6 MICROCOPY RESOLUTION TEST CHART NAflO NA ~ E~UREAU OF STANDARDS 960A

~~ ~ The current damage—risk criteria (10) which define thelimitations for exposure to continuous noise are specified in terms of the A—weighted levels of noise rather than the unweighted, abso-lute sound pressure levels of noise. A—weighted levels are derived LU via an electrical network found in sound measurement equipment. The ~~ ~ effect of A—weighting is the de—emphasis or the measurement reduc—LL... tion of the levels of the low—frequency components of a noise. For example, absolute sound pressure levels or octave—band levels appear C.3 systemically lower by as much as 70 dB at 10 Hz, 26 dB at 63 Hz, and ~~~ ~.8 dB at 800 Hz when measured through an A—weighting network (1). By ~~~~~~~ ~ 9~~ ~ specifying damage—risk criteria in terms of A—weighted levels, theimplicit assumption is that high—intensity, low—frequency sounds are not as harmful to hearing as are high—intensity, high—frequency sounds. This is of particular concern to the Army because of the large number of vehicles within the Army inventory which generate low—frequency noise at very intense levels. For example, analysis of the running noise generated by the Infantry Fighting Vehicle (IFV) or Cavalry Fighting Vehicle (CFV; formerly designated as the Mechanized Infantry Combat Vehicle, MICV) indicate the presence of high—intensity, low—frequency noise. In general, the greatest amount of acoustic energy occurs below 250 Hz with many low— frequency components exceeding a sound pressure level of 100 dB. The unweighted intensity levels found at 63 Hz are frequently in excess of 120 dE sound pressure level (9)