Downloaded from https://www.everyspec.com MIL-STD-810D. s 19 July 1983 1-3.2.5, source. The fluctuating pressure environment experienced in service is a complex combination of progressive wave and reverberant acoustic fields. Becauae of practical limitations, equipment testing is generally, accomplished in reverberant.laboratory facilities. If an appropriate laboratory facility is not available, a jet engine may be able to provide the required acoustic field. I-3.2.5.1 Eeverberan t testiqg. This technique is ueed when the pressure fluctuation source ia distributed. Such a source is turbulent boundary layer flow along a vehicle!s skin.
Reverberant testing ia also used for equipment located in closed spaces inside a vehicle exposed to strong localized acoustic sources. I-3.2.5.2 esslve wave testti. This technique is used to simulate a strong localized acoustic source. The accoustic energy aweeps over the test item like ripp~es spreading on water. This test environment ia appropriate for externally-carried components on all types of vehicles which are directly exposed to localized acouatic sources such aa rocket or reverberant test environment can be used to approximate environment but the level should be adjusted to account vibration efficiency of the.two types Of fields. ., jet engines.
A a progressive wave for the difference in 1-3.2.5.3 avitv rea~ tea ln~. t. It is recommended acoustic energy be used in the cavity resonance test. that sinusoidal I-4,.1 Accert.abe 1 measured data.1 Measured operational or flight teat data can be in either of two forms: (1) acoustic pressure levels, or (2) vibration I-4.’ SWUWWW& response of the teat item in.the field environment. For either form of the data, the data shall be analyzed using a constant percentage bandwidth no greater than one-third octave with a minimum of 50 statistical degrees of freedom. The data shall be over a frequency range of at least 20 to 2,000 -- and pre,ferably to 10,000 --..Hertz.
IY4.1.1 Acoustic operational or flight test data.. The 1evels and spectra measured in flight cannot be used directly as the test conditions for the flight conditions under which they were measured. They must be adjusted (recognizing that aerodynamic noise is less efficient than reverberant acoustic noise at frequencies before the first structural resonance and more efficient above that frequency) to compensate for the differences between flight turbulent boundary layer pressure fluctuations and a reverberant acoustic test environment. The parametric relationships specified in tables 515.3-II and 515.3-III can be used to extrapolate to other operational or flight conditions. .
1-4.1.2 ~a 1 or ~. ht test dti . Vibration can be ‘used indirectly to establish acouatic test levels. The teat item is instrumented with accelerometers,mounted identically as those used in operational or flight testing. The acoustic test levels and spectra are varied until the reproduced vibration matchea the previously measured operational or flight test data within MIL-STD-81OD method 514.3 tolerances for random vibration. This acoustic test environment can.be considered,as the operational or flight environment. The parametric relationships specified in tables 515.3-II and 515.3-III can be used to extrapolate to other conditions. . METHOD 515.3 19 July 1983 515.3-5