Dependence of acoustic attenuation of hearing protectors on incident sound level.

The relationship between incident sound level and acoustic attenuation for four types of earplug and four types of earmuff have been investigated using freshly prepared and instrumented cadaver ears. Pure tones and 1/3-octave bands of random noise in the frequency range 125-8000 Hz were employed as steady-state stimuli with sound pressure levels between 75 and 125 dB. Impulses with peak sound levels in the range 135-175 dB(P) were also presented. For the steady-state signals employed, the eight hearing protectors have been shown to have constant attenuation characteristics over the range of incident sound levels investigated. This was also the case for the six conventional protectors (with intentionally linear characteristics) for the impulse stimuli. The two intentionally amplitude-sensitive protectors provided attenuation which increased with incident sound level for impulse noises. Comparison of the protector attenuation-frequency characteristics determined for steady-state sounds shows good agreement with those obtained from subjective (threshold shift) national standard measurement procedures. It may be concluded, therefore, that the six conventional hearing protectors studied here have attenuation characteristics that are equal for incident sound levels at about 40 and 75 dB, and that they are constant for levels between 75 and 175 dB. Consequently, the results of national standard threshold-shift procedures, although measured at low sound levels, may be applied with confidence to occupations where hazardous high-level noises are present.