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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 flange to fit a standard Type AZ-4090 pro-
gram 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 ad-
justable 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 micro-
volts 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 milli-
watts.
4. Quality of Response.—e operating range of the micro-
phone extends from 60 cycles to 10,000 cycles. When the mi-
crophone 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 dis-
tance 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-direc-
tional property. On the front, or operating side, of the micro-
phone 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 ve-
locity-operated and a pressure-operated microphone, and re-
sults 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 volt-
ages, it is possible to obtain a picture of the action from a con-
sideration if the three patterns shown in Figure 2. In this
illustration (a) is the directional pattern of a velocity micro-
phone, (b) is the directional pattern of a pressure microphone.
While these figures are the theoretical or idealized patterns,
they correspond, for ribbon microphones, quite closely to ac-
tual measured characteristics.
When these patterns are added, the forward lobe of the figure
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 be-
gins 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 reflections, and the increased possi-
bilities of obtaining better balance, clarity, naturalness and se-
lectivity in sound pick-up. Extraneous direct or reflected sounds
approaching the microphone from side directions and from the
rear will have little or no effect and therefore background noises
and reflected sounds in the broadcast are considerably re-
duced, which increases, by comparison, the quality of direct
sounds reproduced. e amount of sound-proofing necessary
for sound originating in the "dead zone" can be greatly re-
duced—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 pur-
poses, the directional characteristic is of considerable value in
reducing feedback effects between the microphone and the
loudspeaker.
Sound concentrators and baffles used with condenser micro-
phones are unnecessary with and inapplicable to the uni-direc-
tional microphone because of the fundamental difference in
the principle of its operation.
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