MIL-PRF-26542F
4.5.2.2 Sensitivity at ground level (see 3.5.1). The sensitivity shall be determined by the voltage
developed by the microphone across a resistive load (as specified, see 3.1) at 1,000 Hz. The output
voltage developed by the microphone shall be measured on an electronic voltmeter. Measurements shall
be made at ground level.
4.5.2.3 Sensitivity at altitude (see 3.5.2). On microphones which are used in high altitude conditions,
the sensitivity shall be measured as specified in 4.5.2.2, except the measurements shall be made at a
simulated altitude of 25,000 feet, unless otherwise specified (see 3.1).
4.5.2.4 Frequency response at ground level (see 3.5.3). The output voltage of the microphone shall be
measured across a resistive load (as specified, see 3.1) over the frequency response range. The
moisture barrier shall be in place during the test.
4.5.2.5 Frequency response at altitude (see 3.5.4). The frequency response shall be measured as
specified in 4.5.2.4, except the recordings shall be made at a simulated altitude of 25,000 feet, unless
otherwise specified (see 3.1).
microphone and the open circuit voltage VO at the microphone output shall be measured. A calibrated
variable resistor shall then be connected across the microphone output terminals. The variable resistor
shall then be adjusted until the voltage across it is VO/2 (one half of VO). The resistance of the variable
resistor shall then be recorded as the microphone impedance.
4.5.4 Distortion (see 3.5.6). The total harmonic distortion of the microphone shall be measured by
means of a distortion analyzer of the wave analyzer or total harmonic distortion type; or, at the option of
the contractor, the total harmonic distortion may be determined by the following formula:
2
2
2
E2 + E3 + ....EN
Percent harmonic distortion = 100 x
2+
2
2
2
E2 + E3 + ....EN
E1
Where E1, E2, E3, ... EN are the output voltage of fundamental frequency and the second, third, and
higher harmonics, respectively, developed across a resistive termination equal to the source impedance.
In performing harmonic distortion measurements, precautions shall be taken to ensure that the sound
pressure at the "nontalking" port has a level not less than 15 dB below the sound pressure level at the
"talking" port. Unless otherwise specified (see 3.1), the measurement shall be made over the frequency
range of 200 Hz to 6,000 Hz with an acoustic input having a pressure level of 114 dB relative to 20 µPa at
the talking port.
4.5.5 Signal-to-noise (see 3.5.7). For the purpose of qualification, the noise-cancellation
characteristics shall be determined with the test procedures outlined in 4.5.5.1 through 4.5.5.1.3.2,
inclusive. For the purpose of quality assurance, the noise-immunity characteristics shall be determined
using the test procedures outlined in 4.5.5.1 through 4.5.5.1.3.2, inclusive or 4.5.5.2 through 4.5.5.2.2,
inclusive at the option of the manufacturer.
4.5.5.1 Noise-cancellation characteristics (see 3.5.7.1). The noise-cancellation characteristics of the
microphone shall be determined by a comparison of the sensitivity and frequency-response
characteristics to a "close" sound source with its sensitivity and frequency-response characteristics to a
"distant" sound source.
4.5.5.1.1 Frequency-response and sensitivity to a "close" sound source for noise-immunity
characteristics measurements. The frequency-response of the microphone at ground level shall be
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