This may or may not be helpful...

A dynamic microphone uses a diaphragm to capture the air-pressure (sound) waves and convert that into a physical movement that in turn moves a coil in a magnetic field to create an electrical. The system is the exact reverse of a conventional loudspeaker, which changes an electrical signal to air pressure waves.

Amongst the challenges with these processes is getting the mechanical and electrical parts to behave as cleanly as possible, with no resonances to cause peaks or notches in the sound.

The mic/speaker designers will do their best to minimise any such resonances, but there will always be some.

Resonances in this context are like a weights on springs. When you put energy into them they bounce around, and once they start, they can continue for some time unless there is something to take the energy out again.

A car has the same problem and car makers solve it with shock absorbers "damping" the bounce.

In a mic/speaker, the diaphragm, its support and the air around them will form part of that damping. There's also the coil within the microphone that acts as a kind of spring in an electrical sense. There are actually several different little resonances in every such assembly.

One of the few places left after that mechanical damping for a microphone to get rid of the energy in that resonance is to dissipate it into the load resistance, so the load resistance is a significant part of the damping.

The microphone designer will plan that the final resonances will be damped as best they can be with a particular load resistance, but as is obvious, load resistances vary, so all they really can do is to aim for a "best chance" load.

Years and years ago, telephone designers discovered that telephone wire pairs have natural impedance(*) of around 600 Ohms, and as early microphones often had quite long cables from mic to pre-amp, it made sense to use that natural impedance as best one could.

For ideal signal transmission, the three components, mic, cable and pre-amp, would all have the same impedance, but in practice that's not really necessary and it also happens that matching too well is usually not ideal for sound quality.

A kind of consensus formed for a microphone impedance of around 300 Ohms, the cable at around 600 Ohms and the pre-amp around 1500 to 2000 Ohms.

The final piece of the equation is how much of that damping the listener wants. Should the microphone sound bright (lower damping) or dull (higher damping)?

Condenser microphone have their own pre-amp, so little of the above applies to them.

(*)Impedance is approximately analogous to resistance, but accounts for the effects of capacitance and inductance in the circuit.

Jazz relative beginner, starting at a much older age than was helpful.
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