Recording - July 2017 - 52

HOW
MICS
WORK
PART 3

Dynamic Mics-
a tale of coils
and ribbons
BY PAUL J. STAMLER

I

n our last two installments (May and June 2017), I talked about how
condenser mics work-how a microphone diaphragm is built as part
of a capacitor, and how vibrations of that diaphragm in response
to sound waves are converted into varying electrical signals. Now I want
to shift gears and talk about the other microphones commonly found in
today's studios, microphones that work by electromagnetic induction-that
is, dynamic and ribbon microphones.
Let's get the semantic issue out of the way first: ribbon microphones are
dynamic microphones. An AEA Nuvo ribbon mic's internal workings are,
in principle, the same as a Shure SM57's. It would be more accurate to call
the Nuvo a "ribbon dynamic mic" and the SM57 a "moving-coil dynamic
mic", and I'll refer to them as "ribbon mics" and "moving-coil mics" for the
rest of this article. As I said, they work on the same basic principle; here's
what that's about.

Science class revisited

Let's go back to grade school: hopefully your science teacher
once did a demonstration where she used a magnet, a loop of
wire, and a meter called a galvanometer to show when current
was flowing in the wire. When the science teacher moved
the magnet, or moved the wire, the meter jumped, indicating
that electric current was flowing. (You say never had such a
demonstration? Then you weren't lucky enough to have Miss
Wehr as a science teacher.)
If you did the experiment, you'll remember that when the
wire and magnet were both stationary, no current flowed and
the meter stayed at the zero position. Only when the lines of
magnetic force were brushing across the wire (either because
52

RECORDING July 2017

of the magnet's or the wire's motion)
did they kick charges inside the metal
wire into flowing as current. What
was happening is called electromagnetic induction, because the magnet is
inducing a current to flow in the wire
when the lines of force brush across it.
One way the experiment might be
configured is shown in Figures 1-3.
The magnet, with its lines of force, is
shown in Figure 1; the wire and gal- 1
vanometer are shown in Figure 2. When you put the loop of
wire between the magnet's poles, and shove it into and out
of the gap, current flows in the wire,
which will register on the galvanometer,
as shown in Figure 3.
What does this have to do with
mics? Just this: if you replaced the galvanometer with a male XLR connector
(eventually connected to a console or
mic preamp), and attached the loop of
wire to a diaphragm that vibrated in
response to sound waves, moving the loop
of wire in and out of the magnet's field,
2 you'd have a (very crude) microphone.
That loop of bare wire is a pretty inefficient way to generate
electric current, but fortunately there are relatively straightforward
ways to turn it into something we can use in a studio. Instead
of one loop of wire, you could wind the wire into a cylindrical
coil, so that more "turns" of the wire would fit inside the most
intense part of the magnetic field.
Then you could improve the efficiency
of the magnetic field itself by making
the magnet into a sort of doughnut
shape and placing the wire coil into
the gap. Finally, because the coil should
be tightly wound for compactness, you'd
use insulated wire to prevent adjacent
turns of the coil from shorting out
against each other. Put all of that
together, and you have a whole lot
more current generated in a very
3
compact space.
Figure 4 shows just such a refinement; in the exploded view
at the top of the drawing, you can see the doughnut-shaped
magnet on the left. Note that this is nothing like a conventional
bar magnet like you'd use in a science demonstration; the
bulging middle part is one of the magnet's poles, while the
rim of the doughnut is the other. A cross-section of the whole
assembly is shown at the bottom. This drawing comes from the
original patent for a moving-coil microphone, which was filed
by E. C. Wente in 1928. (For those of you who read Part 1
back in May-yes, this was the same E. C. Wente who had
patented the first practical condenser microphone a dozen
years earlier. He was an ingenious and productive guy.)
Put the whole thing together, add some parts to suspend
the diaphragm/coil assembly inside the magnet, tack on a
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