MIL-PRF-26542F
4.5.20 Altitude, nonoperating (see 3.5.22). Following the initial readings and examination as stated
below, the equipment shall be tested in accordance with method 500, procedure I, of MIL-STD-810, with
a pressure in a standard atmosphere of 16.89 inches of mercury (corresponding to an altitude of 15,000
ft.). The frequency response shall be measured at 300 Hz, 600 Hz, 1,000 Hz, 2,000 Hz, and 3,50 0 Hz.
When operated at procedure I - step 4, the frequency response shall not vary more than 3 dB.
4.5.21 Moisture barrier seal (see 3.5.23). The microphone element shall be immersed in water and
pressure of plus one-quarter pound per square inch shall be applied to the pressure-equalization hole to
determine whether there are any leaks through the microphone element and around the moisture barrier
seal.
4.5.22 Immersion (see 3.5.24). The microphone shall be immersed to a depth of 3 feet in fresh water
at room temperature for a period not less than 2 hours. Upon completion, remove the unit. Shake
excess water from the microphone. Frequency response shall be measured and checked within 1 hour
for compliance with paragraph 3.5.3.
4.5.23 Fungus (see 3.5.25). The contractor may refer to 6.8, to certify that the materials used meet the
requirements for fungus-inert materials or shall test the materials used in accordance with the
requirements of method 508 of MIL-STD-810. The following details shall apply:
a. The complete assembly shall be tested.
b. Within 1 hour after the test the components shall be examined for evidence of physical damage.
4.5.24 Gun blast (see 3.5.26). Mount the microphone under the test on the carriage of the simulated
gun blast producing equipment in accordance with U. S. Navy Sketch SK-N-864 with the front edge of the
element in the test plane and with its axis coincident with that of the explosion chamber. Subject the
element to 30 rounds of blast at a peak pressure of 9.5 pounds / square inch (psi).
microphone bracket leakage shall be determined utilizing the apparatus shown on figure 3. Connect the
flow meter to the vacuum source and into the text fixture. Remove the front plate of the bracket and
insert the bracket into the gasket, install plate, and fasten securely. The contact area between the gasket
and the fixture shall be coated with petrolatum jelly to ensure proper sealing of the gasket and the test
fixture. With the vacuum adjusted to simulate 50,000 feet or 3.44 inches of mercury (corresponding to an
altitude of 50,000 feet above sea level), slowly apply the suction by opening the stop valve, check that the
gasket is properly seated, and allow the test fixture cavity to stabilize. The ball indicator in the flow meter
should rise initially and start dropping as the test chamber reaches stabilization. When the ball is
completely stabilized, take the reading of where the center of the ball sits on the scale. Then refer to the
calibration chart supplied with the flow meter to determine the rate of leakage in milliliters per minute
(ML/MIN). Repeat testing procedure for 80,000 feet or 0.92 inches of mercury (corresponding to an
altitude of 80,000 feet above sea level). The gasket shall simulate the mask-type as specified (see 3.1).
4.5.26 Boom finish (see 3.5.28). After finish has been applied, the part shall be wiped with a clean,
white cloth. There shall be no indication of residue of any kind on the cloth. Time between application of
the finish (or other finishing treatments to it), and this test, is optional.
4.5.27 Boom operating force (see 3.5.29). The boom shall be positioned such that the non-
microphone end is fixed, and the microphone end is holding a weight of 100 g. Under such loading, the
microphone end of the boom shall not move. The weight shall be replaced with a 250 g weight. Under
such loading, the microphone end of the boom shall move smoothly and slowly downward.
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