Military operations are inherently noisy. Warfighters in all branches of the United States military are exposed to steady-state and impulsive noise at levels and durations far exceeding what has been deemed safe for the civilian work force in industrial occupational settings. Steady-state noise exposures for various combat systems are 98–120 dBA for shipboard diesel-driven systems, 85–126 dBA for high-speed watercraft, 115–167 dBA for on-deck operations related to aircraft, 90–118 dBA for tracked vehicles, and 85–121 dBA for the interior of cockpits in aircraft.
Permissible exposure times range from 1 second for unprotected ears up to several hours with double hearing protection (earplugs and earmuffs). Under normal military operational conditions, actual exposure times far exceed those permitted by OSHA or recommended by NIOSH.
In a similar fashion, warfighters are exposed to impulsive noise from weapons systems such as handguns, rifles, rockets, and howitzers. These range in intensity from 157 dBP (peak unweighted decibels) for a 5.56mm M16 rifle to 183 dBP for a 105mm towed howitzer.
Warfighters are required to wear hearing protection during training and should do so during combat. Military regulations require double hearing protection for some exposures. Unfortunately, double hearing protection is rarely worn because it significantly isolates the wearer from his environment and its use requires the audio level of communications systems to be increased, thereby partially negating its benefit. For some military occupational specialties, communications equipment is integrated with hearing protection. In these cases, warfighters are more apt to wear hearing protection because the combined hearing protection and communications systems are required to perform the task at hand. Dismounted warfighters (infantry, for example) often wear no hearing protection because it is perceived to make detection of the adversary’s movements and actions more difficult. Wearing hearing protection compromises auditory awareness of the surrounding environment and hinders direct person-to-person speech.
OSHA regulations on noise exposure focus on minimizing hearing damage accumulated during continuous daily exposure over a working lifetime and assume a recovery period between occupational noise exposures. Except for setting a maximum permissible peak sound pressure level of 140 dBP, OSHA standards include no language referring to the protection from acoustic trauma or permissible impulse noise exposure while wearing hearing protectors. The non-auditory effects (such as lung damage) of impulsive noise are not addressed at all. Many weapons systems produce noise levels greater than 180 dBP and few, if any, produce noise levels below 150 dBP at the ear of the operator, which makes both the OSHA and NIOSH noise limits unsuitable for military applications.
Military standards (MIL-STD) document materiel objectives of the U.S. Department of Defense (DoD) by addressing unique military requirements. Since military missions are quite often “one-time chance” actions and military environments are harsher than most civilian environments, equipment failure may cause injury, loss of life, and mission failure. Equipment deemed safe for civilian applications may cause injury or health hazards to warfighters when used under harsh combat conditions. Noise is one of these hazards.
The DoD published its first MIL-STD on noise limits in 1984. It was revised to MIL-STD-1474D in 1997. The most recent revision was published in April 2015 as MIL-STD-1474E,
Design Criteria Standard: Noise Limits
. This standard specifies the maximum permissible noise levels produced by military systems and the test requirements for measuring these levels. Through the efforts of a DoD multi-service working group, every aspect of MIL-STD-1474 has been revised to improve readability, reduce conflicting guidance, and consolidate requirements common to steady-state and impulsive noise produced by weapons systems and ground-, air-, and water-borne platforms.
MIL-STD-1474E applies to typical operational conditions and treads a fine line between mission effectiveness and hearing hazard. Its purpose is to provide criteria for designing and fielding materiel that minimize noise-induced hearing loss, promote personnel safety, permit intelligible speech in noisy environments, and minimize acoustic detection and recognition by the adversary while improving warfighters’ overall performance.
Recently Revised Standard Addresses Noise from Military Operations