Recording - April 2015 - 49

a signal when something conductive-in this case a thin strip of corrugated
metal foil-moves in the field of a magnet (see Figure 3). However, like a
condenser mic, the piece that actually generates the signal in a ribbon mic
(the foil) is the same piece that moves in response to vibrations in the air,
while in a dynamic mic the moving diaphragm is connected to a separate,
more massive coil assembly. Because the strip of foil in a ribbon mic has
very low mass, it can potentially start and stop quickly, leading to good transient response.
The signal produced by a ribbon mic's working parts is usually quite
small, so it's typically stepped up by a transformer. Classic ribbon mics were
persnickety about loading, which means their sound varied a lot when it
was plugged into different impedances. We won't mess around with impedance here (too big and complex a topic); ribbon mics care a lot about what
they're plugged into, and let's leave it at that for now.
Or rather, some do; about 13 years ago the clever engineers at Royer
Labs figured out that, since most of the preamps/consoles/interfaces in the
world have phantom power available to drive the head amps of condenser
mics, it would be reasonable to build a phantom powered head amp into a
ribbon mic. That way, the ribbon mechanism would always operate with the
impedance load it liked, and the head amp could provide a little extra gain
for low output ribbon mics. (The Cloudlifter accessory by Cloud
Microphones does the same thing, but it's separate from the microphone.)
Ribbon microphones with head amps are called active ribbon mics.

Pickup patterns
At the beginning of this article I used words like
"cardioid" and "hypercardioid" to describe the
mics I used. Those words refer to a mic's pickup pattern-how it responds to sounds from the front, sides
and back. Take a look at Figure 4: this is a diagram
of how a cardioid microphone responds (it's a
Neumann KM 184; thanks to the folks at Neumann
for letting us use the picture). This is called a polar
diagram; here's how to read it.
Around the sides you'll see numbers that correspond to the degrees in a circle. The concentric circles denote levels, and the upside down heartshaped curve drawn over the circles is the pickup
pattern ("cardioid" comes from the Greek word for

2
Figs. 1-3: Exploded views of condenser (1), dynamic (2), and ribbon (3) mics.

Doing your level best

One big difference between different microphones is their sensitivity, which
means the amount of signal voltage they put out for a given amount of sound
(sound pressure level, or SPL). Different manufacturers specify this in different
ways, leading to much confusion; here's a very rough ballpark guide that will
hold you until Mike Rivers covers the topic in an upcoming issue.
Let's take a common dynamic microphone like the Shure SM57 as a reference point. Typically a transformer-coupled condenser mic will be about
10 decibels more sensitive-that is, it will put out a signal that's about 10
decibels (dB) higher than the reference mic, when both are subjected to the
same SPL. Transformerless condenser mics will typically produce signals that
are about 5 dB hotter still.
Standard ribbon mics without head amps (passive ribbons) generally produce signal levels that are 5-10 dB lower than a typical dynamic mic, given
the same SPL. Active ribbons, on the other hand, usually generate signal levels that are comparable to those from a dynamic mic, given similar SPLs.
I'm talking strictly ballpark figures here; in each class of mic there are
exceptions. Shure's transformerless condenser KSM9 vocal mic, for example, has about the same sensitivity as a typical dynamic mic, since it's meant
to be substituted for the common Shure SM58 in a stage rig. And ElectroVoice's RE20 has low output like a ribbon mic, although it's a dynamic.
Another difference between the common mic types is ruggedness: as I
said before, dynamic mics are usually the toughest, while condenser mics
require gentler handling. Ribbon mics, thanks to that thin piece of foil, can
be very delicate; sometimes the instructions tell you that the mic must be
stored in an upright position, or the ribbon will sag out of shape. Do it.
Finally, testing a mic by blowing into it is a Really Bad Idea-in the case
of a ribbon, you might dislodge or deform the thin foil, which means a costly repair. Don't take chances-don't blow into any mics. (The traditional
"One-two" is a much better test anyhow.)
RECORDING April 2015

Recording - April 2015

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