A question on the meaning and model of microphone impedance

I’m a little perplexed by the circuit model and meaning of a high or low impedance microphone. Here is an example where someone at this page is trying to investigate the effects of low or high mic impedance:

But my question is more primitive and fundamental here. The dynamic microphone is made of moving coil and the electret mic is made of capacitor. But Rm above is modeled as the output resistance of the mic.

My questions are:

1) What is Rm in real? Is it the DC resistance?? Or is it the inductive reactance of the coil or the capacitive reactance of the capacitor at particular frequency? If so isn't it wrong to model it as a pure resistor(Rm) in the circuit analysis? Is there better way to model the microphone impedance as a source and Thevenin equivalent?

2) When we say high impedance or low impedance microphone do we mean the output impedance hence Rm in the above diagram?

Edit:

• If you "match", you lose 50% of the voltage. – analogsystemsrf Apr 17 '19 at 0:23

1: Zm is possibly better then Rm, but actually usually what we care about is something pretty close to |Zm| being as the reactance is generally small enough to not risk forming audio band resonances with Cc and the preamp input capacitance, and the Q is low. Note that is this is NOT the case in guitar pickups or phono cartridges where stray capacitance very much can form resonances in the audio band....

2: Generally yes, but note that nearly no capacitor microphone connects the capsule directly to an external cable, they all have an impedance converter (generally a source follower or cascode or something) right at the capsule to convert the few Gohms you need at that point to get reasonable bass into something in the few hundred ohms region that can drive a cable. Thus a capacitor microphone is usually a low impedance mic in spite of the fact that the capsule needs to see a few Giga ohms of load.

About the only real high Z mics were the old crystal (piezo) microphones once popular with cheap tape recorders (no need for an input transformer when using a valve as the preamp), and some slightly later dynamic designs that generally has a transformer in the body to give a level usable with said tape recorders.

These days almost everything you find will be low Z because gain is cheap and low Z sources are better behaved over long cables (Especially if the cable is balanced).

• I dont get the part about why we can model it as Rm instead of Zm. You saying small enough but the document is talking about a value betwern 600 Ohm and 10k. Can you show it with a model example with a source and a reactance? – user16307 Apr 16 '19 at 19:55
• Real low impedance mics are generally a few hundred ohms (mostly resistive, reactance is usually <20 ohms across the audio band) and are intended to drive a load of a couple of k ohms in parallel with maybe up to a nF or so of cap (cable plus input RFI suppression) which is what most modern mic input stages look like. High Z mics are about the same but seen thru somewhere between a 1:3 and a 1:10 transformer or thereabouts (Yea there is a reason nobody goes there these days). – Dan Mills Apr 16 '19 at 20:10
• Please see my edit. I tried to model a dynamic mic as L and R. If correct at 100Hz audio to neglect the inductive part coil needs to have 2mH inductance. Does that make sense? So I took the DC resistance of the coil is 600 Ohm. Im also asking whether I can model this way. – user16307 Apr 16 '19 at 20:40
• Assume Xl is say 20j at 20KHz and work your values from there, but note that 600 + 20j is pretty much the same as just 600 ohms resistive when driving into a few k ohms of load impedance which is the usual design. 2mH would be VERY large for an air cored coil having maybe a 20mm diameter, I for example make a 20mm diameter, 100 turn single layer coil, 5mm long, right about 300uH (Wild guess, but it feels reasonable). – Dan Mills Apr 16 '19 at 21:07

What is $$\R_m\$$ for real?

It is the effective, resistive part of the microphone impedance, assuming that any parallel capacitance or series inductance is too small to matter, and any series capacitance or parallel inductance is too large to matter. This means that over the audio band, for all intents and purposes it acts like a pure resistance.

When we say high impedance or low impedance microphone do we mean the output impedance hence $$\R_m\$$ in the above diagram?

Yes, except that because of capacitive or inductive effects, you probably can't trust an ohmmeter reading -- if you want to measure it you'll need to do it with some sort of audio-frequency impedance analyzer.

• I still dont get why we can model it as pure resistance. – user16307 Apr 16 '19 at 19:56
• You can if you make sure to capacitively couple to it (because it may have a DC path to ground, or have/need phantom power), and if you remember that the assumption of pure resistance is only valid when it's solidly in it's advertised operating frequency range. – TimWescott Apr 16 '19 at 20:16
• Please see my edit. I tried to model a dynamic mic as L and R. If correct at 100Hz audio to neglect the inductive part coil needs to have 2mH inductance. Does that make sense? So I took the DC resistance of the coil is 600 Ohm. Im also asking whether I can model this way. – user16307 Apr 16 '19 at 20:40