Noise of military weapons, ground vehicles, planes and ships

All Military personnel are going to be exposed to loud sounds. In fact, they are likely to have exposure to some of the most intense sounds that can be found in any occupation. For example, with the exception of the bayonet and the crossbow (which are used today by Special Forces), every Military weapon system makes more than 140 dB peak sound pressure level (dBP), a level generally considered the maximum for a single safe unprotected exposure for impulse noise (OSHA, 1983; U.S. Department of Defense, 2010) at the operator's ears. Several classes of weapon systems expose crew to impulse noise levels that exceed 180 dBP. Almost all of the ground and air transportation platforms expose crew and passengers to steady-state noise higher than 85 dBA while operating. An 8-h average level of 85 dBA triggers the need for Hearing Conservation Programs (Occupational Health and Safety Administration, or OSHA) and any exposure in the Army and Air Force to levels of 85 dBA (regardless of average levels) mandates hearing protection be worn (OSHA, 1983; U.S. Department of Defense, 2010; U.S. Army, 2015). Several shipboard, ground, and air transportation and weapon platforms create interior 110 dBA environments during operation (James and McKinley, 2004; Tufts , 2009).

With some exceptions having to do with weapon simulators or other special training devices, Military personnel are exposed to the same intense sounds during training missions as they would be when deployed. Even during training, there are potentially serious consequences to hearing health. Personal protective equipment is relied on to mitigate against the loud sound levels.

Determining the time-histories of service members' overall noise exposures during deployment is difficult. Because service members perform jobs that are extremely dangerous, and because their jobs usually involve exposure to impulse sounds (which are notoriously difficult to accurately measure in field settings), far fewer exposure studies have been conducted in the Military than in industry. Battlefield noise exposures are estimated from paper studies of mission profiles and noise measurements that are done as part of the acquisition process that brings materiel into the inventory (Chan , 1999; Davis , 2019). For these reasons, this paper is limited to general information about military noise and serves as an introduction. Most of the information to follow is derived from military system qualification testing.

A hazard associated with high-level noise is that it may physically damage the delicate cells and structures in the inner ear, causing tinnitus (ringing in the ears) and temporary and possibly permanent hearing loss (Coles , 1968). With steady-state noise, these effects may manifest only after long-term repeated exposures, but with impulse noise, a single unprotected exposure can produce damage that is irreversible. These effects depend on individual susceptibility, which, unfortunately, is something that cannot yet be associated with any observable human characteristic (Dobie and Berger, 2019; Kil , 2017). Accordingly, hearing conservationists trying to prevent any harmful exposure seek to warn that the damage could easily be permanent and irreversible, and treat all noise-exposed personnel as if they were part of the noise-sensitive group (CAOHC, 2015).

The risk of hearing loss associated with intense impulse noise has long been thought to be correlated to the number of rounds to which an individual may be exposed, the peak level of the noise, and to the B-duration of the impulse (a measure of the time it takes for the impulse noise to decay to levels 20 dB lower than the peak level; Garinther , 1975). These characterizations of the impulse sound were used in the Department of Defense (DOD) Design Criteria Standard for Noise Limits (MIL-STD 1474 D) (U.S. Department of Defense, 1997) for many years to determine how many rounds per day (Allowable Number of Rounds, or ANOR) could be fired when either single (ear plugs or ear muffs) or double (both ear plugs and ear muffs) hearing protection were worn. Newer ANOR assessment methods are part of MIL-STD 1474E (U.S. Department of Defense, 2015). Even though the assessment methodology has changed, the goal has always been to protect all but the most susceptible 5% of the exposed population from having their hearing permanently impaired (Patterson , 1985). For typical small arms weapons, ANOR values have permitted many thousands of rounds to be fired per day with hearing protection. Actual missions or training exercises with high firing counts less than the ANOR could be performed and still comply with recommended exposure guidelines. However, some weapon systems have single-digit ANOR values, which limit the potential to reduce risk through administrative controls by firing fewer rounds.

