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I've been researching some mics and aside from reviews I've been looking at the specs. I'm not sure I understand what impedance is and why I care about it when purchasing a mic.

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Definition
Impedance is essentially a measure of how much a device resists, or impedes, the flow of alternating electrical current (AC). It is measured in ohms, the symbol for which is Ω.

Lower is Better
With microphones, a lower impedance is usually desirable. High-impedance microphones can cause a loss of high-frequency signal and are more prone to signal noise (hum or buzz), especially over long distances.

Generally, microphone impedance of less than 600Ω is considered "low impedance" and anything greater than 10,000Ω is considered "high impedance".

Impedance matching
Microphones should generally be connected to devices (i.e., mixers) with a higher impedance than the microphone. Connecting a mic to a device of lower impedance causes a loss of signal.

Resources

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Yes, has BenV said, impedance is a measure of how a device resists or impedes the flow. The catch is, it's also frequency dependant.

Some more expensive preamps will even allow you to select several input impedances for the device allowing you to approximate the one for your microphone, or if you are in the mood to try something different, well...

I usually notice problems straight away with older ribbon microphones, or some dynamic. Condenser microphones have built in circuitries or a dedicated transformer so you will usually find that they have an pretty consistent average of what their impedance value might be.

Thing is, a difference between 100ohms to 1kohm can change drastically the sound! not only the loss of high frequency but the low end response gets slightly modified. An example is the actual physical resistance that a microphone has. The damper the microphone is usually the thicker the bottom end will sound.

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