Hearing protection against high-level shooting impulses in relation to hearing damage risk criteria

The earmuff attenuation of acoustic impulses produced by large-caliber weapons was measured with a high-speed microcomputer controlled unit. The estimated accuracy was +/- 1 dB in peak sound-pressure level measurements. The peak levels outside earmuffs were 184 dB for the heavy bazooka and 172 dB for the hand-held bazooka (re: 20 microPa). Heavy bazooka impulse peak levels were attenuated from 7 to 19 dB by the earmuffs depending on the mass and volume of the measured three types of earmuffs. Hand-held bazooka impulse peak levels were attenuated by the earmuffs from 9 to 15 dB. The risk limits for hearing loss from a single impulse were exceeded in spite of the use of earmuffs when the criteria of CHABA (USA) or Pfander (Germany) were applied. The unexpectedly low attenuation was due to the low-frequency waveform of the high-level impulses. The earmuffs were found to prolong the impulse duration, which may reduce the benefit otherwise achieved by attenuation of the peak levels.