MIL-DTL-81963C
4.10.7 Audible noise, structureborne. When specified in the applicable documents and the
specification sheet that cite MIL-DTL-81963 as a direct reference, the servocomponent shall be mounted
on the noise test fixture shown on figure 15, unless specified otherwise in the applicable general
specification, by means of the appropriate standard mounting clamps. A calibrated accelerometer shall
be fitted to the mounting plate successively in each of the positions shown on figure 16. The surfaces of
the accelerometer and its attachment area shall be coated lightly with suitable grease. The
accelerometer shall be attached to the mounting plate with a 10-32UNF-2A stud or bolt whose threads
shall be coated with suitable grease and engaged at least 0.25-inch deep in the tapped hole in the plate.
The torque used in the method of attaching the accelerometer to the plate shall be as recommended by
accelerometer manufacturers. The fixture, complete with servocomponent, shall be suspended as shown
on figure 16, in such a manner that the shaft axis and the top edge of the mounting plate are horizontal to
within 5 degrees. With the servocomponent unenergized, the prevailing ambient noise level for the
broad, one-third octave and narrow bands shall be at least 14 dB below the specified maximum noise
limit for the servocomponent in that band or at that frequency. Those windings designated in the
applicable general specification shall then be energized as specified therein and the vibration level of the
servocomponent shall be determined and be recorded for each frequency band. Unless otherwise
specified, the one-third octave and narrow band measurements shall be conducted in that axis of the
servocomponent for which the maximum broadband vibration level is recorded. The structureborne noise
generated in the frequency range of 20 Hz to 9,600 Hz, expressed in terms of acceleration dB above a
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reference acceleration of 0.001 cm/second (9.8 x 10 g) shall not exceed the limiting value or values
specified in the applicable documents and the specification sheet that cite MIL-DTL-81963 as a direct
reference. Broadband structureborne noise, expressed in terms of acceleration dB, shall be measured by
means of a calibrated transducer and vibration-level meter which, including cables and accessories, have
a frequency response known to within ± 2 dB over the frequency range of 20 Hz to 9,600 Hz. The
equipment used shall possess a low response to magnetic, electrical and acoustic fields. No correction
for extraneous response shall be permitted in determining the structureborne noise level of a
servocomponent. The response of the equipment to extraneous fields shall be at least 14 dB below the
maximum noise limits specified for the servocomponent in the applicable documents and the specification
sheet that cite MIL-DTL-81963 as a direct reference. One-third octave band structureborne noise shall be
measured with the same equipment that is used for the broadband noise measurements, but with the
addition of one-third band filters covering the range 20 Hz to 8,000 Hz. The filters shall have rejection
characteristics beyond the band edges at least as good as those required by MIL-STD-740 and
ANSI S1.11 for octave band filters. The complete system, including cables and accessories, shall have a
response known to within ± 2 dB at all band centers within the range. The scanning rate used for the test
shall be not less than 5 seconds per band. Narrow-band structureborne noise shall be measured with the
same equipment as is used for the broadband noise measurement, but with the addition of a vibration-
level recorder and a narrow-band analyzer capable of being turned continuously or of being swept
through band-center frequencies from 20 Hz to 9,600 Hz. The complete system, including cables and
accessories, shall have a response known to within ± 2 dB at all band-center frequencies in the range.
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The scanning or sweeping rate used for the test shall not exceed (one-half x filter width in Hz) or 200 Hz
per second. The servocomponent shall meet the requirements as specified in 3.7.7.
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