Downloaded from https://www.everyspec.com MIL-STD-810D 19 July 1983 METHOD 515.3 ACOUSTIC NOISE SECTION II ,, noise tests can be performed using a reverberant power and size. A reverberant teet ohamber should times the test item volume, With the chamber empty, the distribution of overall sound pressure levels should be uniform to within -2 and +4 dS of the desired value. If no test chamber is available, the noise field behind a jet engine can be used for acoustic noise testing, provided desired uniformity of test environment can be achieved.
The spectrum and overall level should be meaaured and the test item placed in a suitable location to achieve best approximation to desired test conditions. It is difficult to achieve as uniform an acoustio environment as with a reverberant test chamber. This approach may be suitable primarily for development, test- analyze-fix, and environmental worthiness tests. However, because of.the difficulty in controlling test conditions, this approach is not alwaya suitable for qualification testing. u-l test have AEEMMUS. Acoustic chamber of sufficient a volume at least ten II-1.1 ~. The acoustic test facility shall be able to produce acoustic noise at the desired levels and frequency range:
Frequencies outside the desired frequency range may be inadvertently produced by the acoustic test facility but do not need to be controlled. The acoustic environment shall be within the tolerances for the particular test procedure. Step 1. Choose which test procedure shall be conducted (I-4.1). Step 2. Determine overall acoustic test levels, duratione, and spectra shapes to be produced during$testing. Step 3. Prepare test item in accordance with General Requirements, 4.5.2. ~he item shall be in the.operational configuration. Step 4. Mount test item. For Procedur; IV, go to step 7. Suspend test item by means of springs or cords.
If a mounting structure is required between the soft suspension and the test item or to hold the soft suspension, care must be exercised to assure that no spurious acoustic or vibratory inputs are introduced. The natural frequency of suspension shall be less than 25 HZ. The test item shall be exposed on every surface to the sound field by centrally locating it in the test chamber. The test volume shall be no more than 10 percent of the test chamber volume. When the test chamber is rectangular, no major surface of the test item shall be installed parallel to the chamber wall. Step 5. For t~sting of asaembled externally-carried stores, go to step 6.
Position at leaat three microphones to measure the sound pressure field. These microphones shall be located in proximity to each major diaeimilar tegt item eurface, at least 45 cm (18 inches) from the test item surface or one-half the distance to the nearest chamber wall, whichever is less with the chamber empty. The average overall sound pressure distribution around the test item shall be measured and be uniform witbin -2 and +4 dB of the desired value. MSTHOD 515:3 19 July 1983 515.3-13