lower cost.  However there are many disadvantages.  The cable length is critical.  A 10,000 ohm microphone is already experiencing loss at 10,000 Hertz with only 6' of cable!  The maximum cable length should be around 10'.  A 50,000 ohm microphone will have lost half of its high frequencies with only 10' of cable.  As a result, the sound is muffled.

High impedance microphones also use unbalanced cable.  This means that the very small signal is being carried in one inner conductor and the outside shield.  As the cable length increases, the unbalanced cable will have an increasing chance to pick up stray signals from dimmers, fluorescent lights and radio interference (RFI).

As can be seen, there is really no application for high impedance microphones in high quality sound systems.

Low Impedance

Low impedance microphones have an impedance between 50 and 600 ohms, with the most common range being 150 to 250 ohms.  A major advantage of low impedance mics is their ability to drive very long cables.

Low impedance microphones usually use a balanced output.  This means there are two inner conductors to carry the microphone signal with an outer shield.  the outer shield provides an electrical shield for the microphone signal, which results in better immunity to noise and interference over single conductor cable.  Due to these factors, a balanced, low impedance microphone cable can go great lengths without degradation.  A cable like Belden 8412 can be run 437' with a 150 ohm microphone with a very slight loss (1 dB) at 10,000 Hertz.   

Low impedance, balanced lines are the best option when high quality sound is important.