Here's a discussion I had with a mic manufacturer about dynamic mic design at my PSW forum. My parts are in bold type:
Quoted from quite a while ago in a forum far far away...
"Dynamic Mics
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!"
------------------------------------------------------------ --------------------
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 150 Hz 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.
--------------------------------------------------------------------------------------------
Well, yes and no, kinda, not exactly. Above the resonant peak (and up to where the diameter of the diaphragm equals 1/2 wavelength), the mic can be reasonably smooth. But the games that manufacturers play to get the resonant peak down to an acceptable level, and trying to extend the high end and low end is where the problems start.
Unfortunately, my ASCII drawing didn't allow for much detail to show the finer points, but the concept is still pretty valid for the extremes of the response curve. And the point that I was making is still pretty valid: A lot of it is done with smoke and mirrors.
I knew when I was writing this, that some of the analogies I made weren't perfectly accurate, but overall, the thread holds up pretty well as an introduction to microphones and why things aren't always as they seem to be. But you're right; I did paint a somewhat bleaker picture than necessary to get some attention.
--------------------------------------------------------------------------------------------
The resonant peak is indeed damped by things around it, but to a knowledgeable manufacturer (yes, I work for one), there's no real voodoo involved - it's mostly acoustical resistances that can be measured, modeled, etc. Extending the low frequency end isn't really ever done beyond the engineering of the diaphragm's mass and compliance - the low frequency limit is set by the main resonant frequency (the pistonic one of the coil and diaphragm mass on the spring of the diaphragm surround), and below that, it's a 12 dB per octave dropoff. At the high frequency end, managing diffraction is a large part of the job, and doing some extension with a resonator is a well established part of the art.
So there's a second intentional resonance (beyond the low frequency one) in a typical design - for a net of one at each end of the pass band. In most designs, any other resonances are things that are engineered out of the design, not intentionally put in, and for a reputable manufacturer, that means you won't see (or hear) them much, if at all.
Where some of the "chambers" actually come in is more in the area of polar pattern management - making the mic act as a cardioid over a wide range of frequencies. A typical cardioid mic will have front and rear entries that, left to their own devices, will tend to want to make the mic a bi-directional at some frequencies. A chamber behind the diaphragm is often used to bring in some "omni" component, and steer things towards a consistent cardioid pattern.
-------------------------------------------------------------------------------------------
And that explanation would have left most of the readers at home recording in the dust. Yeah, I made too big a deal of the major peak, but it was easier to draw in ASCII, and the concept of other little stuff in there (chambers, slots, and resonators) I did try to cover, probably a little too simplistically in my description.
There were some other areas where I screwed up even worse, but I was pretty ill during a lot of the writing. Not much of an excuse, but it works for me. Most of the questions there were along the lines of "Which is better, a cardioid or a condenser?".
Is the entire thread useful (warts and all)? I think so - but only time will tell.
-------------------------------------------------------------------------------------------
I wasn't trying to undermine the usefulness of your thread - just one sticking point that got to me. You are in remarkably good company, though - the guys at Neumann (who know a heck of a lot about condenser mics, are really smart, and nice guys to boot) even share some misconceptions about the workings of dynamics (as witnessed in the referenced material). So you don't have to feel too bad Wink
Now, if you want to talk major peaks, we could move on to condensers, but for now, let's just leave it lie. OK?
Have a great New Year!
..............if you're not prepared to do that then the wise move would be to STFU.
