MIL-PRF-24751A(SH)
4.6.2.2 Stall recovery capacity. This method is capacity at which the fan recovers from a stalled condition
when the fan is operating in a stalled condition and the discharge throttle is slowly opened while moving towards
free delivery.
4.6.2.3 Stable start-up capacity. This method is capacity at which stable, stall-free operation first occurs, as the
fan is started from a no-spin condition with the throttling device fixed as progressively larger openings.
4.6.3 Airborne noise. Octave band sound power levels for a fan-motor unit shall consist of the measurement of
sound pressure levels and the computation of sound power levels in accordance with procedures and instrumentation
requirements defined in AMCA 300, and amendments to those procedures as identified herein. The fan-motor unit
shall be operated at the design point during airborne noise testing, see 3.15.3. All sound measuring instrumentation
shall be laboratory calibrated within one year of the date of test, excluding the reference sound source, which shall
comply with the laboratory calibration requirements stated in AMCA 300. A random incidence response
microphone and a type I (precision) sound level meter conforming to the requirements of AMCA 300 shall be used.
The minimum distance between the extreme points of the microphone swing as described in AMCA 300 shall be 27
ft., and the speed of traverse of the microphone shall not exceed 2.4 ft./sec. Sound pressure readings shall be a time-
weighted average over an integral number (one or more) of microphone swings. Sound pressure level readings shall
be averaged over a minimum period of observation of 30 sec. The fan shall be mounted on resilient mounts, and
flexible duct sections shall be installed between the fan flanges and any duct sections required for testing. Resilient
mounts shall be loaded within the design load range for the type of mount selected. The frequencies of the natural
modes of vibration of the fan mounted in the vertical direction shall not exceed 11 Hz or one quarter of the lowest
forcing frequency of the mounted unit, whichever is lower. When a duct section is attached to a fan-motor unit to
facilitate measurements, it shall have the same interior, open cross sectional dimensions as that of the fan-motor unit
to which it is attached. There shall be no exposed sound absorbing material on the interior or exterior surfaces of the
attached ducts. No sound attenuating devices shall be installed within attached ducts. An orifice plate shall be
installed if necessary to obtain design pressure at a known operating point. If an orifice plate is installed, it shall be a
quiet type, which does not produce excessive flow-induced noise.
4.6.3.1 Total sound power. Test procedures and calculation procedures to determine total sound power levels
for a fan-motor unit shall be in accordance with Appendix H of AMCA 300, installation type B: free inlet/ducted
outlet, and Figure 4 of AMCA 300. The fan-motor unit and attached discharge duct with orifice plate shall be placed
entirely within the reverberant test room.
4.6.3.2 Casing sound power. 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 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.
4.6.3.3 Inlet sound power. Test procedures and calculation procedures to determine inlet sound power levels
for a fan-motor unit shall be in accordance with Figure 2 of AMCA 300, installation type D: ducted inlet/ducted
outlet. No orifice plate shall be installed in the inlet duct. The fan-motor unit and attached discharge duct with
orifice plate shall be placed entirely within the chamber, and sound pressure level measurements shall be made
within the reverberant test room. Acoustical treatments shall be placed within the chamber to provide an acoustic
seal at the penetration in the wall separating the chamber from the reverberant room as necessary to ensure that no
noise contributions are transmitted from the chamber into the reverberant test room.
4.6.3.4 Outlet sound power. Test procedures and calculation procedures to determine outlet sound power levels
for a fan-motor unit shall be in accordance with Figure 3 of AMCA 300, installation type B: free inlet/ducted outlet.
An orifice plate shall be installed within the discharge duct if necessary to obtain fan-motor unit operation at the
design point. The fan-motor unit with attached discharge duct shall be placed within the chamber. A section of the
discharge duct, approximately 2 to 3 duct diameters long and housing an orifice plate, shall extend into the
reverberant test room, and sound pressure level measurements shall be made within the reverberant test room.
Acoustical treatments shall be placed within the chamber as necessary to provide an acoustic seal at the penetration
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