MIL-DTL-21393B
4.5.1.3.1 Sampling plan. A sample of parts shall be randomly selected in accordance with table V. If
one or more defects are found, the lot shall be rescreened and defects removed. After screening and
removal of defects, a new sample of parts shall be randomly selected in accordance with table V. If one
or more defects are found in the second sample, the lot shall be rejected and shall not be supplied to this
specification.
TABLE V. Group B sampling plan.1/
Sample size
Lot size
Group B
2 to
25
2
26 to
50
3
51 to
90
4
91 to
150
5
151 to
280
6
281 to
500
7
501 to 1,200
8
1,201 to 3,200
9
3,201 to 10,000
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1/ For quantity related to reliability
test, see table VIII.
4.5.1.3.2 Disposition of samples. Samples, which have passed the group B inspection, may be
delivered on the contract, if the lot is accepted and the samples are still within specified electrical
tolerances. When the life test is specified in the contract, the samples used for this test shall not be
delivered on the contract.
4.6 General test procedure.
4.6.1 Test circuit apparatus. The fixtures used to position the calibrating microphone or the test
microphone element, with respect to the sound source, shall be designed to minimize any possible error
in acoustical measurements owing to vibration, sound transmitted from the voice tube, unstable
positioning of the microphone elements, reflections from fixtures or environment (such as the interior of
an altitude chamber), and ambient noise. No baffles shall be used near the sound sensitive parts of the
microphone elements. The test circuit apparatus shall be assembled and connected as shown on
figure 2 and shall consist of the equipment described in the following paragraphs.
4.6.1.1 Audio oscillator. The audio oscillator shall have a waveform distortion not greater than 0.5%. It
shall be used with a high quality amplifier, capable or driving a Western Electric 555 speaker, or
equivalent, up to a level of 120 dB above 20 µPa.
4.6.1.2 Voltmeter. The digital voltmeters (DVM) used in the test circuit shall have a flat frequency
response (±1 dB) from 100 to 10,000 Hz and shall be capable of measuring voltages from 0.001 to 10
volts root mean square (rms).
4.6.1.3 Sound source. The sound source shall be a Western Electric 555 speaker, or equivalent. The
distortion in the output of the sound source shall be such that the second harmonic is at least 35 dB, the
third at least 40 dB, and the fourth at least 45 dB below the fundamental.
4.6.1.4 Signal-to-noise radio test apparatus (see 3.6.6 and 4.6.3.4). The signal-to-noise ratio test
apparatus shall be similar or equal to that detailed on figure 3 and the test circuit similar or equal to that
shown on figure 4.
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