Quoted from quite a while ago in a forum far far away...
"Dynamic Mics
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By far, the most popular mic on the market today is the dynamic cardioid mic, so that's as good a place as any to start. "How does it work, what exactly is a cardioid, and how and where would you use it" will be our focus today. Let's look inside one and see what we find:
Well, it has a cone (like a small speaker), a voice coil (like a small speaker), and it sits in a magnetic gap (like a small speaker), so isn't it just a small speaker in reverse? Yes, and no. The operating principle is the same, but the execution is very different. When's the last time you saw a 3/4" speaker that went down to 30 or 40 Hz? Here's how it's done:
The system resonance is chosen for a mid band frequency. By itself, the capsule's response looks something like this:
......./\
....../..\
...../....\
..../......\
.../........\
../..........\
./............\ - just one big resonant peak, with the response falling off rapidly on each side of the peak. Now you can tame that peak by putting in a resonant chamber that's tuned to that peak, which will give you two smaller peaks on either side, like this:
..../\..../\
.../..\../..\
../....\/....\ And if you add two more resonant chambers, tuned for each or those peaks, you wind up looking more like this:
./\../\../\../\
/..\/..\/..\/..\ And if you make the chambers a little more broad band, the response starts to really flatten out:
._..._.._..._
/..\/..\/..\/..\ but remember, it's still a lot like a bunch of tuned coca cola bottles inside there.
Now ya gotta do all of this stuff JUST to get the response usable - never mind about the mic pattern yet!
A lot more to come!! Everybody still with me at this point? Any questions?
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Last edited by Harvey Gerst : 05-04-2001 at 14:46. "
OK, I quoted this because I've seen it one too many times, and it just plain bugs me at this point.
My objection to this "common wisdom" is that, in plain truth, it's plain wrong.
Dynamic mics are like little moving coil loudspeakers, yes, but... like moving coil loudspeakers, they have one primary resonance - not in the middle of their operating range, but at the bottom (typically 50 to 150Hz for a modern dynamic microphone). Above that bottom resonance, up to 5 to 10 KHz, they are remarkable linear devices that are NOT relying on resonances to work. Yes, at the very highest frequencies, there are peaks and dips on the curve, some due to housing diffraction (which is also present in large diameter condenser mics) and diaphragm breakup modes (which make more dips than peaks.
I know this is counter to common wisdom, but it's true - dynamic mics, though not perfect, do not rely on multiple resonances stacked on each other carefully (like a house of cards) to work. They're actually a little simpler (and hence more robust) than "common wisdom" implies.