MIL-DTL-21393B
3.6.4 Microphone current (see 4.6.3.2.4). For test purposes, the microphone shall be connected as
shown in the standard test circuit on figure 2, and the microphone DC current shall not exceed 35
milliampere (ma).
3.6.5 Voltage transients susceptibility (see 4.6.3.3). There shall be no degradation of performance or
any change in DC current of the microphone, as a result of the voltage transients test. Following such
exposure, the product shall exhibit the following performance:
Sensitivity and frequency response:
No more than ± 3 dB variation at any frequency.
3.6.6 Signal-to-noise ratio (see 4.6.1.4 and 4.6.3.4). Under standard conditions the signal-to-noise
ratio shall be at least 15 dB in favor of the signal.
3.6.7 Radio interference control (see 4.6.3.5). The microphone, in terms of the generation of radio
interference by the microphone and the vulnerability of the microphone to radio interference, shall be
designed to meet the requirements of MIL-STD-461. Audio susceptibility requirements, as applicable to
direct power input leads from either AC or DC aircraft power sources, shall not apply to the equipment
specified herein.
3.6.8 Distortion (see 4.6.3.6). At any frequency between 400 and 6000 Hz, when the sound pressure
at the face of the microphone is 115 dB above 20 µPa, the harmonic distortion in the output of the
microphone shall not exceed 4% under standard conditions or 5% under service conditions, except at
temperatures below -40ºC and above +55ºC where the harmonic distortion shall not exceed 8 percent.
3.6.9 Vibration (see 4.6.3.7). The microphone shall resist failure and degradation in performance due
to repeated excessive vibration. Following such exposure, the microphone shall exhibit the following
performance:
Visual examination:
No evidence of physical damage, which would cause a
malfunction.
Sensitivity and frequency response:
No more than ± 3 dB variation at any frequency.
3.6.10 Impact (see 4.6.3.8). When tested, the product shall exhibit the following performance:
Visual examination:
No evidence of physical damage, which would cause a
malfunction.
Sensitivity and frequency response:
No more than ± 3 dB variation at any frequency.
3.6.11 Altitude.
3.6.11.1 Pressure cycling (see 4.6.3.9.1). When tested for pressure cycling, the response
characteristic of the microphone unit shall not show a decrease in output in excess of 4 dB.
3.6.11.2 Pressure equalization (see 4.6.3.9.2). The response of the microphone at 1,000 Hz shall not
vary by more than 4 dB from the reference established at 15,000 feet while the microphone is subjected
to a pressure increase corresponding to a descent from an altitude of 15,000 feet (16.7 inches of
mercury) to sea level (30.0 inches of mercury) at a rate of 5,000 feet per minute.
3.6.12 Reliability test (see 4.7). The reliability test will be considered satisfactorily completed, when
the required operating time for all microphones under test has been achieved and all failures have been
resolved with corrective actions, which have been approved by the government inspector and the
procuring activity.
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