I have recently purchased a mic preamp with variable input impedance for my studio. So as to correctly understand and use this feature I am logging the impedance of all mics I currently own, many of which are Shure. The question is...What is the deal with rated, and actual, impedances?
Rated impedance is an obsolete concept, but still widespread. It lumps all mics into: low impedance (150 ohms rating); medium impedance (600 ohms rating); high impedance (15,000 ohms rating). The rating is the "typical" impedance for a mic in that category.
Actual impedance is what is says: the actual measured impedance of the microphone.
As a general rule, the impedance setting of a mic preamp should be about 10 times the actual impedance of the microphone.