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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. A STANDARD FOR OPERATIONAL CONDITIONS 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.
Unfortunately, double hearing protection is rarely worn because it significantly isolates the wearer and its use requires the audio level of communications systems to be increased, thereby partially negating its benefit.
This military standard applies to noise levels emitted during the full range of military operations regardless of duration and availability of recovery periods; therefore, the design criteria contained in MIL-STD-1474E are more conservative than the OSHA exposure regulations (29 C.F.R. §1910.95) and are used by the military whenever feasible. However, MIL-STD-1474E is neither a hearing damage risk criterion nor a hearing conservation criterion. It is a set of design criteria. The medical community of each military branch sets standards for implementing and enforcing a hearing conservation program for their personnel exposed to noise hazards.
MIL-STD-1474E requires steady-state noise levels to be less than 85 decibels A-weighted (dBA) and peak-pressure levels of impulsive noise to be less than 140 dBP at the ear (protected or unprotected) at all occupied locations during normal operations. Reducing noise at the source is the primary means to protect personnel from hazardous noise. Hearing protectors and administrative measures such as warning signs should not be relied upon unless all design approaches for noise reduction have been exhausted. MIL-STD-1474E requires posting hearing hazard signage whenever steady-state noise exceeds 85 dBA or when impulsive noise exceeds 140 dBP.
The main body of MIL-STD-1474E contains all requirements, references, definitions, and measurement techniques common to its five appendices. Each appendix mandates unique requirements germane to specific types of noise, equipment, or operational environments. Appendices A through C are generic and apply to specific aspects of noise generated by military equipment and systems. Appendices D through F provide additional criteria addressing requirements unique to aircraft and shipboard noise. STEADY-STATE NOISE Appendix A establishes acoustical noise limits and prescribes testing requirements and measurement techniques for determining conformance to steady-state noise limits in personnel-occupied areas. These areas include those for operators, maintenance personnel, and observers. Although the goal is to limit steady-state noise to 85 dBA, it is recognized that noise from military equipment and operations typically exceeds 85 dBA. Because intelligible face-to-face or electrically enhanced communication is critical to effective military operations, appendix A specifies various acceptable noise levels based on the need to communicate under various operational conditions. Appendix A requires measurement of noise contours showing the distance and direction (from the noise source) at which the sound pressure level equals 85 dBA for each item tested. IMPULSIVE NOISE Appendix B addresses impulsive noise from weapons firing, impacts, or explosive devices at levels equal to or exceeding 140 dBP at personnel-occupied areas. It provides two methods for assessing impulsive noise limits: the auditory risk unit (ARU) metric or the LIAeq100ms method. The U.S. Army uses the ARU metric to determine noise limits for systems procured for Army use. Other military services select the appropriate method on a case-by-case basis depending on the specific noise exposure conditions and environment. For joint service programs (equipment procured by one service for use by multiple services), assessments of noise limits are made in accordance with individual service requirements.
The ARU is a unit of exposure that quantifies the mechanical damage caused by impulsive noise in the cochlea of the human ear as determined by the computer-based Auditory Hazard Assessment Algorithm for Humans (AHAAH) model. AHAAH was developed by the U.S. Army Research Laboratory and is available for download.
The LIAeq100ms method employs the equal energy metric characterizing the equivalent total energy of the impulse calculated for 100 milliseconds. This method is an impulsive noise extension of the equal energy metric Leq used to measure fluctuating (steady-state or impulsive) noise. The LIAeq100ms method can be used to compute a noise dose relative to a single occurrence of impulsive noise and to assess the noise exposure dose from combined steady-state and impulsive noise exposure. MIL-STD-1474E always requires independent measurement of steady-state and impulsive noise. Noise contour measurements showing the distance and direction (from the noise source) at which 140 dBP pressure is obtained are required for each item tested in order to establish hazard zones.
Aural non-detectability is critical to successful military operations. Noisy equipment and vehicles can provide advance warning to the adversary of impending activity.
