Theatre Design & Technology - Winter 1981 - 11

bons, or condensers, have a very smooth and uniform frequency response throughout the audible range.
The principal problem with omnidirectionals is that they offer
no discrimination against unwanted sounds-room reverberation, audience noise, nearby instruments, etc. In the early
days-the 1920's-quality microphones were developed for use
in the controlled environment of the radio studio. There omnis
were quite satisfactory. But at the end of the twenties and
beginning of the thirties, the movies learned to talk. Now there
was a demand for a microphone that could be hung from a pole
over the actors' heads to pick up the actors' voices while not
getting too much room reverberation and crew noise.

Bi-Directional
Quite a number of different techniques were tried, but nothing
seemed very practical except the ribbon microphone which
could be left open front and back giving a bi-directional pickup
field. As seen in Figure 4, sound pressure arriving from the front
or back will cause the ribbon to move. Pressure from the side
will tend to hold the ribbon in place. The total pickup area of the
bi-directional microphone is only one-third that of an omnidirectional. This is a very substantial increase in directivity. A bidirectional microphone does not have to be a ribbon; it can be a
moving coil (though rare) or a condenser. Today there are
several very fine bi-directional condenser microphones made in
Europe. The off-axis response of most modern bi-directional
microphones is excellent. Some do have more off-axis coloration than an omni, but generally the coloration is not a problem.

Cardioid

r-------i

co,,,,,'" """ I

I
I+
I

SHIELD

I
I

~IC

I
I

PHANTOM

POWER

ICONDENSER MICROPHONE i
--.J

L-

Figure 1

Figure 2
MAGNET

VOICE COIL

The bi-directional microphone, introduced in 1931, with its
two-thirds area of reduced sensitivity was a great help in
controlling unwanted pickup, but the rear lobe was often a
problem and the front lobe was often a bit too narrow. Engineers
theorized that a cardioid would be a better pattern (Fig. 5). It has
the same two-thirds area of reduced sensitivity because the rear
lobe of the bi- is replaced by a larger front lobe in the cardioid
pattern.
To a mathematician the cardioid is just a combination of a
circle (omnidirectional) and a figure eight (bi-directional). In
1933, RCA introduced the 77A, which applied this mathematical
approach. It is a microphone with two ribbon elements: one is a
conventional bi-directional while the other is closed off in the
back forming an omnidirectional pickup. These two microphone
elements are combined electrically to produce a cardioid pattern. For over twenty-five years this microphone and others like

DIAPHRAGM

DYNAMIC MOVING COIL MICROPHONE

IMPEDANCE

CORRUGATED RIBBON

MATCHING
TRANSFORMER

Table 1
MOVING COIL

RIBBON

GOOD

BETTER

DISTORTION

BEST

BETTER

TRANSIENT
DISTORTION

GOOD

BEST

BETTER

SELF-NOISE

BETTER

BEST

GOOD

RUGGEDNESS

BEST

GOOD

BETTER

SPECIAL POWER

YES

""OBLEMS?

SOMETIMES

COST

LOW-MID

MID-HIGH

HIGH

FREQUENCY
IlANGE

CONDENSER
BEST

OVERLOAD
CAN BE A PROBLEM

MAGNET

HUMIDITY

COMPARISONS OF ELECTRICAL TYPES

USITIlWinter, 1981

Figure 3
DYNAMIC RIBBON MICROPHONE
Theatre Design & Technology 9

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