BRUEL & KJAER 4111 - Electrostatic Actuator for Control

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Fig. 15. Electrostatic Actuator type 4113.
an Electrostatic Actuator type 4113 for the control of both the fre-
quency response and sensitivity of the instrument.
This accessory, see fig. 15, consists of a fixed slotted plate which
can be placed parallel to the microphone diaphragm, good electrical
insulation existing between the two.
The distance between the car-
tridge membrane and the slotted plate is determined by three support-
ing glass points which may be adjusted by means of a screwdriver
from the upper side of the unit. (Normally the screws are sealed
at the factory after adjustment).
In fig. 16 the set-up is shown for recording the microphone' s frequency
response as well as for measuring its sensitivity. A large d. c. volt-
age E
0
,
together with a small superimposed a. c. voltage eeff' is ap-
plied between the actuator grating and the microphone diaphragm. The
d. c. supply is de rived from the Calibration Apparatus type 4119 to
which instrument the attenuator output of the B. F. 0., part of the Audio