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ACOUSTIC CONSIDERATIONS
A. To modify the 488A for a normally open microphone
switch, proceed as follows:
1. Remove the three #5-40 round head screws
from the back of the microphone case. (Caution:
the microphone cartridge is spring loaded,
and care must be taken to hold the two halves
of the microphone case firmly together).
2. Separate the case front and back and remove
switch button. See Figure C.
3. Remove the #5-40 round head switch mounting
screw and lock washer, holding the switch
assembly in the case back.
4. Disconnect or clip the white cable lead from the
switch solder lug.
5. Strip the insulation from the white lead ap-
proximately 6 mm
(VI
in.) and solder to unused
lug at the bottom end of the switch.
NOTE:
In certain instances, if switch sequence is
critical, it may be necessary to remove the effect
of the shorted switch on the cartridge. This can be
done by insulating the shorting contacts with a
piece of "spaghetti" tubing or a piece of insulating
tape.
6. Re-assemble the switch in the case back, using
screw and lock washer removed previously.
7. Replace switch button in case back. Also be cer-
tain that the cartridge-shield retaining springs
are in the proper position.
8. Place case back on case front and firmly press
cases together. Fasten with the three #5-40
screws and washers previously removed.
The Model 4888 is supplied with the microphone
circuit opened by the switch in the normal position of
the switch. The microphone may be modified for use
in applications requiring the microphone circuit to be
shorted in the normal position of the switch.
B. To modify the 4888 for normally closed microphone
switch, proceed as follows:
1. Remove the three #5-40 round head screws
from the back of the microphone case. (Caution:
the microphone cartridge is spring loaded and
care must be taken to hold the two halves of the
microphone case firmly together).
2. Separate the case front and back and remove
switch button. See Figure C.
3. Remove the #5-40 round head switch mounting
screw and lock washer, holding the switch as-
sembly in the case back.
4. Disconnect or clip the white cable lead from the
switch solder lug.
5. Strip the insulation from the white lead approx-
imately 6 mm
(1/4
in.) and solder this white lead
to the switch blade next to the blade with the
green lead connections. This is the blade that
the switch button pushes against. The outside
solder lug may be bent out of the way to aid
soldering.
6. Re-assemble the switch in the case back using
screw and lock washer removed previously.
7. Replace switch button in case back. Also be
certain that the cartridge shield retaining springs
are in the proper position.
8. Place case back on case frogt and firmly press
cases together; fasten with the three #5-40
screws and washers previously removed.
The important characteristics contributing to the
total noise reduction of the 488 Series Microphones
are shaped frequency response, directionality and
distance and direction discrimination.
The acoustical elements are arranged to optimize
performance in the frequency range of 200 to 4000 Hz.
The results achieved are the elimination of all sounds
outside of the desired frequency range and proper con-
trol of those sounds within the desired range.
The 488 microphones reject noise through their in-
herent directional properties and discriminate against
unwanted sounds arriving from a distance in favor of
sounds arriving from a near source.
The microphones operate on the pressure-gradient
principle and have two sound entry ports, spaced a
small distance apart.
In order to obtain the best noise cancelling perform-
ance with the Model 488 Series, the microphone must
be used as "close talking." The lips should be as close
to the microphone grille as comfort will permit, with a
distance of approximately 6 mm to 25 mm ('/4 in. to
1 in.) providing the greatest discrimination between
the voice sounds and background noise.
No special precautions beyond ordinary care are
necessary in operation of the 488 Series microphones.
They will operate very satisfactorily under all ordinary
conditions of humidity, heat, and cold. Dropping the
microphones or other severe mechanical shocks
should be avoided.
SPECIFICATIONS
Frequency Response:
200
-
4000 HZ
Output Level (at 1000 Hz):
Model 488A
Open Circuit Voltage
.
.
. .
.
.
-
17.0 dB* (.141V)
EIA Microphone Rating
Gm (Sensitivity)
. . . . . . . .
-
147.0 dB***
Model 4888
Open Circuit Voltage
. . . . . .
-
37.0 dB* (.014V)
Power Level (into 200 ohms)
-
56.0 dB* *
EIA Microphone Rating
Gm (Sensitivity)
. . .
.
.
.
.
.
-
148.5 dB*
*
* O dB
=
1 volt per 100 microbars
"'0 dB
=
1 milliwatt with 10 microbars
***Reference, EIA Standard SE-105, August, 1949.
Microbar
=
one dyne per square centimeter.
Measurements at 9.5 mm (% in.) from sound source.
Recommended Load Impedance:
Model 488A
. .
. . . . . . . . . .
. .
.100,000 ohms or more
Model 4888
. .
. .
.
. . . . . .
.
. .
. . . . .
.I50 to 250 ohms
Cables:
Attached, recoils to 0.3m (1 ft) extends to 1.5m (5 ft)
Case:
High impact gray "Armo-Dur"
Dimensions:
See Figure D
Net Weight:
340 grams
(3/4
Ib)
Shipping Weight:
510 grams (1 ' Y E Ib)
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