The updated MIL-STD 1474E has two impulse noise criteria (U.S. Department of Defense, 2015). One criterion depends on the overall energy or the noise produced by the weapon. The energy-related criterion uses a dBA value associated with the impulse, and (similar to steady-state noise) integrates the exposures to estimate the risk for a given mission. The other criterion depends on the waveform shape and uses an electro-acoustical model of the acoustic energy transmitted into the cochlea to calculate a number of Auditory Risk Units (ARUs) associated with the sound. An ANOR value is produced based on the ARUs. Having two distinctly different design criteria in the revised MIL-STD is necessitated by lack of consensus about the technical merits of the two, and this is a recognized complication. In fact, the medical community in the Army, which previously used the design criteria limits expressed in MIL-STD 1474 D as the medical criterion, is waiting for additional research to validate the new design criteria. Meanwhile, the Army medical community has settled on a third, interim, criterion which is a modification of the peak level/B-duration characterization for risk assessment purposes as explained in the interim impulse noise criterion memo (U.S. Army Public Health Center, 2015). For small arms fire, there is no difference in the medically determined ANOR value between what was calculated under the old design standard and the interim medical standard. The ANOR value for small arms fire is reduced considerably, however, when the newer design criteria in MIL-STD 1474E are used, from thousands of rounds to perhaps hundreds. At the same time, the ANOR value for large caliber weapons has increased (Chan , 2001; Price, 2007).

Small arms ammunition comes in a variety of calibers. Cartridges consist of a shell, primer, propellant, and a bullet (projectile). The amount of propellant in the cartridges largely determines how much noise is generated when the weapon is fired. When the weapon is fired, the exploding propellant creates a pressure spike that travels down the barrel of the gun, eventually becoming a spherically expanding shock wave (muzzle blast) after it leaves the weapon. The greatest noise exposure is directly downrange from the muzzle, with lesser exposures to the sides and rear.

For small caliber firearms (rifles, pistols, and shotguns), peak sound pressure levels at the shooter location may range from 150 to 175 dBP range, and are from 8 to 10 dB higher at adjacent shooter locations (Lobarinas , 2016; Murphy and Tubbs, 2007; Schulz , 2013). For that reason, a service member on the firing line at a range may actually get exposed to more noise from the adjacent weapon being fired than from firing their own weapon (Flamme and Murphy, 2019b; Routh and Maher, 2016; Wall , 2019). Exact levels depend on how far the muzzle is from the shooter (some guns support multiple barrel lengths), and particularly on what attachments are fit to the muzzle. If the muzzle has a suppressor (silencer) attached, then pressure levels can be reduced between 15 to 25 dB peak sound pressure level (SPL). The design of the firearm loading mechanism, particularly semi-automatic versus bolt action, can affect the amount of observed suppression (Lobarinas , 2016). If the muzzle has a muzzle brake on it, energy that would normally be directed downrange is diverted back towards the shooter, increasing shooter and adjacent personnel exposure levels by 15–20 dB (Kyttälä and Pääkönen, 1995; Murphy , 2018).

Indoor firing ranges are commonly found at Military installations. Reflections of the impulse at indoor ranges create an audibly louder experience than what is experienced at outdoor ranges. However, peak levels at the shooter's position or at positions adjacent to the shooter in indoor ranges do not change appreciably from those identified for outdoor ranges (Kardous, 2013). The muzzle blast expands spherically, reaching the shooter's ears regardless of the acoustic environment (Nikolaos, 2010; Rasband , 2019).

At indoor or partially covered ranges, the pressure wave reflects off the ground, walls, and overhead surfaces. Reflected gunshots will have lower peak levels because the sound will have traveled further. It is the reverberation that is mainly responsible for the difference in how weapon firing sounds indoors and outdoors (Murphy and Tubbs, 2007). The reflections will affect the pressure envelope duration, specifically the B-duration. The B-duration typically varies from around 5 to 10 ms at outdoor ranges and upwards of 100 ms at indoor ranges, depending on acoustical treatment applied to reflective surfaces. Large indoor ranges, with many shooters firing simultaneously or near simultaneously, make the range noise persist for more than 1 s, which is often taken as the point that distinguishes between steady-state and impulsive sounds (Hamernik , 2003; Murphy and Xiang, 2019). When that happens, the risk associated with incurring hearing loss may be greater using steady-state risk criteria than with impulse noise criteria. The question remains, “which criterion is more accurate?”

