U.S. Department of Health and Human Services

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health
Page 2 Health Hazard Evaluation Report 2013-0124-3208

Contents

Highlights...............................................i

Abbreviations ..................................... iii

Introduction......................................... 1

Methods ............................................... 1

Background on Impulse Noise and 

Damage Risk Criteria ...........................4

Results and Discussion........................ 5

Conclusions........................................ 14

Recommendations............................. 14

Appendix A: Methods........................ 16

Appendix B: Tables.............................18

References.......................................... 21

Acknowledgements........................... 25

The employer is required to post a copy of this report for 30 days at or near the 

workplace(s) of affected employees. The employer must take steps to ensure 

that the posted report is not altered, defaced, or covered by other material.

The cover photo is a close-up image of sorbent tubes, which are used by the HHE 

Program to measure airborne exposures. This photo is an artistic representation that may 

not be related to this Health Hazard Evaluation. Photo by NIOSH.
Health Hazard Evaluation Report 2013-0124-3208 Page i

We measured firearms 

instructors’ exposures to 

impulsive noise during live 

fire training exercises. When 

using firearms and flash bang 

grenades, instructors were 

exposed to impulsive noise 

levels greater than 150 decibels, 

which is above the NIOSH 

ceiling limit. Exposures were 

greater than recommended 

by some damage risk criteria. 

We recommended using dual 

hearing protection during all 

live fire training exercises and 

installing additional noise 

controls.

Highlights of this Evaluation

The Health Hazard Evaluation Program received a request from a federal agency in 

Tennessee. The requestors were concerned about firearms instructors’ exposures to high 

intensity impulsive noise during tactical training exercises.

What We Did

● We measured instructors’ impulsive noise exposures when training with several 

different firearms and weapons systems.

● We calculated the number of gunfire exposures that would be permitted per day without 

incurring a significant risk of hearing loss. 

What We Found

● During most training exercises, instructors 

were exposed to peak sound pressure levels 

greater than 150 decibels. NIOSH recommends 

a ceiling limit of 140 decibels for peak sound 

pressure levels.

● Peak sound pressure levels for the Dillon 

M134D minigun, 12-gauge shotgun, and 

full-load flash bang grenade were sometimes 

greater than 170 decibels.

● Instructors could exceed the number of gunfire 

exposures permitted per day.

What the Employer Can Do

● Install noise controls at the outdoor and 

indoor ranges.

● Use noise suppressors on firearms, if feasible.

● Use the most protective criterion to limit the 

number of daily gunfire exposures

● Require use of dual hearing protection during all live fire training exercises.

● Fit test employees for the hearing protection they use.

What Employees Can Do

● Wear dual hearing protection during all live fire training exercises.
Page ii Health Hazard Evaluation Report 2013-0124-3208

This page left intentionally blank
Health Hazard Evaluation Report 2013-0124-3208 Page iii

Abbreviations

AHAAH Auditory Hazard Assessment Algorithms for Humans

ARU Auditory risk units

ANE Allowable number of exposures

CHABA Committee on Hearing, Bioacoustics, and Biomechanics

CFR Code of Federal Regulations

dBA Decibels, A-weighted

Hz Hertz

LeqA8hr A-weighted 8-hour equivalent sound level

msec Milliseconds

NIMS NIOSH Impulsive-noise Measurement System

NIOSH National Institute for Occupational Safety and Health

OSHA Occupational Safety and Health Administration

U.S. DOD U.S. Department of Defense
Page iv Health Hazard Evaluation Report 2013-0124-3208

This page left intentionally blank
Health Hazard Evaluation Report 2013-0124-3208 Page 1

Introduction

The Health Hazard Evaluation (HHE) Program received a request from the safety and health 

manager of a federal agency concerning firearms instructors’ exposures to high intensity 

impulsive noise during weapons qualifications and tactical training exercises. The firearms 

instructors train the agency’s security personnel. The safety and health manager asked the 

National Institute for Occupational Safety and Health (NIOSH) to characterize impulsive 

noise generated by several different weapon systems. The weapons were used during 

employee tactical training and firearms qualification exercises at indoor and outdoor firing 

ranges at the agency’s security training complex. The agency was particularly interested in 

knowing peak sound pressure levels, B–duration of the impulse noise waveform, and how 

much weapons fire instructors could be exposed to per day.

Two NIOSH HHE Program industrial hygienists and a NIOSH research engineer visited 

the security training complex in June 2013. During the site visit we met with agency safety 

and health staff, union representatives, and firearms instructors to discuss the HHE request. 

We observed workplace conditions and work activities, and informally spoke with firearms 

instructors. We measured impulsive noise generated by several different weapons at three 

outdoor firing ranges, one indoor firing range, the live fire “shoothouse” facility, the tactical 

training facility, and the virtual training facility during tactical training exercises. 

At the end of our visit, we met with employee and employer representatives. We summarized 

our activities, and shared preliminary observations and recommendations. In July 2013, we 

sent a letter to employer and employee representatives. The letter summarized our preliminary 

results and showed the peak sound pressure levels, the B–duration of the impulse waveform, 

and the allowable number of rounds of weapons fire that instructors can be exposed to per day 

when wearing dual hearing protection.

Methods

Impulsive Noise Measurement System

We measured impulsive noise during live firing exercises using the NIOSH Impulsive-Noise 

Measurement System (NIMS) and software platform. The system was designed to acquire, 

characterize, and analyze impulsive noise generated by weapons fire [NIOSH 2013a]. The 

measurement system acquired data and used a graphical interface to display the time domain 

waveform, one-third octave band frequency spectra, peak sound pressure level, equivalent 

average sound level, kurtosis, time duration, number of impulses, and temporal spacing 

between impulses. The system calculates potential risk to hearing based on the United States 

Department of Defense (U.S. DOD) MIL-STD-1474D, A-weighted 8-hour equivalent sound 

level (LeqA8hr) equal energy criterion, and the Auditory Hazard Assessment Algorithms 

for Humans (AHAAH). Additional details about NIMS and calculations using damage risk 

criteria are provided in Appendix A.
Page 2 Health Hazard Evaluation Report 2013-0124-3208

For each tactical training exercise we placed two or three stationary microphones on 

tripods within 3–4 feet of the instructor or representative instructor positions (Figure 1). 

