Helmet-Mounted Displays: - USAARL - The - U.S. Army

Acoustical Performance 225
Figure 8.4. Noise level distribution of U.S. Army helicopters
with noise exposure levels for aviators while
wearing the HGU-56/P alone, with spectacles and
with CB Mask.
Ambient noise in dBA and estimates of noise levels at the ear shown in
Figure 8.1 provides insight as to the extent of hazard present in the aviation
environment and what potential the hearing protection schemes have to
adequately protect the aviator. The estimate should include data collected
under conditions the hearing protectors are normally used by the aviator.
If spectacles are commonly used, then sound attenuation must be
determined while using spectacles as shown in Figure 8.3 and noise
exposure effects as shown in Figure 8.4.
Speech Intelligibility (SI)
Speech intelligibility is generally determined with human listeners
evaluating word sets (ANSI, 1989). SI is a measure of ones ability to
recognize these words when presented through a system under test. Word
sets (Newby, 1972) are standard and comprised of phonetically balanced,
monosyllable words that occur often in everyday use of the English