The Army Public Health Command reported the ANOR values for an M4 rifle evaluated for single and multiple shooters fired in an acoustically treated indoor firing range (U.S. Army Public Health Command, 2013). Using the 8-h, 85 dBA Leq criterion (⁠ $L A Eq 8$⁠), the number of rounds that could be fired with single hearing protection was estimated to be 64, 5-round bursts when multiple shooters (N = 14) were firing simultaneously. However, with only one person firing, the ANOR for the same weapon increased to 155, 5-round bursts. Double protection for the shooter increased the ANOR estimates from 64 to 243, 5-round bursts for multiple shooters, and from 155 to 491 bursts for a single shooter. In the multiple-shooter condition, the more conservative estimates were provided by the steady-state $L A Eq 8$ criterion. In the single-shooter condition, the MIL-STD 1474 D criterion provided the more conservative estimates. While the M4 rifle is one example, $L A Eq 8$ yielded the more conservative ANOR values for multiple shooters for other small caliber firearms (U.S. Army Public Health Command, 2013).

Machine gun firing also has characteristics of both steady-state and impulsive noise. Each shot fired by a machine gun carries with it the hearing hazard that would be commensurate with the acoustical characteristics of that shot, just as it would be for a rifle or hand gun. However, when fired in bursts, the hazard takes on physical attributes more typically associated with steady-state noise. In fact, when the bursts last for one or more seconds in duration, the dBA level of that burst is separately evaluated by steady-state and by impulse noise criteria to estimate risk. Often, the dBA level risk assessment suggests that the steady-state aspect of the noise may cause greater health risk than the risk determined from the peak level examination (Brueck , 2014). The health risks for these combinations of noise exposures have not been thoroughly researched yet, so it is possible health risks for impulse noises superimposed on high-level steady-state background noise could be better, the same, or worse than what our current evaluation methods indicate. Current practice is to base risk estimates on which assessment is more medically conservative.

Large caliber weapon systems such as mortars, howitzers, shoulder-fired rockets, crew-served weapons, are used by service members across the Armed Services. These systems use greater amounts of propellant and are far more energetic than small caliber firearms. The crews using these systems occupy positions around the weapon and thus receive different noise exposures. The mortar gunner's head, for example, after dropping the round into the mortar tube, is very close to the noise source emanating from the mortar tube. The gunners firing a shoulder-fired weapon are exposed to two blast waves: the one coming from the front of the tube, the other from the back end of the tube. At times, these systems are fired from areas protected against enemy fire with walls and other structures, and thus, operators may also be exposed to high-level sound reflections.

Both shoulder-fired weapons and howitzers fire cartridges with specific propellant weights (the charge) selected according to where the firing team wants the warhead to land. The weapon noise will depend on the charge weight. At the top charge, the pressures associated with the muzzle blast may be high enough to cause more harm than just hearing loss. The concussive sounds may be capable of causing eardrum rupture or damage to internal organs containing air, particularly the lungs (Desmoulin and Nolette, 2018; Stuhmiller, 1989).

Howitzers and mortars (among other weapon types) may also be subject to secondary detonation or flashing. Material expelled from the barrel includes some unburnt propellant that can generate a new fireball and blast wave when exposed to more oxygen. That explosion is unrestrained by the weapon casing and may have sound levels higher than the original muzzle blast at crew positions.

Army, Marine Corps, and Special Forces personnel may also be exposed to loud impulse sounds due to exploding ordnance. Breachers seeking to gain forced entrance by blasting a hole in structural impediments blocking their access by using explosive charges may intentionally be very near the explosion, with minimal protective shielding. Levels can certainly be at or near those associated with large caliber weapon systems. With exploding ordinance, there are also the sounds associated with the super-sonic shock waves generated by the outwardly moving explosion fragments. These shock waves travel faster than the muzzle blast and act as precursors to the main blast (Flamme and Murphy, 2019a). These may, according to analysis with one of the newer design criteria in MIL-STD 1474E, be more harmful to hearing than the actual blast wave (Fedele , 2018). The medical significance of these shock waves is a new finding. Improvised Explosive Devices (IEDs) can cause hearing loss (Greene , 2018; Patterson and Hamernik, 1997; Stuhmiller, 1997). These are traps planted by enemy forces to kill and maim. Service members may be exposed to IEDs while on foot patrol (thus without protection from the exploding device), or inside vehicles (thus receiving some protection). IEDs are a potential source of high-level noise exposure for deployed service members. DOD-funded research has resulted in pharmaceutical interventions that may mitigate hearing loss due to high-level noise exposures (Campbell , 2007; Kopke , 2007; Le Prell and Bao, 2015).