We positioned the microphones at a height matching instructors’ ear level (standing, sitting, 

or kneeling). We checked the calibration of each microphone prior to measurements. 

For impulsive noise measurements of the Dillon M134D Aero minigun, we placed the 

microphones behind the turret on the top of a modified special weapons and tactics (SWAT) 

vehicle at the kneeling instructor’s ear level. Using the NIMS, we took several impulse 

noise measurements for each type of weapon or weapon system at each location to capture 

a representative sample of impulsive noise exposures (Figure 2). Table 1 summarizes the 

locations of weapons or weapons systems at each location where NIOSH took impulsive 

noise measurements.

Figure 1. NIOSH investigator positioning and calibrating a one-eighth inch microphone for impulsive 

noise measurements. Photo by HHE requestor.
Health Hazard Evaluation Report 2013-0124-3208 Page 3

Figure 2. NIOSH investigator acquiring and analyzing impulsive noise data on laptop computer using 

NIOSH Impulsive Measurement System. Photo by HHE requestor.

Table 1. Weapons used at each location during NIOSH impulse noise measurements

Location Firearms or weapons system

Range 1 M240 machine gun; M249 machine gun; M4 rifle

Range 2 Dillon M134D Aero minigun with and without noise barrier

Range 3 M240 machine gun; M249 machine gun

Indoor range M4 rifle; 9 mm pistol

Shoothouse M4 rifle; 870 shotgun; flash bang (full-load and reduced load); 

Sledgehammer on door; Battering ram on door

Virtual training facility M249 machine gun

Because of the rate of fire for some of the automatic weapons systems, one to dozens of 

impulsive noise exposures (i.e., gunshots) occurred during each “trigger pull.” The elapsed 

time for each trigger pull was approximately 1 second or less. To account for this sometimes 

rapid succession of gunfire impulses, we considered each trigger pull as an impulse noise 

exposure analogous to one round of gunfire or one impulsive noise exposure event. We 

combined the separate gunfire impulses per trigger pull and calculated damage risk and the 

allowable number of exposures (ANE) for each weapons system and each training exercise. 

The ANE is equivalent to the allowable number of rounds calculated for single shot weapons. 
Page 4 Health Hazard Evaluation Report 2013-0124-3208

Instructors wore Moldex Pura-Fit insert ear plugs and Peltor Model MT15H69FB ear muffs. 

We calculated ANE assuming use of dual hearing protection with an effective attenuation of 

34 decibels (dB). This is the attenuation given to this combination of earplugs and earmuffs 

by U.S. DOD MIL-STD-1474D.

Background on Impulse Noise and Damage Risk 

Criteria

Impulsive noise is considered to be more damaging to hearing than continuous sounds [Dunn 

et al. 1991; Starck et al. 2003]. Unprotected exposure to high intensity impulsive noise can 

cause acute acoustic trauma, resulting in symptoms such as ringing in the ears (tinnitus) and 

temporary hearing impairment [Salmivalli 1967; Mrena et al. 2002]. Permanent hearing loss 

may also occur from exposure to high intensity impulsive sounds that exceed a critical sound 

pressure level by causing direct mechanical damage to the inner ear [Ward et al. 1961; Luz and 

Hodge 1971]. 

Although military agencies in the United States and Europe have studied the effects of exposure 

to impulsive noise from weapons [Ylikoski 1994; Dancer et al. 1998], these studies lacked 

the data needed to quantify the relationship between impulsive noise and auditory damage. 

As a consequence, experts in the occupational, military, and scientific communities have not 

developed a consensus on the extent of hearing loss risk from exposures to impulsive noise.

In 1968, the Committee on Hearing, Bioacoustics, and Biomechanics (CHABA) of the U.S. 

National Research Council proposed a damage-risk criterion for exposure to impulsive 

noise [CHABA 1968]. The CHABA criterion was developed on the basis of measurements 

that could be made using available instruments at the time. The U.S. DOD criterion, MILSTD-1474D, was based on the 1968 CHABA criterion. This standard provided a method 

to calculate an allowable number of impulsive noise exposures without a significant risk of 

hearing loss. The allowable number of impulse noise exposures varies with the use of single 

or double hearing protection, peak sound levels, and the B-duration of the impulse waveform 

[U.S. DOD 1997]. B-duration refers to the length of time that pressure fluctuations in the 

noise wave are within 20 dB of the peak sound pressure level. The U.S. DOD criterion 

does not account for the characteristics of impulsive noise such as the spectral or temporal 

content, or for combined exposure to continuous and impulse noise. Additionally, it does not 

incorporate the effectiveness of hearing protectors or the protection provided by the nonlinear 

acoustical reflex and the peak clipping that occurs in the middle ear [CHABA 1992]. The 

standard does not require hearing protection for impulses with peak pressure levels less than 

140 dB. 

The French Committee for Weapons Noises advocates the use of the LeqA8hr for impulsive 

noise exposure [DTAT 1983; Dancer et al. 1995]. It recommends a limit for unprotected ears 

of 85 decibels, A-weighted (dBA) for an 8-hour, A-weighted, equivalent level. Most current 

noise instruments are capable of measuring LeqA8hr. The criterion integrates continuous and 

impulsive noise and allows for the assessment of exposure to multiple impulses regardless of 

whether the impulse happened in the free-field or a reverberant environment. It also allows 
Health Hazard Evaluation Report 2013-0124-3208 Page 5

for measuring the effectiveness of hearing protectors. However, some studies have shown 

that the LeqA8hr method may underestimate the actual protection efficiency of certain 

hearing protectors by 5 to 20 dB [Dancer et al. 1995]. A NIOSH report on the LeqA8hr 

and other damage risk criteria highlighted the advantages of using LeqA8hr to characterize 

exposure to impulse noise [NIOSH 2012].

The AHAAH damage risk criterion is based on an ear model developed by the U.S. Army 

Research Laboratory. The AHAAH model simulates the behavior of the middle and inner 

ear in response to high intensity impulse noise and calculates Auditory Risk Units (ARU). 

