Proposed Aural Nondetectability Limits for Army Materiel
HUMAN ENGINEERING LAB ABERDEEN PROVING GROUND MD
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The parameters which affect the propagation and detectability of sound are discussed and assessed for the purpose of revising the aural nondetectability limits of MIL-STD-1474 1979. The factors considered are geometric spreading, atmospheric absorption, ground effect, atmospheric turbulence, refraction due to wind and temperature gradients, barriers, foliage, threshold of hearing, psychoacoustic factors, and background noise at the listeners location. Examples are given of the effect on sound propagation of varying the parameters, and several suggestions are offered for field expedient measures to control or detect sound. Standard conditions for sound propagation and detectability are proposed, along with two assumed background noise levels. These conditions and background noise levels form the basis for two proposed nondetectability limits. These limits, expressed in 13-octave bands, are provided for critical and typical military scenarios and a rationale for selecting between them is developed. Measurement procedures for determining conformance with the limits and an explanation of the methods and rationale used in computing the limits are also presented. The proposed limits are intended to provide nondetectability under most conditions likely to be encountered. However, sound propagation conditions are identified which might allow materiel meeting the limits to be detected. Keywords Aural Detection Sound Propagation, Background Noise, Ground effect.