Almost all ground vehicles in the Military expose crew and passengers to hazardous levels of noise during normal operating conditions. Wheeled transports will expose occupants to hazardous steady-state noise when traveling at high speeds with climate controls systems on and/or with windows or hatches open. Spartan interiors and noise sources are mostly designed with function in mind; noise control treatments are not used. Some specialty equipment, including vehicles that detect or destroy mines, may operate at safe noise levels because the crew member operating the vehicle is located high up, in a protected space, far from the vehicle noise sources. Generally speaking, wheeled vehicle noise reaches into the mid- to high-90 dBA range. Single hearing protection is generally required when in or operating these vehicles.

Occupants of tracked vehicles are exposed to higher sound levels than wheeled ones when the vehicle is in motion, and the levels rise with increased travel speed. There is some variability in levels associated with occupant location and hatch condition, but levels exceeding 110 dBA are often reached. Double hearing protection is often required when in these platforms, and in some cases, there may be restrictions in permitted travel distances (which is a simpler alternative to limits expressed as permitted operating time at specific speeds each with a defined noise level). That approach works well for missions that have been described in terms of travel time at different speeds.

Vehicular noise may be compounded by noise from weapon systems such as mortars, missiles, grenade launchers, or machine guns that may be integrated into the vehicle platform. Our current medical criteria do not address such mixtures of sounds; each is treated on an individual basis as if the other sounds are not present. The development of new noise dosimeters capable of sampling complex noise may inform our knowledge about complex exposures (Davis , 2019; Smalt , 2018). Wearable sensors permit 24-h sampling in hazardous environments.

Rotary wing aircraft platforms are inherently noisy. All interior environments are in the high-90 to low-100 dBA range and can reach close to 115 dBA in the noisier platforms. Double hearing protection is the norm because maximum protection is generally required (Ribera , 1995).

Rotary wing aircraft designed as offensive weapon systems have additional noise issues. Some may be flown with crew unprotected by shielding (blockage of sound propagation) otherwise provided by the fuselage. Worse still, open-door flying on some platforms not only places the platform-mounted weapon systems (machine guns, missiles, or rockets) with the noise sources unshielded, but the muzzles or rocket engines are closer to the crew in axial and lateral directions, and thus they are very noisy. Typically, low ANOR values are associated with these systems, even with double hearing protection.

Fixed wing aircraft may also have high (from 105 to 122 dBA) cabin noise levels (Bjorn , 2005). Sources include engine and propeller noise and airflow noise around the fuselage. In tactical aircraft, a primary noise source is the high air flow velocities associated with cooling the avionics.

Flight lines include noise environments that are typically well above double hearing protection requirements (Yankaskas and Komrower, 2019). The Services have different aircraft operating protocols. For example, fixed-based installations may have 10 000-ft runways, and operating protocols appropriate to that situation are different for the 1000-ft “runway” on an aircraft carrier. For fixed-base operations, minimal personnel are adjacent to aircraft upon engine start. Aircraft then taxi to the runway which is clear of personnel. For aircraft carriers, those operations occur in the confines of four and a half acres. During aircraft operations, a variety of personnel are present on the flight deck, including launch and recovery, re-fueling, ordnance, maintenance, and occasionally medical personnel. Numerous aircraft operate in close proximity requiring directions from aircraft directors. There can be about 150–200 personnel on the flight deck. The “quiet” spots are about 126 dBA (Yankaskas and Fast, 1999). For a carrier launch, aircraft are hooked to the catapult and restrained while the aircraft goes to full power for final aircraft checks. These noise levels are about 148 dBA for tactical aircraft and somewhat lower for propeller aircraft (Smalt , 2018). Practical engineering noise controls do not exist that will make these extraordinarily high-level noise exposures non-hazardous.