The AHAAH model recommends a daily dose maximum of 500 ARUs [Price and Kalb 

1991]. Doses greater than 500 ARUs are predicted to produce permanent hearing loss. Other 

researchers recommend reducing this limit to 200 ARUs for daily or near daily occupational 

exposures [Fedele et al. 2013]. The model takes into account protective nonlinearities of 

the middle and inner ear and explains why some short impulses can cause more hearing 

damage than longer impulses, even when the longer impulses contain more acoustic energy 

than the short impulses. The model incorporates “warned” and “unwarned” conditions for 

calculating ARUs. The “warned” condition occurs when an individual anticipates intense 

noise exposures. In contrast, the “unwarned” condition occurs when an individual does not 

know or expect intense noise exposure. The model also incorporates use of hearing protectors 

for calculating ARUs. 

Regulations established by the Occupational Safety and Health Administration (OSHA) and 

NIOSH recommendations state that no exposure to impulsive sound should be permitted if 

the peak sound pressure level exceeds 140 dB [OSHA 1992; NIOSH 1998]. The European 

Union directive 86/188, the International Organization for Standardization in ISO 1999:2013, 

and the American National Standards Institute ANSI S3.44-1996 also state that no exposure 

should be permitted above peak sound levels in excess of 140 dB [ECD 1986; ISO 1990; 

ANSI 1996].

The agency currently uses the U.S. DOD MIL-STD-1474D damage risk criterion to determine 

the allowable number of gunshot exposures per day. The U.S. DOD has proposed a revised 

design limit criterion for noise, MIL-STD-1474E, that uses a variant of the equivalent 

A-weighted sound level based on the first 100 milliseconds (msec) of an impulse (LeqA100msec) 

and the AHAAH model.

Results and Discussion

In this evaluation, we assessed firing range instructors’ exposures to high-intensity impulsive 

noise from gunfire and weapons systems during tactical training exercises. Tables B1, 

B2, and B3 in Appendix B show detailed results of our noise measurements and ANE for 

impulsive noise from each firearm or weapons system based on the three damage risk criteria 

with hearing protection. 

Peak impulsive noise levels during shooting of the Dillon M134D Aero minigun at outdoor 

range 2 reached a maximum of 171.4 dB at the instructor’s position next to the shooter. 
Page 6 Health Hazard Evaluation Report 2013-0124-3208

Figure 3 shows the acoustic waveform and rapid succession of 42 gunfire impulses that 

occurred in less than 1 second during firing of the Dillon M134D. One-third octave band 

measurements show that the highest noise levels occurred at frequencies of 50, 63, 100, 

and 160 hertz (Hz). The highest peak bands correspond to the fundamental frequency and 

harmonics of the firing rate of the weapon. A single impulse peaks at higher frequencies, near 

the 1000 Hz band.

Figure 3. Screen capture of impulsive noise waveform and one-third octave band noise frequency 

results from NIMS display of one trigger pull from the M134D Dillon Aero minigun showing 42 gunfire 

noise impulses.
Health Hazard Evaluation Report 2013-0124-3208 Page 7

Depending on the damage risk criterion, the ANE (i.e. trigger pulls) for the Dillon M134D 

per day ranged from 5 using the AHAAH model to 276 using the U.S.DOD criterion (with 

dual hearing protection). To reduce instructors’ noise exposures from the Dillon M134D, 

agency personnel positioned a wooden barrier between the gun muzzle and the shooter and 

instructor (Figure 4). With the barrier in place, the maximum peak sound pressure level was 

160.7 dB, a decrease of about 10 dB. The ANE with the barrier in place increased to 37 based 

on AHAAH criterion and to 17,131 based on the U.S. DOD criterion. Additional acoustical 

treatment of the current barrier design could further increase attenuation of peak sound 

pressure levels.

Figure 4. Firing range instructor kneeling between microphones and behind a noise barrier on top of 

modified SWAT vehicle during firing of Dillon 134D Aero Minigun. Photo by HHE requestor.
Page 8 Health Hazard Evaluation Report 2013-0124-3208

At outdoor ranges 1 and 3, three shooters in a prone position fired their weapons 

simultaneously. One or two instructors usually stood 4–6 feet behind the firing line between 

the shooters (Figure 5) or sat next to the shooters. An additional instructor stood about 10 feet 

behind the shooters. Maximum peak levels at the instructor 1 position (nearest the shooters) 

during shooting of the M4, M240, and M249 ranged from 159–164 dB. The M249 had 

slightly lower peak sound pressure levels. The ANE for instructor 1 ranged from 19 using 

the AHAAH criterion to 412 using the U.S. DOD criterion (with dual hearing protection). 

Because instructor 2 stood farther away from the shooters, maximum peak sound pressure 

levels were less, ranging from 149–151 dB. ANE for instructor 2 was 154 using the AHAAH 

criterion, and > 40,000 using the U.S. DOD criterion.

Figure 5. Two instructors standing between shooters at the outdoor range. Photo by HHE requestor.

At the outdoor ranges each set of shooters usually had 16 trigger presses per live fire 

exercise with each weapon. Figure 6 shows the impulsive noise waveform pattern for two 

separate trigger presses of an M240. The impulsive noise waveform for each trigger press 

had seven distinct noise peaks, corresponding to seven gunshots, occurring in a timespan of 

approximately 0.75 seconds. The same pattern was repeated one-half a second later. Peak 

impulse noise levels ranged from 2000 to 3000 pascals (160–164 dB). Noise levels at all 

frequencies higher than 100 Hz were above 120 dB. However, at 1250 and 1600 Hz noise 

levels were 130–135 dB. Acoustical treatment of the underside of the corrugated metal roof 

and the wall behind the shooters might reduce some of reverberant noise during shooting. 

Based on the AHAAH criterion, instructor 1 would exceed ANE after observing only two 

sets of shooters (assuming 16 trigger presses per set of shooters), but could observe about 25 
Health Hazard Evaluation Report 2013-0124-3208 Page 9

sets of shooters before ANE was exceeded using the U.S. DOD criterion. At the instructor 2 

position, an instructor would exceed the ANE after observing 9 sets of shooters based on the 

AHAAH criterion, but would not exceed the U.S. DOD criterion during a workday because 

the ANE was greater than 40,000.