AURAL NON-DETECTABILITY Appendix C establishes noise limits criteria and testing requirements for aural non-detectability of military materiel. It applies to all systems and equipment that emit acoustic noise in order to minimize aural detection by an enemy. It does not apply to aircraft on the ground. Aural non-detectability is critical to successful military operations. Noisy equipment and vehicles can provide advance warning to the adversary of impending activity.
The non-detectability requirements are based on conservative estimates of sound propagation. They assume a young adult listener with normal hearing sensitivity and consider environments varying from extremely quiet wilderness sites to urban centers. Criteria are based on propagation of the most dominant one-third octave band frequency. Appendix C provides tables and graphs showing detectability distance (in meters) versus the dominant frequency.
Fielding equipment meeting these non-detectability criteria also minimizes the risk of noise-induced hearing loss, permits intelligible speech in noisy environments, and provides acceptable habitability of personnel quarters including berthing spaces aboard ships. AIRCRAFT AND AERIAL SYSTEMS Appendix D establishes testing requirements and measurement techniques for determining noise limits in personnel-occupied areas of DoD aircraft, including fixed- and rotary-wing aircraft and unmanned aerial systems. The intent of these requirements is to ensure that occupied spaces aboard aircraft, and all occupied work spaces either on the ground, aboard ships, or in the vicinity of aircraft operations, have suitable sound pressure levels consistent with a safe acoustic working environment.
This appendix applies to all systems, subsystems, component hardware, and support equipment that can contribute to the noise within personnel-occupied areas during aircraft operation and maintenance. It requires analysis of noise produced by aircraft operations for each mission profile and at all places where flight crew, mission crew, passengers, and ground crew are located. Each mission profile is composed of specific mission segments (for example, take-off, climb and descent, level flight, landing, refueling, and so on). This appendix does not apply to ground-level noise from aircraft in normal flight. SHIPBOARD NOISE On ships, each compartment or on-deck work area has a noise requirement based on the purpose of the compartment and the communications requirements within it. Each compartment contains permanently mounted and portable equipment, all of which contributes to the overall noise level. To better limit the noise level in a specific fully outfitted compartment, MIL-STD-1474E addresses shipboard equipment and compartment noise separately.
Appendix E prescribes the maximum acceptable sound pressure level for airborne sound, and the instrumentation, procedures, and locations for the measurement and analysis of the noise generated by various types of shipboard equipment. Two types of sound pressure measurements are required: weighted levels (time-averaged for all measurements and peak for selected measurements), and octave band sound pressure levels. Equipment is acceptable for use in a specific shipboard compartment when none of the airborne levels measured at the designated locations exceed the applicable acceptance criteria.
Appendix F, the final appendix in MIL-STD-1474E, establishes acoustical noise limits (for both steady-state and impulsive noise) for shipboard compartments and on-deck work stations. It includes testing and measurement procedures, and administrative and reporting requirements for demonstrating conformance to airborne noise limits for compartments accessible by shipboard personnel and for on-deck work stations.
The criteria and procedures in appendix F are designed to achieve acceptable spoken communication, minimize or eliminate noise-induced hearing loss, and improve warfighters’ overall performance by improving the acoustic environment in the shipboard compartment in which they operate. NECESSARY GUIDANCE Weapons systems developers must have appropriate design criteria to develop and field effective military equipment, vehicles, and systems. These criteria must include objective means of testing, measuring, and evaluating noise levels using state-of-the-art techniques and computational models to certify weapons systems as safe for use while minimizing hearing loss, maximizing intelligibility of speech, and reducing aural detectability. Revision E of MIL-STD-1474, published in April 2015, provides the necessary guidance to accomplish these objectives while maximizing the effectiveness of weapons systems. BRUCE E. AMREIN, CTS, is principal at WorshipTech in Bel Air, Md. He is a guest researcher at the U.S. Army Research Laboratory, and he chaired the Department of Defense MIL-STD-1474 Working Group from 2011 to 2015. He can be reached at amrein@gmail.com.
NOISE LIMITS FOR
WARFIGHTING
Recently Revised Standard Addresses Noise from Military Operations
BY BRUCE E. AMREIN
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