Theatre Design & Technology - Winter 1981 - 10

A major problem with the condenser microphone is best
understood by discussing how it works. Figure 1 shows a pair of
plates, one or both of which can be diaphragms. These two
plates are charged with an electrical potential. When the sound
pressure waves causes the diaphragm plate (one or both) to
vibrate, the vibration alters the distance, hence also the capacitance, between the plates, thus allowing a very small electrical
current to flow. For technical reasons, this feeble current from
the condenser plates must be amplified right at the condenser.
No more than a few inches can separate the plates from the
amplifier.
This amplifier must be powered. A few modern microphones
use a battery mounted in the microphone housing. Most use a
scheme called phantom powering in which the microphone
cable carries the operating power from the control board to the
microphone while, at the same time, the audio voltage travels on
the cable from the microphone to the control board. External
power supplies are also available for use with older boards or
tape machines which do not have provisions for supplying
phantom power.
With modern transistor amplifiers, it is no problem to build a
tiny amplifier into the case of a microphone no more than an inch
in diameter. But in the 1910's and 1920's when the age of the
vacuum tube had just begun, one would have to be crafty indeed
to cram that amplifier into a breadbox. These early microphones
were large, heavy, warm boxes with a microphone element in
one end.
Because of the problems of amplifier size and the need for
external power, the early radio engineers were eager to have
physical changes. While some microphone developers-principally the Germans-eontinued to work on reducing the size of
the amplifier, most engineers began work on the electrodynamic
microphone. The condenser microphone passed into an eclipse
and did not return until rather recently. Today the finest microphones made are condensers.

Eisenhower was about to enter the race for the presidency, he
asked his friend General Sarnoff of RCA for a high-quality
microphone that would be small enough to not hide his face.
This was the genesis of the RCA Bk-5, the first small ribbon
microphone, measuring about one inch in diameter and about
six inches long.
The real change was to come from the excellent small
moving-coil microphones which were introduced after WWII.
The decade of the fifties saw the ribbon microphone almost
completely replaced by smaller, lighter, more rugged movingcoil units. So complete was this replacement that today there is
only one American manufacturer and only about three in the
world who make ribbons. Even the term dynamic has come to
be synonymous with moving coil.
So we arrive at the 1980's with many moving-coil and a few
ribbon dynamics and many condenser microphones, each capable of very high quality and available in sizes from a beer bottle
to a bottle cap or smaller, wired and wireless. Size and
portability are no longer problems. But there are still a few subtle
differences among the three types (see Table 1).

ACOUSTICAL TYPES
In the beginning, all microphones were non-directional-also
called omnidirectional (and the world was a simpler place). Then
the ribbon microphone came with a bi-directional (figure-eight)
pattern. There followed a still continuing quest for increasingly
directional microphones.
Before getting into a discussion of the various acoustical
(directional) types, let us pause to meet two important concepts:
polar patterns and off-axis response.
The polar pattern of a microphone is a way of depicting its
directional characteristics. The microphone is at the center of a
group of concentric circles. Each circle represents some change
in output level from the microphone-usually 5 dB per circle.
The spokes radiating from the center represent angles about the
microphone. Zero degrees is on axis of the microphone; 180 is
directly behind; while 900 and 270 0 represent right and left. The
polar pattern is a flat representation of a pick-up pattern which is
actually spherical. Most microphone patterns are uniform about
the sphere, so the pattern is the same for horizontal or vertical or
diagonal slices through the pick-up field.
In the real world, no microphone has an absolutely uniform
frequency response from all directions. The sonic effect of the
variations in response from different angles is called off-axis
coloration. This means simply that a musical instrument will
sound different depending upon its angle from the microphone.
Or, in the case of large instruments such as a piano, different
parts of the instrument will sound different because the sounds
will strike the microphone at different angles. The amount of
variation in off-axis response is one of the most important
measures of a microphone's quality.
0

Dynamic
From the early 1930's until the present, the most common
professional microphone has been one of the two dynamic
types: moving coil and moving ribbon. These microphones
operate on the principle that an electrical voltage will be generated in a conductor moving inside a strong magnetic field. As seen
in Figure 2, sound pressure waves cause the diaphragm to
move. The motion of the diaphragm is passed on to a coil of very
fine wire that is centered around a rod of a strong magnet. The
moving ribbon (Fig. 3) operates on the same principle, except
now the diaphragm and coil are combined into a single element-the ribbon.
The first high-quality moving coil microphone was introduced
by Western Electric in the late 1920's. Because of its large size,
about three inches in diameter, its high-frequency responseespecially off axis-was poor. In 1931 RCA produced the first of
its famous 44 line of ribbon microphones. In 1933 RCA introduced a second form of the ribbon microphone, the 77A. These
two ribbon microphones along with a couple of ribbons and
moving coils from Western Electric and the Shure Unidyne
moving coil dominated the United States market until after World
War II.
While the dynamics were smaller and more portable than the
condenser with its amplifier, most-especially the ribbonswere still big, heavy, and delicate. They were excellent in the
radio or recording studio or on the Hollywood set, but ribbons
were too fragile for reliable use outdoors. For TV or stage they
were too big. No singer can hand hold a five pound microphone-as some ribbons weighted.
New technologies and new magnetic materials developed
during World War II along with changing user demands combined to lead the industry to find ways to make high-quality
microphones that would also be smaller in size. A good example
of user pressure affecting manufacturing: In 1952 as General
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Theatre Design & Technology

Omnidirectional
An omnidirectional microphone (Fig. 4) has a sealed chamber
behind the diaphragm so that sound pressure can apply force
only to the front of the diaphragm. No matter in what direction
the sound originates, as the pressure waves pass, the diaphragm will be subject to the push and pull of the varying sound
pressure.
The first microphones, while basically omnidirectional, were
really not very good or uniform at the high end. These problems
were the result of a large diaphragm and the large box behind
the microphone. The early microphone engineers quickly
learned that a large diaphragm does not operate well at high
frequencies and that a large microphone rear housing will cast a
shadow in the highs arriving from the rear. Modern omnidirectional microphones have diaphragms less than one inch in
diameter and correspondingly small housings. Today's best
quality omnidirectional microphones, whether moving coil, ribUSITIlWinter, 1981

Table of Contents for the Digital Edition of Theatre Design & Technology - Winter 1981