Figure 6. Screen capture of impulsive noise waveform and one-third octave band noise frequency 

measurement results from NIMS display showing two trigger pulls of M240 rifle.
Page 10 Health Hazard Evaluation Report 2013-0124-3208

At the indoor range, shooters used M4 rifles and 9 mm pistols at distances of 50 yards, 

25 yards, and 5 yards from range targets (Figure 7). During our evaluation, 14 shooters 

fired weapons simultaneously at the 50-yard and 25-yard positions. However, the number 

of shooters can vary from ten to twenty. Shooters fire weapons from standing, kneeling, 

and prone positions. On a typical day, each shooter fires 75 rounds from the M4 rifle and 

50 rounds from the 9 mm pistol. For the 50-yard and 25-yard exercises, instructors stood 

about 6 feet behind the firing line and shooters. Maximum peak sound pressure levels at the 

instructor position were approximately 152 dB for the M4 rifle and 146 dB for the 9 mm 

pistol. Maximum peak levels increased to about 160 dB for 5-yard training exercises. This 

increase in peak levels is most likely because the instructors stand within an arm’s length 

distance from the shooter for the 5-yard exercises. Additionally, instructors were exposed to 

more reverberant noise because the shooters were relatively close to the walls and ceiling of 

the nearby bullet trap.

Figure 7. Live fire exercises using M4 rifle from kneeling position at 50 yard position. Note the NIMS 

microphone at the instructor position behind the firing line. Photo by HHE requestor.
Health Hazard Evaluation Report 2013-0124-3208 Page 11

Using the U.S. DOD criterion, the ANE was 40,000 for most of the training exercises at the 

indoor range, except for an ANE of 6,651 for shooting the 9 mm pistol at the 5-yard position. 

Using the AHAAH criterion, the minimum ANE was 91 for live fire exercises at the 25- and 

5-yard positions. At the 50-yard position the instructor’s minimum ANE was 149 for the 

M4 rifle and 476 for the 9 mm pistol. The indoor range had concrete floors and did not have 

acoustical treatment on the walls and ceiling. Installing acoustical treatment could reduce 

some of the reverberation from gunfire but may not have a substantial impact on the ANE for 

instructors because of their close proximity to the shooters. However, at the 50-yard position 

it may be possible to install three-sided enclosures behind the firing line for instructors to 

stand in during shooting.

Live fire training exercises in the shoothouse simulated tactical scenarios in rooms of 

various sizes and configurations inside a building (Figure 8). Shooters used M4 rifles and 

Remington model 870 12-gauge shotguns. Additionally, full-load and reduced load flash 

bang grenades were used for some exercises. Peak sound pressure levels during shooting of 

the M4 rifle reached about 163 dB when fired in a doorway, but were about 3 dB less when 

fired in a walkway or hallway. The 12-gauge shotgun reached peak levels of about 172 dB 

when fired in a doorway and about 155 dB when fired in a hallway. Using the AHAAH 

criterion, the minimum ANE for the M4 rifle was 5 and for the shotgun was 95. Using the 

U.S. DOD criterion, the ANE was more than 11,000 for the M4 rifle and more than 2,500 

for the shotgun.

Figure 8. Two shooters firing M4 rifles in the shoothouse, with an instructor standing in the doorway. 

Photo by HHE requestor.
Page 12 Health Hazard Evaluation Report 2013-0124-3208

The full-load flash bang grenades generated peak sound pressure levels ranging from 158–

172 dB. Figure 9 provides an example of the impulse noise waveform and octave band noise 

frequency from a full-load flash bang grenade. The wave form was characterized by a very 

large initial impulse followed by several reverberant impulses of rapidly diminishing energy 

over the next 0.5 second. Full-load flash bang grenades generated higher peak sound pressure 

levels than reduced load flash bang grenades discharged at the same location. However, even 

reduced load flash bang grenades generated peak levels of 151.1–168.8 dB. The highest 

peak levels occurred when the flash bang devices were exploded in doorways. Peak sound 

levels were lower when the grenades were exploded in the middle of the room and in the 

walkway. These differences in peak levels are most likely related to greater reverberation 

of the explosion shockwave because of the close proximity of the wall and door frame. The 

minimum ANE for the flash bang grenades ranged from 47 using the AHAAH criterion to 

551 using the U.S. DOD criterion.

Figure 9. Screen capture of impulsive noise waveform and one-third octave band noise frequency 

measurement results from NIMS display of a full-load flash bang grenade.
Health Hazard Evaluation Report 2013-0124-3208 Page 13

Our results showed that the AHAAH model was the most conservative criterion (most 

protective against hearing loss) and the U.S. DOD MIL-STD-1474D (currently used by the 

agency) was the least protective. Results for the LeqA8hr criterion fell between the other 

criteria. We calculated damage risk estimates for instructors using dual hearing protection and 

assumed a combined attenuation for earplugs and earmuffs of at least 34 dB, as prescribed by 

the damage risk criteria in U.S. DOD. On the basis of the U.S. DOD criterion and use of dual 

hearing protection, instructors would not likely exceed the calculated ANE during any of the 

live fire exercises. However, on the basis of the AHAAH criterion, instructors would likely 

exceed the ANE for live fire exercises using the Dillon M134D (with or without the barrier), 

and for the weapons used at the outdoor ranges and shoothouse. Using the equal energy 

criterion, instructors would exceed the ANE at the Dillon M134D if the barrier were not used. 

If ANE were calculated using single hearing protection instead of dual protection, instructors 

would exceed ANE for nearly all of the weapons and live fire exercises, except for exercises 

in the virtual training facility.

We observed a few instances at the outdoor ranges of instructors only using single hearing 

protection. We also observed that not all insert hearing protection appeared to be deeply 

inserted into the ear canal, as recommended by hearing protector manufacturers. Additionally, 

employees can appear to have hearing protection properly inserted, but the hearing protectors 

still may not fit effectively. Lack of proper insertion can substantially reduce the ability of 

the hearing protectors to attenuate noise. NIOSH has developed a hearing protector fit test 

system [Murphy 2014] to help ensure proper selection and fit. Methods and systems for fit 

testing of hearing protection are available from several manufacturers.

