RCA 77-A Instructions Manual
I N S T R U C T I O N S
for
UNI-DIRECTIONAL MICROPHONE
TYPE 77-A
(MI-4040)
RCA Victor Division
RCA Manufacturing Company, Inc.
Camden, N. J., U. S. A.
Copyright 1936 by the RCA Manufacturing Company, Inc.
Instructions IB-25838
OPERATING INSTRUCTIONS
FOR
UNI-DIRECTIONAL MICROPHONE
TYPE 77-A
(MI-4040)
PART I — DESCRIPTION
1. Introduction.—e RCA Type 77-A uni-directional microphone is an entirely new type of pick-up device—a microphone
with a directional pick-up pattern wholly different from that of
any other microphone. While it resembles the velocity microphone in appearance and construction, and is, in fact, evolved
from research and development work on the latter, the RCA
Type 77-A uni-directional microphone combines the principles
of velocity and pressure operation. For this reason, it possesses
in a surprising degree the best features of each and overcomes
the disadvantages inherent in both. In view of the fact that the
unit is designed to pick up sound arriving from one direction—
or, more accurately, from one side—while almost completely
rejecting sound from the other side, it is admirably adapted to
studio pick-up, public address and sound reenforcement applications.
Instead of a diaphragm (in the commonly accepted meaning
of the word), the uni-directional microphone contains a thin
metallic ribbon suspended between the poles of a permanent
magnet with its length perpendicular to, and its width in the
plane of, the magnetic lines of force. e ribbon is rigidly
clamped at the center, as well as at the top and the bottom. e
lower half is open front and back and operated as a regular velocity microphone. In order to make the upper half of the ribbon
operate as a pressure microphone, it is, of course, necessary that
the rear of this section of the ribbon be enclosed. At the same
time it is not possible just to block this section off, as such a contrivance would result in a response increasing with the frequency. Rather, it is necessary to present an acoustic
impedance to the back part of the ribbon. An innitely long
tube would be the ideal impedance; but this, of course, is impossible. Instead, an ingenious labyrinth, which gives practically the same effect, is used. While this labyrinth has a nite
length, the desired damping of reection is obtained by ling it
very loosely with sound-absorbing material. e result is that
the upper half of the ribbon becomes an efficient pressure-operated microphone.
e vibration of each part of the ribbon is in exact accordance
with the sound vibrations and, occurring as it does within the
magnetic eld, sets up corresponding alternating electric potentials across the primary of its associated transformer. Since
the two microphones (i.e., the velocity-operated section and the
pressure-operated section of the Type 77-A microphone) are a
part of the same ribbon, the voltages developed in the two sections are, of course, in series, and the output level is obtained
from the ends of the ribbon in essentially the same manner as
in the case of the velocity microphone.
2. Description.—e uni-directional microphone shown in
Figure 1 consists of a microphone unit mounted in a horizontal
swivel on the top of a program stand. “Aiming” is accomplished
partially by means of this swivel and partially by rotating the
vertical column of the program stand. e transmitter is enclosed within a circular, perforated metal casing, so designed
as to conform to the circular construction of the labyrinth,
which occupies the lower part of the unit.
e labyrinth consists of a series of circular sections, the interior of each section having a spiral partition, an opening at the
beginning or the end of which communicates with the beginning or the end, respectively, of the section of the labyrinth that
immediately precedes or immediately follows it. e sections
occupying the upper part of the labyrinth are so designed as to
provide a cavity to accommodate the line coupling transformer,
which thus forms a part of the microphone unit.
Figure 2—Development of Directional Pattern
e Type 77-A microphone unit is supported in a mounting
yoke (containing the aforementioned horizontal swivel), which
permits it to be tilted as desired. is mounting yoke is supplied
with a threaded stand ange to t a standard Type AZ-4090 program type microphone stand. A suspension mounting (Type
UP-4212-A) is supplied to permit the suspension of the unit
overhead when desired.
e microphone program stand (Type AZ-4090) is of the adjustable single vertical column type with a three-point base. e
height of the transmitter may be adjusted to maximum and
minimum heights of 84 inches and 59 inches respectively.
