MIL-PRF-26542F
4.4.2 Periodic inspection. Periodic inspection shall consist of group C. Except where the results of
these inspections show noncompliance with the applicable requirements (see 4.4.2.1.4), delivery of
products which have passed group A and group B shall not be delayed pending the results of these
inspections.
4.4.2.1 Group C inspection. Group C inspection shall consist of the inspections specified (see 3.1) in
the order shown. Group C inspection shall be made on sample units selected from inspection lots which
have passed the group A and group B inspection.
4.4.2.1.1 Sampling plan. Group C inspection shall be performed once each 6 months on six sample
units selected at random without regard to their quality from units produced during the period. The
sample shall be divided equally into two groups and subjected to the tests of group II and group III (see
4.4.2.1.2 Disposition of sample units. Samples which have been subjected to Group C inspection shall
not be delivered on the contract.
4.4.2.1.3 Failures. If one or more sample units fail to pass group C inspection, the sample lot shall be
considered to have failed.
4.4.2.1.4 Noncompliance. If a sample fails to pass group C inspection, the manufacturer shall notify
the qualifying activity and the cognizant inspection activity of such failure and take corrective action on the
materials or processes, or both, as warranted, and on all units of product which can be corrected and
which were manufactured under essentially the same materials and processes, and which are considered
subject to the same failure. Acceptance of the product shall be discontinued until corrective action,
acceptable to the qualifying activity, has been taken. After the corrective action has been taken, group C
inspection shall be repeated on additional sample units (all inspections, or the inspection which the
original sample failed, at the option of the qualifying activity). Group A and group B inspections may be
reinstituted; however, final acceptance and shipment shall be withheld until the group C inspection has
shown that the corrective action was successful.
4.5 Methods of inspection.
4.5.1 Visual and mechanical inspection. Microphones shall be examined to verify that the materials,
interface, design, construction, physical dimensions, markings, and workmanship are in accordance with
4.5.2 Sensitivity and frequency response.
4.5.2.1 General. Unless otherwise specified (see 3.1), the test fixture shall hold the microphone in
such a way that with the moisture barrier guard in place, the sound port is 0.25 inch from and coaxial with
the opening in the artificial voice. For the purpose of this specification, all sensitivity data shall use an
input sound pressure level (SPL) of 103 dB SPL re20µPa. The response of the microphone shall be
measured by means of a suitable ac voltmeter and with an applied sound pressure level of 103 dB re 20
µPa and shall be related back to 74 dB SPL re20µPa. The driver unit used to obtain the desired SPL
shall be Bruel and Kjaer Artificial Voice Type 4227 or equivalent and shall have been previously
calibrated by means of a condenser microphone such as W.E. 640 AA, or equivalent, (calibrated in
accordance with ANSI S1.15).
4.5.2.1.1 Test fixtures. 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
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.
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