Recent NIOSH research on hearing protectors for impulsive noise exposures has shown that 

the combination of ear plugs and muffs worn by instructors can attenuate noise levels by 

36–49 dB, if properly fitted and worn [Murphy et al. 2012; NIOSH 2013b]. If this attenuation 

was achieved, noise levels at the ear of firearms’ instructors could be reduced to below 

the impulse noise exposure limit of 140 dB established by OSHA, and recommended by 

NIOSH, ANSI, ISO, and the EU. Greater noise attenuation of hearing protection should also 

increase the calculated ANE. Although NIOSH research has shown that hearing protectors 

are capable of attenuating more noise than the attenuation levels used for damage risk 

criteria calculations in this report, these higher levels of noise attenuation cannot be assumed 

and hearing protector attenuation should be verified through fit testing. Sound can also be 

transmitted to the inner ear through the skull (bone-conduction), bypassing the hearing 

protector entirely and effectively limiting the attenuation to about 41 dB. 
Page 14 Health Hazard Evaluation Report 2013-0124-3208

Conclusions 

Firearms instructors were exposed to impulsive noise levels greater than 150 dB for most 

of the weapons and weapons systems used during indoor and outdoor live fire training 

exercises. Thus, instructors were exposed above the occupational exposure limit of 140 

dB. The Dillon M134D and flash bang grenades generated peak levels greater than 170 dB. 

Calculation of ANE for live fire exercises using the U.S. DOD currently used by the agency 

showed that instructors wearing dual hearing protection would not exceed the ANE permitted 

on a typical day. In contrast, calculations using the AHAAH criterion showed that instructors 

would likely exceed the ANE permitted for most of the weapons or weapon systems used. 

The U.S. DOD has proposed a revised standard that incorporates a variant of the LeqA8 

criterion and the AHAAH damage risk model. 

Recommendations

On the basis of our findings, we recommend the actions listed below. We encourage the 

agency to use a labor–management health and safety committee or a working group to 

discuss our recommendations and develop an action plan. Those involved in the work can 

best set priorities and assess the feasibility of our recommendations.

Our recommendations are based on an approach known as the hierarchy of controls. This 

approach groups actions by their likely effectiveness in reducing or removing hazards. In 

most cases, the preferred approach is to eliminate hazardous materials or processes and 

install engineering controls to reduce exposure or shield employees. Until such controls 

are in place, or if they are not effective or feasible, administrative measures and personal 

protective equipment may be needed. 

Engineering Controls 

Engineering controls reduce employees’ exposures by removing the hazard from the 

process or by placing a barrier between the hazard and the employee. Engineering controls 

protect employees effectively without placing primary responsibility of implementation on 

the employee. 

1. If feasible and legally permissible, attach noise suppressors to firearms to reduce peak 

sound pressure levels. 

2. Continue to use the noise barrier for all exercises with the Dillon M134D.

3. Install acoustical materials and panels to the canopy and walls at the outdoor firing 

ranges and to the barrier used for exercises using the Dillon M134D. 
Health Hazard Evaluation Report 2013-0124-3208 Page 15

Administrative Controls

The term administrative controls refers to employer-dictated work practices and policies 

to reduce or prevent hazardous exposures. Their effectiveness depends on employer 

commitment and employee acceptance. Regular monitoring and reinforcement are necessary 

to ensure that policies and procedures are followed consistently.

1. Use the LeqA8hr or the AHAAH model to determine the amount of gunfire instructors 

can be exposed to per day. 

Personal Protective Equipment

Personal protective equipment is the least effective means for controlling hazardous exposures. Proper use of personal protective equipment requires a comprehensive program and a 

high level of employee involvement and commitment. The right personal protective equipment must be chosen for each hazard. Supporting programs such as training, change-out 

schedules, and medical assessment may be needed. Personal protective equipment should not 

be the sole method for controlling hazardous exposures. Rather, personal protective equipment should be used until effective engineering and administrative controls are in place.

1. Require instructors to use dual hearing protection for all live fire training exercises.

2. Perform hearing protector fit testing to determine the noise attenuation of the hearing 

protectors used by instructors. 
Page 16 Health Hazard Evaluation Report 2013-0124-3208

Appendix A: Methods

For measurements using the NIMS system, we connected four 1/8-inch condenser pressure 

microphones (GRAS model 40DD) and four 1/4-inch preamplifiers (GRAS model 26AC) 

to two 2–channel power modules (GRAS model 12AA) using LEMO® cables. The outputs 

from the two–channel power modules were connected to a National Instruments (model 

NI–4432) universal serial bus data acquisition board using BNC coaxial cables. The data 

acquisition board operated at a sampling rate of 102.4 kilohertz. The data acquisition board 

was connected to a laptop using a USB 2.0 cable. The NIMS program operated using the 

National Instruments LabVIEW Runtime 2011 64 bit (for Windows 7 or higher) software 

and National Instruments DAQMX 93.5 device drivers, and the Microsoft.Net or Visual 

C redistributable package. We calibrated each microphone using a Brüel and Kjær model 4228 

piston–phone calibrator. 

We calculated damage risk and maximum ANE to gunfire using three different damage risk 

criteria. The NIOSH artificial head fixture was not available at the time of the site visit to 

measure attenuation of actual hearing protectors used by instructors. Therefore, for damage 

risk calculations, we used previously measured values for the single and double hearing 

protectors that instructors used [Garinther and Hodge 1971; NIOSH 2013b].

U.S. Department of Defense Criterion MIL-STD-1474D

We calculated the allowable number of impulsive noise exposures per day using the 

following equation [U.S. DOD 1997]:

N1 = 10X where,

X=1/5 [177-L+6.64 log10200/T]

N2 = 20 x N1 

N1 = allowable number of impulse exposures/day (single protection),

N2 = allowable number of impulse exposures/day (double protection),

L = measured peak sound pressure level, in dB,

T = measured B–duration in milliseconds (msec), if B > 200 msec, use B = 200 msec.

