Objective measurements of sound attenuation by hearing protectors used by Finnish Army personnel during shooting with firearms under field conditions were carried out. Peak impulse sound pressure levels (SPLs) were simultaneously measured outside and inside the earmuff with two microphones. For earplugs and combination earmuff/earplugs a probe microphone method was used. Acoustic impulses were sampled without frequency weighting, simultaneously in pairs with a 50,000 Hertz sampling rate per channel. The unweighted peak levels were the most suitable estimate for risk of damage to the inner ear. Weapons used were some hand held weapons, 105 and 130 millimeter cannons, and antitank bazookas. Results showed that the time delay between outside and inside signals was about 0.5 milliseconds. As rapid pressure fluctuations in the inside signals faded, a low pass filtering effect was seen. The highest peak SPLs were for the bazookas, and were in the range of 184 to 185 decibels (dB). For small, medium, and large volume earmuffs, peak level attenuations were 7, 14, and 19dB, respectively. Rise time and durations got longer, and peak frequency decreased in the earmuffs. For pistol and rifle shots, attenuations was 20 to 30dB. Attenuation decreased with increasing cup volume and mass. The authors conclude that comparison of laboratory and field experiments indicate good agreement in attenuation efficiency of hearing protectors for small caliber weapons, but with large caliber weapons, impulses capable of producing hearing loss even when wearing helmets were generated.
Proceedings, 1992 Hearing Conservation Conference, April 1-4, 1992, Lexington, Kentucky, Office of Engineering Services, University of Kentucky and NIOSH