Insertion loss of hearing protection devices for military impulse noise

Abstract

Military operators are exposed to different kinds of impulse noise sources, including small calibre weapon fire, heavy artillery fire and low-level blast from breaching exercises.  For training purposes, limits for the number of exposures per day are indicated in training doctrines.  However, there is often no background information on how the daily limits are calculated and insufficient guidance on the use of hearing protection devices (HPDs).  With changing guidelines on impulse noise damage-risk criteria and evolving HPD technology, unit commanders often question the validity of the exposure limits and are uncertain about which HPDs to provide to their personnel.  We have measured the impulse noise levels of several different Canadian Armed Forces (CAF) and Israel Defense Forces (IDF) weapons near the operator's ear, as well as the insertion loss of different types of HPDs using an acoustic test fixture (ATF).  The peak levels of small calibre rifles and machine guns were in the range of 153 to 165 dB peak near the ear.  The 1/3 octave band insertion loss results of passive and active earplugs showed that 1) different insertion loss levels were obtained for different types of noise sources and 2) the ATF measurements often exceeded the limits of bone conduction.  The results suggest that it is not sufficient to measure only the peak insertion loss of an HPD; rather, the spectrum must be analyzed for different types of weapons and HPDs.  This complicates the task of including HPD insertion loss in the calculation of daily exposure limits.  Practical concerns for military operators, including compatibility of HPDs with other personal protective equipment and communication requirements, will also be discussed.