To calculate the allowable number of exposures per day for the shooters and instructors 

wearing hearing protection, we divided N2 by the number of impulses in each exposure. 
Health Hazard Evaluation Report 2013-0124-3208 Page 17

MIL-STD-1474D Definitions and Assumptions

A. A single exposure consists of either 

1. a single impulse for non-repetitive systems (systems producing not more than 

one impulse per second, e.g., single shot rifle or shotgun, or 

2. a burst for repetitive systems (systems normally producing more than one 

impulse per second, e.g., automatic weapons. For repetitive systems such as 

the Dillon Aero M134D and M240/M249 belt–fired machine guns, the B–

duration is calculated by multiplying the average B–duration of every impulse 

in a “gunfire burst” by the number of impulses in the first 200 millisecond of 

that burst, as specified in section 5.4.2 of MIL-STD-1474D. 

B. The maximum permissible number of exposures per day provided in Table 4–I of 

MIL-STD-1474D (40,000) was used when the calculated allowable number of rounds 

exceeded those shown in the table.

A-weighted 8-hour Sound Equivalent Level (LeqA8hr) 

[DTAT 1983; Dancer et al. 1995]

Following measurements of gunfire noise exposures, the NIMS provided LeqA8hr values for 

single or multiple shot exposures. Because instructors wore hearing protectors, we reduced 

the LeqA8hr by 29 dBA for single protection or 34 dBA for double protection [Garinther 

and Hodge 1971]. More recent NIOSH research indicates that greater hearing protector 

reductions are possible, ranging from 26–39 dB for single protection and 36–49 dB for 

double protection [NIOSH 2003; 2013b]. However, for this report, we used the 29 dB and 34 

dB values because they represent a more protective measure of instructors’ potential risk.

We calculated the allowable number of impulsive noise exposures per day on the basis of the 

LeqA8hr criterion by using the following formula:

Allowable number of exposures = 10[85-(LeqA8hr with Hearing Protection Device [HPD]/10]

Where, LeqA8hr with HPD = LeqA8hr value from NIMS – 34 dBA of attenuation for dual 

hearing protection (or 29 dB for attenuation of single protection) 

Ear Modeling Method – Auditory Hazard Assessment 

Algorithms for Humans (AHAAH) [Price and Kalb 1991; 

Price 2007]

The AHAAH ear modeling technique is based on an ear model that explains the behavior of 

the middle and inner ear in response to high intensity impulse noise [Price and Kalb 1991]. 

Following gunfire measurements, the NIMS provided ARU values calculated on the basis 

of dual hearing protection and “warned” noise exposure conditions because instructors 

anticipated high impulse gunfire. We calculated the allowable number of impulsive noise 

exposures per day by dividing the model maximum allowable ARU (i.e., 200) by the number 

of impulses per exposure.
Page 18 Health Hazard Evaluation Report 2013-0124-3208

Appendix B: Tables Table B1. Peak sound pressure levels, damage risk, and allowable number of exposures at outdoor ranges 1, 2, and 3 Weapon/Location Peak level (dB) Number of impulse exposures per trigger pull or impulsive event U.S. DOD criterion MIL-STD-1474D Equal energy criterion LeqA8hr AHAAH Auditory Risk Units B-Duration (msec) Allowable number of exposures LeqA8hr w/ HPD (dB) Allowable number of exposures ARU w/ HPD Allowable number of exposures Dillon M134D/Range 2 168.0–171.4 15–45 8–15 276–1,498 63.0–69.9 32–158 11.3–36.9 5–17 Dillon M134D (with barricade)/Range 2 157.8–160.7 28–52 16–20 17,131–40,000 53.8–59.4 363–1,318 1.4–5.4 37–144 M240/Range 1 (Instructor 1) 158.7–163.9 6–16 82–94 412–2,542 58.8–65.4 89–417 2.2–10.3 19–91 M240/Range 1 (Instructor 2) 147.0–150.7 6–16 92–110 40,000 52.4–56.9 645–1,819 0.4–1.3 154–571

