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The reduction of gunshot noise and auditory risk through the use of firearm suppressors and
low-velocity ammunition
1William J. Murphy, 2Gregory A. Flamme, 1Adam R. Campbell
1Edward L. Zechmann, 2,3Stephen M Tasko, 4James E. Lankford,
5Deanna K Meinke, 5Donald S. Finan, 6Michael Stewart
1Hearing Loss Prevention Team, Engineering and Physical Hazards Branch, Division of Applied
Research and Technology, National Institute for Occupational Safety and Health, 1090 Tusculum
Ave. Mailstop C-27, Cincinnati, OH USA.
2Stephenson and Stephenson Research and Consulting, LLC, 2264 Heather Way, Forest Grove,
OR USA
3Department of Speech, Language and Hearing Sciences, Western Michigan University,
Kalamazoo, MI USA
4Allied Health and Communication Disorders, Northern Illinois University, Dekalb, IL USA
5Audiology and Speech-Language Sciences, University of Northern Colorado, Greeley, CO USA.
6Department of Communication Disorders, Central Michigan University, Mount Pleasant, MI
USA
Corresponding author: William J. Murphy, wjm4@cdc.gov, (513) 533-8125.
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Abstract
Objective: This research assessed the reduction of peak levels, equivalent energy and sound
power of firearm suppressors.
Design: The first study evaluated the effect of three suppressors at four positions around four
firearms. The second study assessed the suppressor-related reduction of sound power with a
3-meter hemispherical microphone array for two firearms.
Results: The suppressors reduced exposures at the ear between 17 and 24 dB peak sound
pressure level and reduced the 8-hour equivalent A-weighted energy between 9 and 21 dB
depending upon the firearm and ammunition. Noise reductions observed for the instructor’s
position about a meter behind the shooter were between 20 and 28 dB peak sound pressure
level and between 11 and 26 dB LAeq,8h. Firearm suppressors reduced the measured sound
power levels between 2 and 23 dB. Sound power reductions were greater for the low-velocity
ammunition than for the same firearms fired with high-velocity ammunition due to the effect of
N-waves produced by a supersonic projectile.
Conclusions: Whereas firearms suppressors are an effective engineering control to reduce
firearm noise exposures, the cumulative exposures of suppressed firearms can still present a
significant hearing risk. Therefore, firearm users should always wear hearing protection
whenever target shooting or hunting.
Keywords: Noise induced hearing loss, firearm suppressors, and damage risk criteria
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Introduction
Gunfire is noisy. Peak sound pressure levels have been reported for small caliber rifles, pistols
and shotguns ranging between 140 decibels peak sound pressure level (dB peak SPL) for a .22
caliber rifle to well above 175 dB peak SPL for a .30 caliber rifle with a muzzle brake (Murphy et
al 2012). The impulses from gunfire present a significant hazard to the hearing of the shooter
and nearby shooters or bystanders. The most common approach to protecting the shooter and
bystanders from the high-level impulse exposures has been to provide personal protective
equipment - hearing protection devices (HPDs). However, the standard for industrial hygiene
practice has been to follow a hierarchy of controls, beginning with eliminating or replacing the
process that produces the exposure, then moving to engineering or administrative solutions to
minimize the exposure, and then finally relying on personal protective equipment as a last
resort. Firearm suppressors are an engineering noise control. Limiting the time or number of
rounds or the type of ammunition that a person may fire in a given training session is an
administrative control.
The pull of a gun trigger initiates a chain reaction of events that result in one or more
projectiles being fired down range. The trigger releases a firing pin that strikes a cartridge
containing a primer, powder and the projectile. The primer ignites and combusts the powder
that forces the projectile out of the barrel. After the projectile exits the barrel, the gases and
unburnt propellant follow and produce what is called the muzzle blast. Depending upon the
ammunition characteristics, the projectile may be accelerated beyond the speed of sound in air
thus breaking the sound barrier. If the bullet is accelerated to supersonic speed, the waveform
observed down range will include a ballistic N-shaped wave (N-wave) in addition to the muzzle
blast (See Figure 1). The N-wave will start at the trajectory of the bullet and radiate conically
outwards from that line.
The muzzle blast is characterized by a sharp pressure rise that generally follows an
exponentially decaying oscillation as the gases condense at the muzzle blast and then return to
their quiescent state. As an engineering noise control, the firearm suppressor minimizes the
muzzle blast by breaking up the initial wavefront. The shock front can be disrupted by passing
through a series of baffles in the suppressor. The peak energy escaping the muzzle is
diminished by allowing the expanding gasses to pass through small orifices separating the baffle
sections. Thus, the effectiveness of the suppressor will depend upon the length of the
suppressor, the number of baffles and orifice dimensions within the suppressor.