On board typical ships, high-level noise environments are omni-present 24 h a day, and there is little, if any opportunity to “rest” Sailors' ears. Numerous acoustic measurements have been made directly below the catapults which are typically berthing spaces or ready rooms. Through modeling and verification measurements, it has been shown that the acoustic energy propagates through a ship's structure and re-radiates into occupied spaces. The noise levels in these spaces range from 87 to 102 dBA (Yankaskas and Fast, 1999). Activities in these areas range from squadron briefings to mundane activities such as sleeping. Aircraft recovery operations (landings) are also noisy in that the aircraft land at full power as they capture the arresting gear wire. That wire initially pays out at aircraft speed and slaps the flight deck as the aircraft is stopped. This generates numerous acoustic transients which add to the noise of the aircraft at full power.

In off-hours, personnel will have meals on the mess deck that can have noise levels of 92–94 dBA (Yankaskas and Fast, 1999). Or, they can do their laundry which is in the lower aft end of the ship. The noise levels in such areas have been measured at 105 dBA (Smalt , 2018). Machinery spaces and engine rooms also add to the din of ship noise. Including other classes of ships (besides aircraft carriers) engine rooms have been measured from 85 to 118 dBA. For reference, diesel powered engine rooms tend to be from 108 to 118 dBA (Bjorn , 2005) with lots of low frequency energy from the engine and high frequency energy if the diesel engine is turbo-charged. Other shipboard noise sources are hydraulic systems (elevators), vent fans, pumping systems, and ventilation systems.

Generally speaking, you will find virtually every kind of industrial operation somewhere in the Military. Some are notoriously noisy, such as sandblasting at rework facilities, where levels can reach into the 120-plus dBA range (Komrower, 2013). Other operations have high-level noise that is, unsurprisingly, similar to noise levels found in general industry. They may have, however, different exposure profiles than their civilian counterparts because activity levels may be more variable. There are also many occupations in the Military with similar work environments to their Civilian counterparts; for example, the noisy equipment used in military motor pools can be found in Civilian auto repair shops. Commonality of noise exposures exist for machine shops, food preparation areas, construction and other occupations. Some differences in occupational task profiles may exist in smaller operations compared to their industrial counterparts, because they are serving a smaller population set.

Not all military occupations involve daily exposure to gunfire. However, almost all service members are required to at least demonstrate proficiency in small arms weapon use every year (Garinther , 1975). Therefore, almost all of them are annually exposed to hazardous levels of impulse noise. Currently, all service members are required to receive annual audiograms and counseling about noise. These check-ups help to ensure that they are provided properly-sized hearing protection and know how to use it when firing their weapons (U.S. Army, 2015). The check-ups are lagging indicators of damage and provide an opportunity for refresher training on various aspects of hearing readiness and conservation to help prevent further losses.

Hearing protection is frequently used to mitigate the adverse aspects of noise exposure. Hearing protection can provide a wide range of noise reduction. For some products, the noise reduction is intentionally low to provide better situational awareness (e.g., filtered or electronic ear plugs). For other hearing protectors, the noise reduction can be more than 30–35 dB, especially for double protection (ear muffs and plugs) or active noise reduction protectors under specific types of noise. The attenuation of protectors is limited as a function of frequency between 41 and as much as 60 dB by the bone conduction of the head and skull (Berger , 2003). Even though much of this discussion has emphasized that military operations can be noise-hazardous, most of the situations described can be made non-hazardous through use of hearing protection. Noise-induced hearing loss remains a problem in the Military for reasons discussed below. Many obstacles remain to capturing the full potential benefit of hearing protection.

For example, the complexities of training service members to properly use and fit hearing protection is underestimated. If not trained on how their hearing protectors should be worn, the ear canal seal needed to effectively block sound may not be obtained (Berger , 1998; Murphy , 2011; Royster , 1996). Too shallow an insertion depth is also a commonly observed matter. Some ear plug designs come in different sizes to accommodate differences in ear canal size (Bjorn , 2005). Inattention to ear plug sizing is thus another opportunity for getting less protection than one might think. Mismatches between desired protection levels for specific environments and not maintaining the devices (using them beyond their expected useable life) are additional complexities. New methods for hearing protector testing may help to achieving the required level of protection for all military personnel (Federman and Duhon, 2016).