M249/Range 1 (Instructor 1) 157.0–159.0 7–13 50–60 2,772–17,619 53.0–58.6 436–1,585 0.4–1.8 113–526M249/Range 1 (Instructor 2) 147.0–149.2 7–13 66–75 40,000 50.7–56.3 741–2,692 0.3–1.0 208–625M4/Range 1 (Instructor 1) 158.6–164.2 1–3 56–100 1,509–23,727 53.1–59.0 398–1,549 0.5–2.1 95–417M4/Range 1 (Instructor 2) 147.3–149.0 1–3 64–115 40,000 41.9–48.5 4,467–20,417 0.1–0.2 1,111–2,857M240/Range 3 (Instructor 1) 163.6–164.4 14–15 26–44 554–1,467 62.5–64.1 123–178 5.1–6.2 32–39M240/Range 3 (Instructor 2) 148.2–149.8 14–15 35–70 40,000 51.0–52.0 1,995–2,512 0.3–0.4 500–645M249/Range 3 (Instructor 1) 156.8–159.0 8–13 22–46 12,693–26,556 53.0–58.6 436–1,585 0.4–1. 8 113–526M249/Range 3 (Instructor 2) 147.2–149.0 8–13 34–65 40,000 50.7–56.3 741–2,692 0.3–1.0 208–625
Health Hazard Evaluation Report 2013-0124-3208 Page 19 Table B2. Peak sound pressure levels, damage risk, and allowable number of exposures at the indoor range Weapon/Location Peak level (dB SPL) Number of impulse exposures per trigger pull or impulsive event U.S. DOD criterion MIL–STD–1474D Equal energy criterion LeqA8hr AHAAH Auditory Risk Units B-Duration (msec) Allowable number of exposures LeqA8hr w/ HPD (dB) Allowable number of exposures ARU w/ HPD Allowable number of exposures M4 rifle – 50 yards, Instructor 1 150.4–150.5 8–18 > 200 40,000 47.5–51.8 2,089–5,623 0.4–1.2 167–455 M4 rifle – 50 yards, Instructor 2 150.4–151.6 8–18 > 200 40,000 48.2–52.7 1,698–4,677 0.5–1.3 149–385 M4 rifle – 50 yards, Instructor 3 147.9–150.9 8–18 > 200 40,000 47.7–51.6 2,188–5,370 0.4–1.1 182–465 9 mm pistol – 50 yards, Instructor 1 144.6–146.2 10–18 > 200 40,000 43.2–45.6 8,710–15,135 0.2–0.4 541–1,000 9 mm pistol – 50 yards, Instructor 2 142.9–144.8 10–18 > 200 40,000 43.0–45.7 8,511–15,849 0.2–0.3 588–1,111 9 mm pistol – 50 yards, Instructor 3 141.3–143.4 10–18 > 200 40,000 42.9–46.8 6,607–16,218 0.2–0.4 476–1,176 M4 rifle – 25 yards, Instructor 1 149.4–149.5 28 > 200 40,000 53.8 1,318 1.7–1.8 111–116 M4 rifle – 25 yards, Instructor 2 150.2–151.6 28 > 200 40,000 53.2–55.0 1,000–1,514 1.5–2.2 91–131 M4 rifle – 25 yards, Instructor 3 150.8–152.0 28 > 200 40,000 54.2–54.6 1,096–1,202 1.9–2.1 95–106 M4 – 5 yards, Instructor 1 144.3 2 > 200 40,000 45.2 9,550 0.3 800 M4 – 5 yards, Instructor 2 160.1 2 > 200 40,000 49.8 3,311 0.8 256 9 mm Pistol – 5 yards, Instructor 1 159.7 6 > 200 9,613 54.6 1,096 2.2 91 9 mm Pistol – 5 yards, Instructor 2 160.5 6 > 200 6,651 54.2 1,202 1.9 104
Page 20 Health Hazard Evaluation Report 2013-0124-3208 Table B3. Peak sound pressure levels, damage risk, and allowable number of exposures at the shoothouse Weapon/Location Peak level (dB SPL) Number of impulse exposures per trigger pull or impulsive event U.S. DOD criterion MIL–STD–1474D Equal energy criterion LeqA8hr AHAAH Auditory Risk Units B–Duration (msec) Allowable number of exposures LeqA8hr w/ HPD (dB) Allowable number of exposures ARU w/ HPD Allowable number of exposures M4 rifle, Doorway 159.8–162.7 1–12 100–120 11,524–28,553 54.2–56.9 646–1,202 1.8–3.6 55–111 M4 rifle, Walkway 153.3–156 1–12 148–150 40,000 50.0–52.7 1,698–3,162 0.6–1.4 146–323 M4 rifle, Hallway 159.4 2 125 40,000 51.5 2,239 1.2 174 Shotgun 870, Doorway 166.9–171.8 1 30–62 2,724–9,920 55.1–55.2 955–977 1.6–2.1 95–127 Shotgun 870, Walkway 150.5–154.7 1 172–185 40,000 42.0–45.9 8,128–19,953 0.1–0.4 540–1,818 Flash Bang (reduced load), Doorway 163.8 1 125 16,297 50.2 3,020 0.5 392 Flash Bang (reduced load), Walkway 151.1 1 180 40,000 42.5 17,783 0.1 1,667 Flash Bang (reduced load), Midroom 157.7–162.9 1 154–200 13,213 50.7–53.5 692–2,692 1.4–4.3 47–145 Flash Bang (full load), Doorway 171.8 1 100 551 53.5 1,413 0.4 571 Flash Bang (full load), Walkway 157.8 1 170 40,000 43.0 15,849 0.1 1667 Flash Bang (full load), Midroom 168.4 1 96 2,782 57.0 631 3.3 60 Sledgehammer 137.9–140.4 1 84–125 40,000 66.8 50,000 < 0.01 20,000 M249, Virtual training facility 127.7 1 180 40,000 51.4 50,000 1.9 6,667
Health Hazard Evaluation Report 2013-0124-3208 Page 21

References

ANSI [1996]. Determination of occupational noise exposure and estimation of noise–induced 

hearing impairment, American National Standards Institute. American National Standard 

ANSI S3.44–1996. Acoustical Society of America, Melville, New York. 

CFR. Code of Federal Regulations. Washington, DC: U.S. Government Printing Office, 

Office of the Federal Register. 

CHABA [1968]. Proposed damage risk criterion for impulse noise (Gunfire). Committee on 

Hearing, Bioacoustics, and Biomechanics. National Research Council, Washington, D.C. 

CHABA [1992]. Hazardous exposure to impulse noise. Committee on Hearing, Bioacoustics, 

and Biomechanics. National Research Council, Washington, D.C. 

Dancer A, Plessiet D, Vaillant T [1995]. Damage risk criteria for weapons noises. CD nl 241/

CT/DTB/1995. Etablissement Technique de Bourges, Bourges, France. 

DTAT [1983]. Recommendations on evaluating the possible harmful effects of noise on 

hearing. AT–83/27/28. Direction Technique des Armements Terrestres. Etablissement 

Technique de Bourges, Bourges, France. 

Dunn DE, Davis RR, Merry CJ, Franks JR [1991]. Hearing loss in the chinchilla from impact 

and continuous noise exposure. J Acoust Soc Am 90(4):1979–1985. 

ECD [1986]. Protection of workers from risks related to exposure to noise at work. European 

Council Directive 86/188/EEC. Brussels, Belgium. 

Fedele PD, Binseel MS, Kalb JT, Price GR [2013]. Using the auditory hazard assessment 

algorithm for humans (AHAAH) with hearing protection software, release MIL–STD–

1474E. ARL-TR–6748, U.S. Army Research Laboratory, Aberdeen Proving Ground, MD. 

Garinther GR, Hodge DC [1971]. Small rocket noise: hazards to hearing. Advanced LAW 

Program, Report No. HEL–TM–7–71. Army Material Command, Human Engineering Lab. 

Aberdeen Proving Ground, Maryland.

ISO [1990]. Acoustics—determination of occupational noise exposure and estimation of noise–

induced hearing impairment. International Standards Organization, International Standard ISO 

1999:2013. International Organization for Standardization, Geneva, Switzerland. 

Luz G, Hodge R [1971]. The recovery from impulse noise-induced TTS in monkeys and 

men: a descriptive model. J Acoust Soc Am 49(6):1770–1777. 

