R&D Portfolio - Research Goal 2.3:
Evaluate the effectiveness of hearing protection devices against impulsive noise
Over 1.8 million U.S. workers are exposed to potentially hazardous levels of impulsive noise. This estimate includes Federal, State, and local law enforcement officers, DOD infantry, armor, and artillery personnel, and workers in the construction and mining sectors. In addition, 50 percent of U.S. industrial workers are believed to be exposed to impulsive noise due to recreation use of firearms in activities such as target shooting or hunting.
High-intensity impulsive sounds are considered to be more damaging to hearing than continuous sounds. Exposure to impulsive sound can cause acute acoustical trauma, which can be followed by symptoms such as tinnitus and temporary hearing impairment. Sudden hearing loss may also occur from exposure to impulsive sounds that exceed a critical sound pressure level by causing direct mechanical damage to the middle and inner ear.
Law enforcement officers routinely practice skills with weapons that produce noise levels in excess of 150 dB peak sound pressure levels. Personal protective equipment is essential to preserving both hearing function and situational awareness.
The potential hazard of exposing human test subjects to high impulse levels combined with the lack of sufficiently isolated acoustic test fixtures has limited research in developing hearing protection devices for use in impulsive noise. The EPA labeling regulation states “Although hearing protectors can be recommended for protection against the harmful effects of impulsive noise, the NRR is based on the attenuation of continuous noise and may not be an accurate indicator of the protection attainable against impulsive noise such as gunfire.”1 Accordingly, OSHA established a non-enforceable level to a permissible exposure level (PEL) of 140 dB(A)16 and MSHA has established a PEL of 115 dB(A).31 To date, national guidelines are not available for evaluating the performance of HPDs in attenuating impulsive sounds. However, HLR researchers have addressed the need for updated guidance and regulations related to impulsive noise.
A discussion of physiologic research on the effects of impulsive noise is found in Research Subgoal 4.3. In this section, the focus will be on the measured performance of hearing protection devices.
Hearing protector attenuation is most often evaluated through REAT measurements, but can be assessed with a microphone in real ear or using an acoustic test fixture. The HLR program constructed an acoustic test fixture to digitally record the acoustic transfer function of HPDs in an impulsive environment. In 2001 and 2002, the HLR program staff evaluated the effectiveness of more than twenty hearing protectors at indoor and outdoor firing ranges with impulses generated by small-caliber weapons and peak impulse levels ranging from 140 to 170 dB SPL., We analyzed the impulses using several damage risk criteria and recommended the use of double hearing protection whenever impulses exceed 140 dB SPL. A combination of an electronic level-limiting earmuff and a passive earplug was recommended to improve the communication when using dual protection.
The HLR program used a mannequin specially designed to measure high impulse noise levels to evaluate the performance of hearing protection devices.
 Clark WW. . Noise exposure from leisure activities: A review. J. Acoust. Soc. Am. 90, 175–181.
 Tubbs RL, Murphy WJ, Little MR . Health Hazard Evaluation report 2002-0131-2898. Fort Collins Police Services, Fort Collins, Colorado. National Institute for Occupational Safety and Health, Cincinnati, Ohio.
 Harney JM, King BF, Tubbs RL, Hayden CS, Kardous CA, Khan A, Mickelsen RL, Willson RD . Health Hazard Evaluation report 2000-0191-2960. Immigration and Naturalization Service, National Firearms Unit, Altoona, Pennsylvania. National Institute for Occupational Safety and Health, Cincinnati, Ohio.