Downloaded from https://www.everyspec.com NIL-STD-81OD 19 July 1983 METHOD 523.0 VIBRO-ACOUSTIC, TEMPERATURE SECTION 11 II-1 ~ 11-1.1 Genera 1. facility must include a large acoustical noise test chamber capable of approximately a 155 dB intensity level and associated air- conditioning equipment to provide controlled, fixed air temperatures and rapid .- .V tampe,raturechanges (40c per minute) in the range -40° C to +85° C. Mechanical or hydraulic shakers capable of stressing the store(s) under test may also be required. Adequate instrumentation for controlling, monitoring, calibrating and recording the environment variables will be needed. b II-1.2 I&at chsmbw.
Chamber shape and dimensions shall provide for diffusion and uniform distribution of the acouetic field, and support reverberation of acouatic frequencies of 150 Hz and above. Ports mist be provided for introduction of the acoustic energy, for pressure ,stabilization(exit of modulator air), for entry and exit of temperature-conditioned air and for access by multiple electrical cables and waveguides, light beams, anechoic duct’ing,etc. as applicable. Some stores may require specialized test aPParatus such as artificial targets, r-f anechoic shrouds,.or visible gages which must be incorporated without compromising the combined environments. 11-1.3 tion e~.
A suitable acoustic energy field shall be provided by an “acousticpower source controlled,to.reproduce the acoustic mission profile. Typical apparatus oonsis~s of a constant-pressure compressed air”source such as a reciprocating compressor with pressure regulator feeding an air modulator that is acoustically coupled to the chamber through an exponential horn. The air modulator is excited electrically by an amplified audio signal. Considerable acouatic power is needed to reach required levels, often 10 to 30 KW; multiple modulator-horn units may be necessary to reach desired intensities.
To provide low-frequency vibration below about 100 Hz, electrodynamicsor hydraulic shakers may,be used to augment the acoustic field. Such shakers may also be used to provide limited mechanical shock impulses. To maintain access to the stores by the conditioned air and acoustic energy, suspended stores can be vibrated at low levels using a rod and collar arrangement to conduct the vibration from the shaker(s).. Procedure VI of method 514.3 will furnish some guidelines for this procedure. This arrangement is diagrammed in figure 523.0-7. ,, II-1.4 timDerature e~. Temperature conditioning of the store(s) under test must be accomplished without adversely affecting the acoustic * environment.
One process for accomplishing this is to duct high velocity conditioned air acrosg the stores by means of thin flexible shrouds that are acoustically transparent. They can be supported by light.metal frameworks. 19 July 1983 523.0-13 METHOD 523.0