Downloaded from https://www.everyspec.com MIL-STD-81OD 19 July 1983 ~ - 3 . 2 .. J & w h w u& w . The spectrum shape shall be determined from measured data, if available (see I-4.1). If data are not available,use the spectrum given in figure 515.3-1, with two exceptions: For assembled, externally carried aircraft stores, use the spectrum given in figure 515.3-2.“ For cavity regonanoe testing, uee the spectrum given in figure 515.3-4. 1-3.2.2 lmustic test level. The acoustic test level should be determined from measured data, if available (see I-4.1).. Otherwise, use the values suggested by table 515.3-I. a.
Procedures 1, II, and IV - The most severe acoustic stress conditions shell be used as test conditions. ,.. . b. Procedure III - The measurements shall be taken for the.entire : -. mission, including periods of low stress. For this procedure, the test level is varied over time, but the test does not exactly duplicate a mission in this respect. ‘ Test levels should be determined using the technique described in ~~ paragraph 1 - 3 . 2 . 3 . 1 . I - 3 . 2 . 3 M a t durati. For procedure III, teat duration should be determined as described in paragraph I-3.2.3.1. For all other procedures, determine the test duration by ‘referringto table 515.3-I. ., I-3.2.3.1 ~.
For airborne equipment,“the test mission profile should be based on the dynamic pressure histogram. Test cycle duration should equal the average duration of the missions.used to develop the histogram. The percentage of time spent at each dymmic. Pressure band level in the test cycle should be in proportiorito the relative contribution of the same level to the histogram. The test cycle, therefore, resembles a service mission with varying test levels as a function of time. The test oycle is then repeated until the desired number of ‘tmissionsnhas been accumulated. Each mission profile should be expressed in a dynamio pressure versus time profile rather than Mach number and altitude profiles.
The dynamic pressure profile for each mission is analyzed to develop a histogram of mission time spent at various ranges of dymmic pressure. This is accomplished as follows. Using the highest measured value of dynamic pressure (regardless of mission) as Qwx, sum all of the miesion time for which dynamic pressures were within 5 percent of ~ax. Then, sum all of the mission time for which dynamic pressures were between 0.95 Qmax and 0.90 Qmax. C?ntinue this process of ‘summingmission time for 5 percent increments of dynamic pressure until all values”of meaaured dynamic pressure are included.
For test purposes, the pressure levels can be determined using the midpoint dynamic preaaure value of the appropriate 5 percent dynamic pressure band. Thie value Will be assumed to be constant for the amount of flight time within this band. I-3.2.4 ~. The tolerance can be adjuated to reflect the purpose of the test and the available acouatic noise source. For environmental worthiness testa the upper limit on figure 515.3-1 can be ignored and the tolerances on figure 515.3-2 can be -3dB to plus infinitY. For,qualification testinu the tolerances should be as shown on figure 515.3-1 and for figure I 515.3-; they can be -3dB for each one-third octave band. MSTHOD 515.3 19 July 1983 515.3-4’