MIL-PRF-24755B(SH)
4.7.3.3 Outlet sound power. Test procedures and calculation procedures to determine outlet sound power levels
for a fan-motor unit shall be in accordance with ANSI/AMCA 300 requirements for ducted inlet/free outlet test
configurations for fan outlet sound testing. An orifice plate shall be installed within the inlet duct if necessary to
obtain fan-motor unit operation at the design point. Acoustical treatments shall be placed within the chamber as
necessary to provide an acoustic seal at the penetration in the wall separating the chamber from the reverberant room
to ensure that no noise contributions are transmitted from the chamber into the reverberant test room.
4.7.3.4 Casing sound power. When this test is required, the requirement shall be as specified (see 6.2). Test
procedures and calculation procedures to determine sound power levels radiated from the casing of a fan-motor unit
shall be in accordance with Appendix G of ANSI/AMCA 300. Duct sections attached to a fan-motor unit to
facilitate testing to determine casing radiated sound power shall be rigid, heavy gauge metal ducts and/or solid,
double wall ducts to eliminate noise contributions transmitted through duct walls. The noise transmission loss of the
walls of the reverberant test room shall be such that noise transmitted into the reverberant test room from adjacent
spaces does not contribute to the sound power levels of the fan-motor unit. Penetrations in the walls of the
reverberant room in way of ducts shall be acoustically sealed to prevent noise contributions from being transmitted
into the reverberant test room from the adjacent room.
4.7.3.5 Airborne noise detected by ear. Fan-motor units not undergoing airborne noise testing described in
4.7.3.1 through 4.7.3.4 shall be operated at either their design points or at free flow. Any unusual airborne noise or
any airborne noise thought to be excessive shall be corrected if the cause is apparent. If the cause is not apparent
and the condition is not corrected, the fan-motor unit shall be tested in accordance with 4.7.3.1, and sound power
levels shall be computed to determine conformance with sound power level requirements specified in table III.
4.7.3.6 Airborne noise test facility. The facility wherein measurements of fan-motor unit airborne noise are
performed shall be registered in accordance with AMCA 111 as being a qualified facility in which to perform octave
band sound measurements in accordance with ANSI/AMCA 300.
4.7.4 Balance test. In this test, the fan-motor unit shall be operated at maximum speed and free delivery, with
shaft horizontal. It shall be suspended from a stand by means of two elastic vibration cords, one secured to each end
flange. The cords shall have a natural frequency of less than ¼ of the minimum rotational frequency of the unit in
Hertz. The static deflection shall be within the limits of MIL-STD-167-1 for elastic mounting elements, and this
shall be the basis for determining cord size for each unit tested. As an alternative to this suspension test, rigid
brackets may be bolted to the fan end flanges, and these brackets shall be elastically mounted on a rigid, level floor.
Another alternative is that the fan-motor unit may be resting on elastic mounts on a rigid level floor. Another
alternative is that the fan-motor unit may be suspended from an electric hoist by means of two chains, one secured to
the end flanges of the fan-motor unit (that is, the ends of the chain secured to opposite flanges); one secured to the
electric hoist and to the center of the chain secured to the end flanges. The length of the chains shall be not less than
15 inches nor more than 35 inches, and the link size of the chains shall be not less than ¼ inch nor more than 3/8 inch.
With either method, the internally excited vibrational displacement (single amplitude of vibration) shall be measured
in mils as follows: four axial measurements 90 degrees apart on the face of each end flange, one horizontal
measurement perpendicular to the axis on the edge of each flange, and one vertical measurement on the top edge of
each flange. The maximum amplitude of vibration shall be the maximum single reading, and not an average. The
operating frequency at which the maximum amplitude occurs shall be measured. Exceeding vibration limit
specified in 3.10.2 shall be cause for rejection.
4.7.4.1 Vibration test. Fan-motor units shall be subjected to Type I environmental vibration tests as specified
in MIL-STD-167-1. The exploratory vibration test specified in MIL-STD-167-1 shall include frequencies from
4 Hertz up to and including 33 Hertz at the table amplitude specified therein. The vibration test shall be conducted
in the order specified in 4.7.10. Any unit that fails to meet any requirements shall be rejected.
4.7.5 Casing tightness. When this test is required, the requirements as well as the test conditions shall be as
specified (see 6.2). If conditions are not specified, the test shall be conducted as follows: Blind flanges (one with air
supply and gauge connections) shall be bolted to gasketed fan flanges. Air at a pressure 50 percent higher than the
maximum pressure capability of the fan shall be supplied to the casing interior. The pressure drop in 10 minutes
shall not exceed 5 percent of the test pressure.
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