3. Sensitivity.—With an input sound pressure of 10 dynes per
square centimeter perpendicular in the plane of the ribbon, the
Type 77-A uni-directional microphone will deliver 317 microvolts across a 250-ohm load, which is equivalent to an output
level of –75 dB, as compared with a zero level of 12.5 milliwatts,
or –72 dB as compared with a zero level of 6 milliwatts.
On an open circuit basis of measurement, i. e., with an input
of 1 dyne per square centimeter (1 bar) perpendicular to the
ribbon, the output of the microphone on open circuit is the
equivalent of –89 dB with reference to a zero level of 12.5 milliwatts.
4. Quality of Response.—e operating range of the microphone extends from 60 cycles to 10,000 cycles. When the microphone is located less than 2 feet from the source of sound,
the low-frequency response is increased somewhat, and when
operated at a greater distance (up to 4 feet), the low-frequency
response is slightly attenuated. Beyond the 4-foot operating distance the response characteristic is unchanged by changes in
the operating distance. e frequency response is essentially
unchanged by the direction of the incident sound over an angle
of 150 degrees at the front of the microphone.
5. Directional Characteristics.—One of the most important
characteristics of the Type 77-A microphone is its uni-directional property. On the front, or operating side, of the microphone the response is very uniform, while at the rear of the
microphone sounds are attenuated an average of 20 dB, thus
giving a 10-to-1 ratio of desired to undesired pick-up. Sound
waves originating in front and along an axis perpendicular to
the plane of the ribbon will have the maximum effect.
e outstanding advantage of the Type 77-A microphone is
derived from the fact that the unit combines the action of a velocity-operated and a pressure-operated microphone, and results from the manner in which the velocity-operated and the
pressure-operated parts of the ribbon add together.
Without going into mathematical expressions for these voltages, it is possible to obtain a picture of the action from a consideration if the three patterns shown in Figure 2. In this
illustration (a) is the directional pattern of a velocity microphone, (b) is the directional pattern of a pressure microphone.
While these gures are the theoretical or idealized patterns,
they correspond, for ribbon microphones, quite closely to actual measured characteristics.
When these patterns are added, the forward lobe of the gure
8 pattern adds to the circular pattern, while the rear lobe, which
is 180 degrees out of phase, opposes. the result is the same as
that obtained when the signals of a vertical antenna and a loop
antenna are added; viz., a cardioid of revolution, as shown at
(c). In practice, the actual measured response of the Type 77-A
uni-directional microphone, as shown in Figure 3, approaches
this cardioid very closely. For all frequencies up to 6,000 cycles
the cancellation is very good. At higher frequencies a small
“tail” occurs because of the slight phase displacement that begins to become noticeable in this range.
It is at once apparent that the uni-directional characteristic is
of considerable value in the solution of some of the difficulties
encountered in reverberant locations by the reduction of the
effect of undesired sound reections, and the increased possibilities of obtaining better balance, clarity, naturalness and selectivity in sound pick-up. Extraneous direct or reected sounds
approaching the microphone from side directions and from the
rear will have little or no effect and therefore background noises
and reected sounds in the broadcast are considerably reduced, which increases, by comparison, the quality of direct
sounds reproduced. e amount of sound-proong necessary
for sound originating in the “dead zone” can be greatly reduced—and, in many cases, “dead end” construction can be
entirely eliminated.
For the same allowable reverberation pick-up, the operating
range of the uni-directional microphone is approximately 1.73
times greater than a non-directional microphone having the
same sensitivity.
When used for public address and sound reenforcement purposes, the directional characteristic is of considerable value in
reducing feedback effects between the microphone and the
loudspeaker.
Sound concentrators and baffles used with condenser microphones are unnecessary with and inapplicable to the uni-directional microphone because of the fundamental difference in
the principle of its operation.
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