MIL-DTL-13253E
4.4 Inspection conditions.
4.4.1 Conditions. Unless otherwise specified herein, all inspections shall be performed in accordance
with the test conditions specified in the "GENERAL REQUIREMENTS" section of MIL-STD-202. All
acoustical testing, except operational testing, shall be made in an anechoic chamber, which is the
equivalent to the free field acoustic environment. Free field conditions are those that approach
acoustically those of free space. An anechoic room should be considered satisfactory, if the sound
pressure varies inversely as the distance, within ± 1 dB (decibel), over the measuring frequency range,
under the following conditions: the sound source is effectively a point source (that is, emits spherical
waves) and is located at the position or positions of the reference point1/ of the device under test; the
microphone is located at the position or positions it will occupy during the test; and the distance referred
to is the distance between the reference point1/ of the effective point source and the microphone.
4.5 First article inspection (see 3.2). First article inspection shall be performed by the supplier, after
award of contract and prior to production, at a location acceptable to the Government. First article
inspection shall be performed on sample units, which have been produced with equipment and
procedures normally used in production. First article approval is valid only on the contract or purchase
order under which it is granted, unless extended by the Government to other contracts or purchase
orders.
4.5.1 Sample size. Nine (9) handsets shall be subjected to first article inspection.
4.5.2 Inspection routine. The sample shall be subjected to the inspections specified in table II, in the
order shown. All sample units shall be subjected to the inspections of group I. The sample shall then be
divided equally into three groups and separately subjected to the inspections of groups II, III, and IV.
4.5.3 Failures. More than one failure in group I and any failure in group II, Ill, or IV shall be cause for
refusal to grant first article approval.
1/ For single direct-radiator drivers having plane diaphragms, the reference point should be the center of
the diaphragm; and for those having conical diaphragms, the reference point should be the center of a
plane passing through the outer edge of the cone.
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