Proper use and proper fitting are key elements in effective hearing protection. The most important aspect is whether hearing protection is worn correctly and consistently. Based on observations made by Army Hearing Program personnel, service members seemed reluctant to wear hearing protection, even during training, and Army leadership did not aggressively enforce hearing protector use. Hearing loss has long been an accepted outcome not just for Soldiers in the Army, but for many service members. There were reasons for all this. Dismounted service members on patrol need to hear relatively quiet sounds to be aware of what is going on in their surroundings. Their lives depend on that situational awareness (Casali and Robinette, 2015; Clasing, 2012; Robinette, 2014). Anything put into the ears affects detection and impairs localization (identifying where a noise is coming from). The level-dependent or (non-linear) ear plug and other new hearing protective equipment (such as Tactical Communications and Protective Systems, or TCAPS) were designed to address those problems.

Level-dependent protectors pass quiet sounds to the inner ear while shutting out transmission of high-level noise associated with gunfire. Some devices do this passively, through the action of the sound wave propagation through a small tube embedded in the device that filters out the loud sounds. Other devices do this electronically and have other advantages such as the capability of attaching to radios. Naturally, the electronic devices are much more expensive. All these devices offer improved situational awareness over the ordinary hearing protector (Clasing, 2012). They are not perfect and require a period of training to approach the performance one would get from the unoccluded ear (Casali and Robinette, 2015). Newer models address these issues better. The existence of these new devices has already generated a change in the Army culture with regard to expecting hearing loss as a part of being a Soldier. Service-related hearing loss no longer has to come along with being in the Military; occupational hearing loss is preventable.

The consequence of inconsistent hearing protection use is often overlooked. For example, a service member in a vehicle can accumulate about a third of the daily allowable noise exposure per each minute of unprotected exposure to a level of 106 dBA. That service member should be wearing the hearing protection prior to entering the transport, and not waiting for the vehicle to start moving. There is a similar issue about unprotected exposure to impulse noise. As stated before, unprotected exposure to even one or two rounds can cause permanent damage. The protection should be in place before the impulse occurs. For those instances when an exposure might occur when hearing protection is not worn, the potential for more significant damage exists. Although the Federal Drug Administration has not approved pharmaceutical treatments for blast noise exposure, phase II clinical trials have taken place and may yield effective treatments in the future (Campbell , 2007; Kopke , 2007; Le Prell and Bao, 2015).

To prevent hearing loss, all service branches have Hearing Conservation Programs. Since 1974, the annual rates of significant hearing loss (>H-1 hearing profile) in Infantry, Artillery, and Armor Soldiers have decreased considerably: 34%–39% in 1974, 14%–19% in 1989, and 6%–8% in 2016 (Ohlin, 2000; Robinette, 2016). Hearing health has been maintained and even improved, notwithstanding noise exposures received during our country's several combat operations (Ohlin, 2000; Robinette, 2016). In 2015, the U.S. Army reported the following statistics for Active Duty Soldiers: 78.7% have normal hearing, 4.3% have a clinically significant hearing loss (H-2 or greater hearing profile), 1.0% have an H-3 or greater profile, and 4.5% had a new case of significant threshold shift (STS) (Robinette , 2015). There is still some room for improvement, and the goal is to reduce rates of observed hearing degradation to at least the levels in the general population (Hoffman , 2017). No amount of occupational hearing loss among civilian and military personnel should be acceptable as a result of noise exposure. Rates of hearing loss among all personnel will continue to decrease so long as the importance of communication and hearing during training, combat, and life outside the Military is acknowledged and supported through hearing loss prevention activities.

The authors acknowledge the efforts of William J. Murphy and Colleen G. Le Prell, co-editors of this special issue, for their thoughtful editing and contributions to this manuscript. The findings and conclusions in this report are those of the authors and do not represent any official policy of the U.S. Department of Defense. The authors have no conflicts to declare.