Mrena R, Savolainen S, Kuokkanen JT, Ylikoski J [2002]. Characteristics of tinnitus induced 

by acute acoustic trauma: a long–term follow–up. Audiol Neurootol 7(2):122–130. 
Page 22 Health Hazard Evaluation Report 2013-0124-3208

Murphy WJ, Flamme GA, Meinke DK, Sondergaard J, Finan DS, Lankford JE, Khan A, 

Stewart M [2012]. Measurement of impulse peak insertion loss for four hearing protection 

devices in field conditions. Int J Aud 51:S31–S42.

Murphy WJ [2014]. Comparing personal attenuation ratings for hearing protector fit-test 

systems. CAOHC Update 25(3).

NIOSH [1998]. Criteria for a recommended standard—occupational noise exposure (revised 

criteria 1998). Cincinnati, OH: U.S. Department of Health and Human Services, Centers 

for Disease Control and Prevention, National Institute for Occupational Safety and Health, 

DHHS (NIOSH) Publication No. 98-126. 

NIOSH [2003]. Health hazard evaluation report: Fort Collins Police Services, Fort Collins, 

CO. By Tubbs R, Murphy W. Cincinnati, OH: U.S. Department of Health and Human 

Services, Centers for Disease Control and Prevention, National Institute for Occupational 

Safety and Health, NIOSH HHE Report No. 2002-0131-2898.

NIOSH [2012]. In–depth survey report: a case for using A–weighted equivalent energy as 

a damage risk criterion. By Murphy WJ, Kardous CA. Cincinnati, OH: U.S. Department of 

Health and Human Services, Centers for Disease Control and Prevention, National Institute 

for Occupational Safety and Health, EPHB–350-11a, 1-32, 2012.

NIOSH [2013a]. Engineering report: development and validation testing of an impulse noise 

meter. By Kardous CA. Cincinnati, OH: U.S. Department of Health and Human Services, 

Centers for Disease Control and Prevention, National Institute for Occupational Safety and 

Health, EPHB–349-11a, 1-31, 2013.

NIOSH [2013b]. In–depth survey report: comparison of two acoustic test fixtures for 

measurement of impulse peak insertion loss. By Khan A, Fackler CJ, Murphy WJ. Cincinnati, 

OH: U.S. Department of Health and Human Services, Centers for Disease Control and 

Prevention, National Institute for Occupational Safety and Health, EPHB–350-13a, 1-40, 2013.

OSHA [1992]. Occupational noise exposure 29 CFR 1910.95. U.S. Government Printing 

Office, Office of the Federal Register. Occupational Safety and Health Administration, 

Washington, D.C. 

Price GR, Kalb JT [1991]. Insights into hazard from intense impulses from a mathematical 

model of the ear. J Acoust Soc Am 90(1):219–227. 

Price GR [2007]. Validation of the auditory hazard assessment algorithm for the human with 

impulse noise data. J Acoust Soc Am 122(5):2786–2802. 

Salmivalli A [1967]. Acoustic trauma in regular army personnel—clinical audiologic study. 

Acta Otolaryngol 222(Suppl):1–85. 
Health Hazard Evaluation Report 2013-0124-3208 Page 23

Starck J, Toppila E, Pyykko I [2003]. Impulse noise and risk criteria. Noise Health 

5(20):63–73. 

U.S. DOD [1997]. MIL-STD-1474D Design criteria standard noise limits. U.S. Department 

of Defense, Washington, D.C.

Ward WD, Fleer RE, Glorig A [1961]. Characteristics of hearing losses produced by gunfire 

and steady noise. J Aud Res 1:325–356. 

Ylikoski ME [1994]. Prolonged exposure to gunfire noise among professional soldiers. Scand 

J Work Environ Health 20(2):87–92.
Page 24 Health Hazard Evaluation Report 2013-0124-3208

Keywords: North American Industry Classification System 928110 (National Security), 

noise, impulse noise, impulsive noise, peak noise, damage risk criteria, firearms, weapons, 

guns, gunfire, hearing loss
Health Hazard Evaluation Report 2013-0124-3208 Page 25

The Health Hazard Evaluation Program investigates possible health hazards in the 

workplace under the authority of the Occupational Safety and Health Act of 1970 

(29 U.S.C. § 669(a)(6)). The Health Hazard Evaluation Program also provides, 

upon request, technical assistance to federal, state, and local agencies to investigate 

occupational health hazards and to prevent occupational disease or injury. Regulations 

guiding the Program can be found in Title 42, Code of Federal Regulations, Part 85; 

Requests for Health Hazard Evaluations (42 CFR Part 85).

Disclaimer

The recommendations in this report are made on the basis of the findings at the 

workplace evaluated and may not be applicable to other workplaces.

Mention of any company or product in this report does not constitute endorsement 

by NIOSH.

Citations to Web sites external to NIOSH do not constitute NIOSH endorsement of the 

sponsoring organizations or their programs or products. NIOSH is not responsible for 

the content of these Web sites. All Web addresses referenced in this document were 

accessible as of the publication date.

Acknowledgments

Desktop Publisher: Shawna Watts

Editor: Ellen Galloway 

Logistics: Donnie Booher

Availability of Report

Copies of this report have been sent to the employer, employees, and union at the 

facility. The state and local health department and the Occupational Safety and Health 

Administration Regional Office have also received a copy. This report is not copyrighted 

and may be freely reproduced. 

This report is available at http://www.cdc.gov/niosh/hhe/reports/pdfs/2013-0124-3208.pdf.

Recommended citation for this report:

NIOSH [2014]. Health hazard evaluation report: measurement of exposure to 

impulsive noise at indoor and outdoor firing ranges during tactical training 

exercises. By Brueck SE, Kardous CA, Oza A, Murphy WJ. Cincinnati, OH: U.S. 

Department of Health and Human Services, Centers for Disease Control and 

Prevention, National Institute for Occupational Safety and Health, NIOSH HHE Report 

No. 2013-0124-3208.
Delivering on the Nation’s promise:

Safety and health at work for all people through research and prevention

To receive NIOSH documents or more information about 

occupational safety and health topics, please contact NIOSH:

Telephone: 1–800–CDC–INFO (1–800–232–4636)

TTY: 1–888–232–6348

CDC INFO: www.cdc.gov/info

or visit the NIOSH Web site at www.cdc.gov/niosh

For a monthly update on news at NIOSH, subscribe to 

NIOSH eNews by visiting www.cdc.gov/niosh/eNews.