MIL-DTL-55535A
TABLE VII. Signal spectrum.
Frequency
130
300
600
1,000
2,000
3,000
4,000
in Hz
Sound pressure in dB
relative to sound
0
+7
+8
+5
-3
-7
-9
pressure at 130 Hz
4.7.3.3 Measurement of the signal-to-noise ratio. The microphone shall be mounted in its test rig with
its front grid 0.25 (¼)-inch from the mouth of the "signal" speaker. The microphone shall be connected to
the test circuit shown on figure 4. The noise spectrum shall be applied for 3 seconds, and the output of
the microphone shall be measured. The noise spectrum shall be removed, and the signal spectrum shall
be applied for 3 seconds. The output of the microphone shall be measured. The dB difference between
the "signal" value and the "noise" value is the signal-to-noise ratio, and shall be in accordance with 3.6.3.
4.7.4 Dielectric strength and insulation resistance. The microphone shall be subjected to a voltage of
100 volts dc for 10 seconds, which is to be applied between the terminals of the microphone and the
insulated outer metal parts of the microphone. The microphone element shall be tested for compliance
with 3.6.4, before final assembly.
4.8 Earphone tests.
4.8.1 Earphone response test. Available power frequency response measurements shall be made
starting at 300 Hz and extending through 3,500 Hz, in sufficient detail to establish definitely the shape of
the curve. Response measurements shall be made in accordance with NCSL-Z540.1. The contractor
shall supply a 6 cc coupler. The outside dimensions of the coupler shall be such as to provide the proper
seating and sealing of the earphone.
4.8.1.1 Testing procedure. The earphone under test shall be connected to the test circuit and shall be
mounted on the 6 cc coupler as shown on figure 5. The output from the oscillator at each test frequency
shall be adjusted to 0.245 volts rms as measured by Voltmeter #1. The output from the calibrated
microphone and pre-amplifier unit shall be measured as indicated by the reading of Voltmeter #2. This
reading shall be converted to the equivalent dB value above 20µ Pa using the most recent available
calibration curve for the test microphone in use. The operations set forth above shall be performed from
300 Hz to 1,000 Hz, inclusive, at 100 Hz increments and from 1,000 Hz to 3,500 Hz, inclusive, at 250 Hz
increments. The response of the earphone shall meet the requirements specified in 3.7.1.
4.8.1.2 Test equipment. The test equipment used for the response test shall meet the following
requirements:
a. Calibrating microphone. A Western Electric Co. Q40AA condenser microphone, or equal, shall be
used for measuring sound pressure. It should be calibrated by the reciprocity method in
accordance with ANSI S1.15/Part 2.
b. Audio oscillator. The audio oscillator shall have a frequency range of at least 100 to 10,000 Hz,
shall have a high degree of stability in both output voltage and frequency, and shall have a
waveform distortion of less than 2 percent.
c. Vacuum-tube voltmeter. The voltmeters used must have flat frequency response (± 1 dB) from at
least 100 to 10,000 Hz and must be capable of measuring voltage from 0.0005 volt rms to 10
volts or more.
d. Microphone pre-amplifier. The microphone pre-amplifier shall have a flat response (± 1 dB) over
a frequency range of at least 100 to 10,000 Hz, shall have a high degree of stability, and shall
have a distortion of less than 2 percent.
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