MIL-PRF-26542F
4.5.15 Pressure equalization (see 3.5.17). Following the initial readings and examination as stated
below, the microphone element under test shall be placed within the test chamber and the absolute
internal pressure of the chamber reduced to 3.44 inches of mercury (corresponding to an altitude of
50,000 feet above sea level) and the microphone element shall be allowed to remain at atmospheric
pressure limit for 5 (five) minutes. The rate of change of pressure shall correspond to approximately
50,000 feet per minute. The microphone element under test shall be subjected to 3 (three) each pressure
cycles. The impedance at 1,000 Hz, sensitivity, and frequency response characteristics of the
microphone element shall be measured and a mechanical inspection shall be made before and (1 hour)
after completion of the tests.
4.5.16 Explosive decompression (see 3.5.18). Following the initial readings and examination as stated
below, the microphone element under test shall be subjected to 10 (ten) consecutive explosive
decompression cycles from an altitude of 8,000 feet to an altitude of 35,000 feet (corresponding to
pressures of 22.2 inches of mercury and 7.04 inches of mercury respectively). Each complete
decompression or change of pressure shall take place in 0.1 (one tenth) second or less. The impedance
at 1,000 Hz, sensitivity, and frequency response characteristics of the microphone element shall be
measured and a mechanical inspection shall be made before and (1 hour) after completion of the tests.
4.5.17 Salt fog (see 3.5.19). Following the initial readings and examination, the equipment shall be
tested in accordance with method 509 of MIL-STD-810. The following details shall apply:
a. No operation is required while in the chamber.
b. Physical and mechanical examination: Failure criteria shall be limited to corrosion of finishes and
metal. Such corrosion shall be defined as any visible degradation of microphone surfaces that can
be attributed to flaky, pitted, blistered, or otherwise loosened finish on the metal surface.
c. Measurements after test: Frequency response and impedance.
4.5.18 Vibration (see 3.5.20). Following the initial readings and examination, the equipment shall be
tested in accordance with method 201 of MIL-STD-202. The equipment shall be mounted in a horizontal
position, and shall be vibrated in a direction perpendicular to the plane of the microphone face for a
period of 5 (five) hours.
4.5.19 Bounce (see 3.5.21). Following the initial readings and examination as stated below, the
product shall be tested in accordance with MIL-STD-810, method 514.4 ("Vibration"), Procedure III
("Loose cargo transport"). The test shall simulate conditions in which the part is transported as
unsecured cargo on a military-transport vehicle traversing irregular surfaces, having the freedom to
bounce, scuff or collide with other items of cargo or with the sides of the vehicle. This test is considered
to exceed the conditions of commercial transport during initial product delivery to the Government. The
following test exceptions and details apply:
a. Test apparatus. The specified typical package tester, or equivalent, shall be used, having a base of
plywood and individual test chamber(s) with walls of plywood (no roof), forming a 6X6 inch test
area(s), where the base has been secured with bolts or six-penny nails (tops of heads at, or slightly
below the surface). The bed of the package tester shall move in a circular path in a vertical plane to
the axes of the shafts.
b. Test procedure. The specified package tester shall be operated at not less than 1-inch double
amplitude and 284 rpm, for a total time of 3 hours. At each 1/2 hour interval, the test item shall be
turned so as to rest on a different face, so that at the end of a 3-hour period the item has rested on
each of its six faces (top, bottom, sides, and ends).
Following the test, the part shall be inspected for failure-mode defects, and the sensitivity, frequency
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