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Figure 1. An example of an N-wave preceding the muzzle blast. The peak levels of the N-wave and muzzle blast are indicated
with inverted triangles. Figure adapted from data reported in Rasmussen et al 2009, Figure 9.
Under ideal conditions, the suppressor does not alter the velocity of the projectile. Therefore,
if the propellant charge is capable of accelerating the projectile to a supersonic speed, an N-
wave will still be produced. This component of firearm noise associated with the projectile is
not expected to be altered by the use of a suppressor.
Damage Risk Criteria for Noise Exposure
Several noise exposure damage risk criteria (DRCs) for small caliber firearms exist to protect
persons from hearing hazards. The simplest of these criteria is based on the peak level,
wherein peak levels over 140 dB SPL are considered hazardous to adults and peak levels over
120 dB SPL are considered hazardous to children (WHO 1997). Up until 2015, the U.S.
Department of Defense used the MIL-STD 1474D as a de facto DRC. The peak sound pressure
level of the weapon, the B-duration (the time for the envelope of the gunshot to decay by 20 dB
from the peak impulse level) and the number of shots that were expected to be fired were used
to estimate the allowable number of rounds that a person could “safely” fire. The MIL-STD-
1474D included no limits for impulsive sounds with peaks below 140 dB SPL, and it assumed
that all exposed listeners would use hearing protectors above 140 dB SPL. In reference to this
Muzzle Blast
175 dB peak SPL
N-wave
172 dB peak SPL
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standard, some suppressors are labeled as “hearing safe” if they are not expected to allow
sound levels in excess of 140 dB SPL.
In related damage risk criteria research, Atherley and Martin (1971) first proposed using an
integrated A-weighted equivalent energy as a damage risk criterion. Stevin et al. (1982)
examined a variant of A-weighted equivalent energy, sound exposure level (SEL) for a 1-second
exposure, and concluded that a 135 dB SEL could provide a reasonable DRC for an impulse
exposure with a peak level of 170 dB SPL. Dancer et al. (1995) also proposed a similar DRC limit
value of LAeq,8h = 85 dB. The LAeq,8h criterion is based upon filtering the acoustic signal to
approximate the transfer function of the auditory periphery and integrating its energy. The A-
weighting curve is derived from the iso-loudness curve at 40 phons and it is implemented into
most sound measurement instruments in use today (ANSI/ASA S1.4, 2014).
Price and Kalb (1991) proposed the use of an electroacoustic model of the auditory system, the
Auditory Hazard Assessment Algorithm for Human (AHAAH). Zagadou et al. (2015) proposed a
different electroacoustic cochlear model that purports to represent the integrated energy
received by the cochlea (ICE). In 2015, the US Department of Defense promulgated MIL-STD
1474E for noise limits of military materiel. This standard has previously been used as a de facto
noise criterion because it defined limits for the use of no hearing protection (peak levels below
140 dB SPL), single hearing protection, and double hearing protection. It also defined exposure
limits that should not be exceeded because excessive exposures could produce damage to
other parts of the body (e.g. lungs, gut, or other organs). The revised MIL-STD 1474E standard
includes a modification to the equivalent energy of the LIAeq,100ms and the AHAAH model.
LIAeq,100ms is derived from the LAeq,8h with an adjustment for the duration of the initial peak
overpressure of the impulse.
The choice of a DRC is open to a good deal of interpretation. The peak sound pressure levels
and the 8-hour A-weighted equivalent energy were selected as the metrics to characterize the
suppressor performance because they related to traditional metrics describing exposures and
risk assessments of impulse noise. The exposure limit of LAeq,8h = 85 dB was used to estimate
permissible exposures in this study.
Prior Evaluations of Firearm Suppressors
Suppressor effects have been evaluated by many researchers. Skochko and Greveris (1968)
conducted an extensive study of firearms with and without suppressors. While their results are
difficult to interpret due to a lack of information about microphone locations, they generally
found between 10 and 35 dB peak reductions for the suppressors. They note that the nearfield
levels downrange will generally be dominated by the muzzle blast and the supersonic
projectile’s N-shaped shock wave (N-wave). They also note that low-velocity projectiles will
generate noise as the projectile displaces air along the trajectory and as the turbulence in the
wake produces vortices that are shed at a regular frequency.
Pääkönen (2008) considered the use of firearm noise suppressors to reduce the impact of firing
ranges on community noise annoyance. Shotguns produced peak impulse levels that were
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approximately 65 dBA at 2 kilometers in the direction of shooting and 65 dBA at 1.4 kilometer
to the right and left sides of the shooter. Tactical rifles had less of a noise footprint and
extended to about 1 km to the sides of the shooter. Pistols produced 65 dBA peak levels at
about 1 km in front and 0.4 km to the side of the shooter while small bore rifles (.22 caliber)
produced 65 dBA peaks at about 0.5 km in front and 0.3 km to the side of the shooter.
