MIL-PRF-26542F
4.5.5.2 Noise-immunity characteristics (see 3.5.7.2). When mounted in the test fixture illustrated on
figure 1 and figure 2 with the talking port of the microphone 0.25 inch from the mouth of the signal
speaker, the noise spectrum shall be applied from the "noise source" and the microphone output
measured. The noise spectrum shall be removed and the signal spectrum shall be applied from the
"signal source" and the microphone output measured. The signal-to-noise ratio shall be the quotient of
these measured values.
4.5.5.2.1 Noise source. By means of a suitable test apparatus and circuit (see figure 1 and figure 2),
provision shall be made for the production of the noise spectrum, table III, at a rms sound pressure of 115
dB above a reference level of 20 µPa as measured at the microphone diaphragm. The noise source shall
be a loudspeaker capable of producing a sound pressure of 115 dB above a reference level of 20 µPa at
the rear as well as the front of the microphone and the loudspeaker shall be located as far away from the
microphone as practicable. The calibrating microphone shall be mounted in the test rig, in the box, 0.25
inch from the mouth of the signal speaker, which shall be short-circuited during adjustments of the noise
spectrum.
TABLE III. Noise spectrum.
Frequency in Hz
Sound pressure in dB relative
to sound pressure at 130 Hz
40
0
70
0
130
0
300
-5
600
-9
1,000
- 13
2,000
- 17
3,000
- 19
4,000
- 21
4.5.5.2.2 Signal source. By means of a suitable test apparatus and circuit (see figure 1 and figure 2),
provision shall also be made for the production of the following signal spectrum, table IV, at a rms sound
pressure of 115 dB above a referenced level of 20 µPa. The signal source shall be the driver unit. The
calibrating microphone shall be positioned 0.25 inch directly in front of the driver unit.
TABLE IV. Signal spectrum.
Frequency in Hz
Sound pressure in dB relative
to sound pressure at 130 Hz
130
0
300
+7
600
+8
1,000
+5
2,000
-3
3,000
-7
4,000
-9
4.5.6 Effect of external magnetic field (see 3.5.8). The sensitivity of the microphone to an external ac
magnetic field shall be measured by comparing the induced open-circuit voltage across the terminals of
the microphone, when placed in a 400 Hz magnetic field with the induced open-circuit voltage across the
terminals of a calibrating coil of known sensitivity placed in the same magnetic field.
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