Recording - May 2017 - 38

HOW
MICS
WORK
which is measured in farads. One farad is actually a whole
lot of capacitance; the capacitors that you can buy at Radio
Shack are measured in microfarads (μF), nanofarads (nF)
and picofarads (pF), corresponding to millionths, billionths
and trillionths of a farad respectively. (Our Editor once had a
chance to play with a giant capacitor bank
that could explode tree trunks when discharged abruptly, and even that was only
a hundredth of a farad. I asked him what
he was doing with such a beast, and he
refused to answer.)
Early in the 20th century, inventors
(including Mr. Wente) built microphone
capsules that took advantage of one important
characteristic of capacitors: as the foils of
the capacitor get closer to one another, the
capacitance gets bigger.

Capsules and capacitors

Take a look at Figure 3, which shows a
typical condenser mic capsule. The body of
the capsule (or backplate) constitutes one
plate of the capacitor, while the thin metal
diaphragm is the other plate. (In modern mics
the diaphragm is more likely a thin sheet of
plastic; the front side is coated with metal so
that it can act as a capacitor plate.) As the
diaphragm moves in and out in response to
airborne sound, the capacitance changes.
Here's what happens. First, remember that
sound consists of waves of air compression,
interspersed with waves of decompression
(or rarefaction). The "+" terminal of a battery
(or another source of DC, like a power supply)

is connected to a large resistor. (In early condenser mics this
was typically 100 million ohms, abbreviated 100 Mȍ or sometimes just 100M; in modern mics it might be a billion ohms, or
1 Gȍ.) The other end of the resistor in turn is connected to one
plate of the capacitor; the other plate is connected to the
battery's "-" terminal. Figure 4 shows what this circuit looks
like, and what happens at the instant the battery/power supply
is connected. Charge flows from the battery through the resistor
to fill up the capacitor.
When the capacitor is full of charge, the circuit reaches a
state of equilibrium (Figure 5). Nothing moves; all is calm.
Then a sound wave's region of air compression makes the
diaphragm move inward; this increases the capsule's capacitance.
Because the capacitor now has more capacity, it has room for
more charges, and they promptly rush in (Figure 6).
Charges in motion-that's what we defined as current a while
back. Let's imagine that the amount of current rushing in is 1nA,
which is 10-9A. (It isn't; it's much smaller. But it's a lot easier to
see what's going on if we use a fictional
high number.)
Remember, back when we were talking
4
about Ohm's Law, we said that if you knew
the current and resistance in a circuit, you
could calculate the voltage difference that
would be produced across the resistor:
V=IxR
= 10-9A x 100,000,000ȍ
= 0.1V
0.1V with respect to what? The top of
the resistor is connected to the battery's
positive terminal, which we'll say is at
+40V with respect to ground. (The little
triangular doodad at the bottom of the
picture represents ground, which is defined as
being at 0V.) We know that current flows
5
from a more-positive point to a less-positive
point, so the bottom of the resistor must
be less positive than +40V, and we just
calculated that the voltage difference will
be 0.1V, so the bottom end of the resistor
is at +39.9V. Here's the chain of events:
Air pressure pushes diaphragm in.
Capacitance of the capsule goes up.

RECORDING May 2017

REC-0517-FINAL.indd 38

4/7/17 2:12 PM

Table of Contents for the Digital Edition of Recording - May 2017