Downloaded from https://www.everyspec.com MIL-STD-81OC 3.5.2 Test setup. The store should be softly suspended by means of springs or elastic cord. If a mounting structure is required between the soft sus- pension and the test item or to hold the soft suspension, care must be exercised to assure that no spurious acoustic or Vibrating inputs are intro- duced. The natural frequency of suspension shall be less than 25 Hz. A suitable acoustic noise source can be a jet engine on an open-air stand, a jet engine in test cell, or an acoustic test chamber. Provision for baffles shall be made so that the frequency spectrum can be shaped to the required profiles. 3.5.2.1 Microphone placement.
Three reference planes perpendicular to the longitudinal axis of the store shall be defined. The location of these reference.planes shall be such that they are at positions.which are one- sixth, one-half.,and five-sixths of the length along the store. In each reference plane three microphones, 120 degrees apart, shall be positioned about the store. Each microphone shall be located within 18 inches from the surface of the store but no greater than one-half the distance from the nearest baffle, whichever is less. 3.5.3 Test procedure. The response of the microphones which are in each reference plane shall be averaged giving one output for each reference plane.
The one-third octave band sound pressure level about the entire store shall be shaped so that at the location of each of these three reference planes the frequency spectral profile shall envelope the frequency spectral profile of figure 515.2-4 and the values of table 515.2-11. A : controlled acoustic environment below 100 Hz and above 2,000 Hz is not . required but these frequencies may be present because of the nature of the test facility. For low power acoustic facilities which cannot excite the entire frequency range at one time, it is permissible to break the test into smaller frequency segments.
The testing time for each of these segments shall be the same as required if the entire spectrum is excited simultaneously. ) 3.5.3.1 Captive flight test. Two tests, functional and endurance, shall be conducted using their respective test levels given in table 515.2-11. No endurance test is required if, for a l-hour endurance test, the test level Lo is equal to or less than the corresponding functional test level. During the functional test, the store shall perform according to the system specification operation requirements (see General Requirements, 3.2). This test shall be:for 1 hour.
Proper performance of the store is required only after conclusion of the endurance test (see General Requirements, 3.2). 515.2-5 METHOD 515.2 B , i.