Downloaded from https://www.everyspec.com MIL-STD-81OC high ,frequencylimitations of the shaker and vibration attenuation of the fixture and the equipment under test. Of importance is the fact that some equipment may possess both sensitive and insensitive characteristics,and it,may be difficult to ascertain; before a test is.performed, whether the equipment is sensitive to acoustic stimulation. The following criteria are presented as a guide to initially determine whether equipment is sensitive to acoustic stimulation. Such criteria cannot be considered as the only determining factors.
The final decision, whether to perform an acoustical noise test, “mustbe supplementedby such additional factors as a description of the characteristicsand duration of the end use acoustic environment, the location of the equipment within the vehicle structure, and a consideration of special mounting means or protective enclosures employed for the equipment. 3.2.1 Equipment insensitive to acoustic stimulation. This equipment is likely to have small surface areas, high mass to area ratios, and high internal damping. Examples are as follows: 1 a. High density modules, particularly the solid or encapsulated type. b.
Modules or packages with solid state elements mounted on small con- strained or damped printed circuit boards or matrices. I Valves, hydraulic servo controls, or auxiliary power unit pumps, d. Equipment surrounded by heavy metallic castings, particularly those that are potted or are encased within the casting by attenuatingmedia. I 3.2.2 Equipment sensitive to acoustic stimulation. This equipment is normally classified as microphonicsor usually having large compliant areas I of exposure, low mass to area ratios, or low”intern~l damping.- Examples are as follows: I a.
Equipment containing microphonicselements with high frequency reson- ances such as electron tubes, waveguides, klystrons, magnetrons, piezo- electric components, or relays attached to thin plate surfaces. b. Equipment containing or consisting of exposed diaphragmatic elements such as pressure sensitive transducers, valves, switches, relays, or flat spiral antenna units. 3.3 Selection of test intensity. The noise level; are divided into four intensity categories as listed in table 515.2-1. The categories are in order of”increasing severity (overall sound pressure level) from A through D. The category should be selected as appropriate for the expected noise level of the end use environment.
METHOD 515.2 515.2-2