Acoustic Considerations: The front response-frequency
characteristic of the Model 55s is shown in Figure B. The smooth,
wide-range characteristic is excellent for high-quality reproduction
of music and speech.
“Ultra-Cardioid” describes the horizontal polar characteristic of
the Model 55s. This polar characteristic resembles a cardioid but
is slightly more directional. There is a wide, useful pickup angle
at the front of the microphone, while the response et the sides is
down 6 db from the front response. The rear response is down ap-
proximately 15 db. The Model 55s fulfills these requirements over
a broad range of frequencies. The true unidirectional character-
istic of the microphone should not be confused with the relatively
slight directional effect at high frequencies which can be produced
by baffle effects in the conventional pressure microphone.
Fig. B.
Response-Frequency Characteristic Model 55s Microphone.
27A62. 5-51.
The result of this unidirectional characteristic is elimination of
acoustic feedback at volume levels which would cause considerable
feedback with conventional semidirectional or omnidirectional mi-
crophones. In practically all cases it is possible to increase loud-
speaker levels when a Unidyne is installed. By directing the dead
side (rear) of the microphone towards the audience or other
source of interfering sound, pickup can be concentrated on the de-
sired source. Reverberation energy pickup is decreased approxi-
mately two-thirds. The microphone can be placed close to reflect-
ing surfaces without objectional effects if the rear side of the mi-
crophone is toward the reflecting surface. This is particularly valu-
able in small broadcast studios. It is desirable to experiment with
microphone placement end orientation in order to secure the
greatest benefits from the unidirectional characteristic.
Specifications
1000 C.P.S. Response:
Model 55s "L" Position
Open Circuit Voltage Level. . . . . . . . . . . . . . . . . — 87.9 db*
Loaded with 50 ohms . . . . . . . . . . . . . . . . . . . . . — 92.4 db*
Power Level into 50 ohms . . . . . . . . . . . . . . . . . . . . — 59.4 db**
RTMA Microphone Rating GM (Sensitivity)
. . . — 153.6
db***
Model 55s "M" Position
Open Circuit Voltage Level . . . . . . . . . . . . . . . . . — 81.0 db*
Loaded with 250 ohms. . . . . . . . . . . . . . . . . . . . . . . — 86.1 db*
Power Level into 250 ohms. . . . . . . . . . . . . . . . . . — 60.1 db**
RTMA Microphone Rating GM (Sensitivity). . .
— 152.8
db***
Model 55s "H" Position
Open Circuit Voltage Level. . . . . . . . . . . . . . . . . . — 60.5 db*
Loaded with 100,000 ohms. . . . . . . . . . . . . . . . . . . — 62.3 db*
(*)
RTMA Microphone Rating GM (Sensitivity). . .
— 157.0
db***
0 db = 1 Volt Per Microbar
(**)
0
db = 1 Milliwatt with 10 Microbars
(***) RTMA Standard SE-105, August 1949.
Recommended Load Impedance:
Model 55s "L" Position 30-50 ohms.
Model 55s "M" Position 150-250 ohms.
Model 55s "H" Position 100,000 ohms or more.
Code Word
Net Wt.
MODEL 55s
RUDOT
3 1/8 lb.
Shipping Weight
4 1/4 lb.
Cable
20 ft. Two-Conductor
Height, Overall * 7 3/8"
Height, Case
*
3 15/16"
Width
*
2 3/16"
Depth
3 1/16"
Finish
Satin Chrome
*See Fig. C.
Guarantee:
Each microphone is guaranteed to be free from
electrical and mechanical defects for a period of one year from
date of shipment from factory, provided all instructions are com-
plied with fully. In case of damage, return the microphone to the
factory for repairs. Our guarantee is voided if the microphone is
subjected to accident or abuse or if the case is opened.
MODEL 55s
Architect’s Specification
The microphone shell be a moving coil type microphone
with a frequency range of 50 to 15,000 c.p.s. This unit shall
have an “Ultra-Cardioid” horizontal polar characteristic. The
cancellation et the sides shall be approximately 6 db and
the cancellation at the rear shell be in the order of 15 db.
The microphone shall be equipped with a three-position im-
pedance change switch for adjusting the microphone rating
impedance to 38 ohms, 150 ohms or 40,000 ohms. The micro-
phone rating GM (sensitivity) at 1000 c.p.s. shell be within
± 3 db of the following levels:
"L" Position of switch — 153.6 db
"M" Position of switch — 152.8 db
"H" Position of switch — 157.0 db
RTMA Standard SE-105 August 1949
The microphone shall be provided with a swivel adjust-
able from 0° to 80° and it shell have a detachable cable
connector capable of connection to a two-conductor shield-
ed cable. The microphone will mount on stand having 5/8"-
27 thread. The overall dimensions shell be 7-3/8 ± 1/4 inches
in height, 2-3/16 ± 1/8 inches in width, and 3-1/16 ± 1/8 inches
in depth.
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