Recording - April 2015 - 48

By Paul J. Stamler

Part 4: About Microphones 1
Over the past three months, I've introduced you to the idea of
learning about audio through a sample recording session, and
digressed briefly to discuss room preparation and monitoring.
Now it's finally time to get to the actual recording session.
When Ottoman Underground came in to record "Arbetslosen
Marsch" (our example audio file for this whole series, available
at http://is.gd/RECFirstStepsClip1), I pulled from my suitcase
three small-diaphragm cardioid condenser mics (one for Dana's
clarinet, two for room mics), two large-diaphragm cardioid condenser mics (for Will's voice and tenor banjo) and a mediumdiaphragm cardioid condenser mic (for Shlomo's bass amp). I
brought along, but wound up not using, a hypercardioid dynamic mic and a hypercardioid ribbon mic. (I'll name names soon,
I promise.) What does all that mean?

You already know this part
Most recordings are made from sounds captured in rooms,
and what they sound like depends a lot on the microphones
used. (Recordings that are entirely sequenced in a computer are
the exceptions, but even those, if they use samples, are dependent on the mics used to record the samples.)
When something vibrates (a string, a column of air in a wind
instrument, a loudspeaker in an amp, or a set of vocal cords) it
creates sound waves in the air-patterns of compression and
decompression (rarefaction) that spread out across the room. A
microphone senses these patterns and generates electrical signals; with luck, the ups and downs of those electrical signals
match the compressions and rarefactions in the air.
Once the mic has generated an electrical signal, we can do
things with it, like amplify or record it. This article, and the next,
will talk about how microphones do what they do.

How condenser mics work
The business (capsule) end of a condenser mic, sometimes
called a capacitor mic, is essentially a pair of metal plates: one
thick and stationary, the other thin and flexible. These days the
flexible "plate" is usually a thin sheet of plastic, coated with
metal (see Figure 1).
The capsule of a condenser mic, with its stationary and moving plates, is a capacitor (or condenser in old-fashioned terminology): it stores electrical charge on its plates, and the amount
of charge that the plates will store depends on the distance
between them. As the flexible plate (the diaphragm) moves in
and out in response to sound waves, the amount of stored
charge increases and decreases; as the charge rushes in and
out of the capsule, it generates a changing electric voltage.
That's a signal.
Unfortunately, it's a weak and fragile signal; if you tried to run
it down a cable, most of the signal would leak away before it
arrived. To prevent that, every condenser mic has a head amplifier built in, which beefs up the signal enough to drive a long
cable. In the earliest days, this head amp was made from a vacuum tube, and some still are. In recent years, head amps have
also been made using field effect transistors, or FETs.
The output section of the head amp may feed the cable via a
transformer (in some FET-based head amps and most tubed
48

RECORDING April 2015

head amps); other FET head amps connect to the world with no
transformer. Transformer-coupled head amps put a lid on how
high the microphone's signal can get with a loud sound source;
too loud, and the transformer saturates, which means it creates
distortion. Transformerless head amps don't have this limit, since
you can't saturate a transformer that isn't there. When sound levels are moderate, though, some engineers prefer the sound of
transformer-coupled mics, which tend to sound smoother, while
others prefer the clarity and sharp focus of a transformerless condenser mic. I used both in our recording session.
The head amp needs power; most tubed mics use a separate
power supply in an external box, while FET mics use phantom
power, a clever scheme that puts +48V on each wire of the connecting cable. The phantom power comes from the
preamp/console/interface, and there's usually a switch to turn
it on and off. Take my word: you don't want to plug in or unplug
either end of the cable when phantom power is on. Really.
Condenser mics are often classified by the capsule's diameter: small, medium or large (1/2" or less, 1/2" to 1", and 1" or
greater, respectively); the main difference for our purposes is
that smaller capsules have more uniform pickup patterns at different frequencies. More on that later.
Because the light, low-mass diaphragms of condenser mics
follow rapid changes and small details of sound waves (transients) faithfully, and they usually have low distortion, condenser
mics have acquired the reputation as the most accurate mics in
the business. Like a lot of legends, it ain't necessarily so (dynamic and ribbon mics can also follow transients faithfully), but condenser mics have become the workhorses of most studios.

How dynamic mics work
Remember last month I talked about how most loudspeakers
place a coil of wire (connected to a cone or diaphragm) in the
field of a magnet and run current through it? How the current
makes the coil move in the magnetic field and takes the cone or
diaphragm with it? Well, a dynamic mic is just a speaker run in
reverse: vibrations in the air move a diaphragm in and out; the
diaphragm moves the coil in the magnetic field, and current
flows in the coil, creating a signal. The actual assembly is more
complicated, but the principle is straightforward (see Figure 2).
Dynamic mics don't need head amps or power supplies,
which makes them small and portable; often they're tougher
than condenser mics, and can take harder use. Some incorporate transformers; others drive the cable directly from the moving coil.
Dynamic mics, as Figure 2 shows, are more complex mechanisms than condenser (or ribbon) mics, with lots of pieces in the
capsule. These pieces resonate at various frequencies, and
designers must juggle the resonances to get good performance
from a dynamic mic. Some dynamic mics can perform very well,
including good transient response; the Sennheiser MD 441 and
Electro-Voice RE20 are good (if expensive) examples.

How ribbon mics work
Ribbon microphones share some characteristics with their condenser and dynamic siblings. Like dynamic mics, they generate

http://www.is.gd/RECFirstStepsClip1

Recording - April 2015

Table of Contents for the Digital Edition of Recording - April 2015