Pääkönen (2008) also found that suppression was largely ineffective beyond 30 meters in front
of the shooter. At distances of about 10 meters to the side of the shooter, the reductions in C-
weighted levels due to the suppressor ranged from about 15 to 20 dB.
Lobarinas et al. (2015) considered the performance of several suppressors with semi-automatic
rifles of two different calibers: the widely used .223 caliber Armalite 15 (AR-15), and the .300
caliber Blackout. The attenuation that they reported varied with the suppressor that was used
and the measurement location. Generally, they reported between about 20 and 30 dB of peak
reduction for the muzzle or the left ear. For the weapons that had a gas ejection port
associated with the cycling of the semi-automatic weapons, the reduction at the right ear of the
shooter was less, about 10 to 18 dB.
Nakashima (2015) conducted a series of measurements of small-caliber firearms with and
without suppressors. For the three firearms evaluated at 0.5 to 1 m to the side of the shooter,
the levels of peak reduction were 22 dB for the 5.56 mm C8 semi-automatic rifle, 29 dB for the
8.6 mm C14 medium range sniper rifle, and 32 dB for the 12.7 mm C15 long range sniper rifle.
Nakashima (2015) did not report the reductions in terms of other metrics such as equivalent
energy, although they did consider the effects of hearing protection devices on the allowable
number of exposures.
In 2015, the North American Treaty Organization (NATO) working group published the NATO
AEP-4785 Standard for testing suppressors and measuring the acoustic signature of small arm
suppressors (NATO, 2015). The purpose of this standard was to accurately measure the far-
field acoustic characteristics of a suppressor for small-caliber firearms from an elevated firing
position. The method focuses primarily on the reduction of the muzzle blast and excludes two
components of the acoustic signature, the N-wave and the ground reflection. The elevation of
the platform helps to ensure that any reflection from the ground will be separated by about 13
ms from the initial muzzle blast. The NATO method uses a 12.5 ms time window to isolate the
muzzle blast from any N-wave that might be present and the arrival of the ground reflection.
The NATO method will only characterize the effect of the suppressor on the blast wave. The
ground reflection contributes to the acoustic hazard of a firearm. In some conditions where
troops or public safety officers might be advancing toward an objective, the N-wave could
contribute significantly to the acoustic hazard.
Sound Power Calculations
In addition to the exposures that might be received at the ear of the shooter, this paper reports
the sound power for the suppressed and unsuppressed firearms measured with a 3-meter
radius hemispherical array of microphones. The sound power characterizes the total energy
radiated from a noise source and yields the directivity and power as a function of frequency
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bands (ANSI/ASA S12.54, 2011). Microphones are spaced about the hemisphere and the sound
energy passing across each microphone is summed to estimate the total energy radiated
through the hemispherical surface. The sound power level is first calculated in one-third octave
bands by integrating over the surface, then correcting for the reflections, and lastly summing
across the one-third octave bands to obtain the overall sound power level in dB relative to 10-12
Watts. Because the radiation of a gunshot is highly directional, the sound power may provide
insight regarding the total energy of noise radiated by a firearm and of the noise reduction
afforded by the suppressor because it is integrated over the entire hemisphere and not just a
single location of a microphone.
Purpose
Exposure to firearm noise is the leading cause of hearing loss among military, law enforcement,
and public safety officers (Ylikoski & Ylikoski, 1994). The prevalence of hearing loss among
youth and adult recreational firearm users who engage in target shooting or hunting is greater
than that observed for the general public (Stewart et al 2002; Stewart et al 2009; Stewart et al.
2014; NHCA, 2017). Because the hearing loss associated with firearm noise exposure often
presents as a precipitous loss of high frequency hearing, the impairment can be difficult to
remediate. Audiologists frequently see these configurations among their clients and need to
have effective solutions to prevent further hearing loss. Relying upon hearing protection alone
does not provide sufficient protection to the shooter because the hearing protection is
frequently improperly fit or not worn at all. Hunters do not typically use protection because
their ability to hear their quarry is dramatically reduced. Wearing typical earmuffs or earplugs
causes the hunter to lose situational awareness. Electronic hearing protection offers the ability
to hear environmental cues while still affording protection against the firearm noise. However,
Stewart et al. (2014) reported that hunters typically use protection only about 20% of the time.
Often the hunters are using larger bore rifles or shotguns with sufficient energy to harvest large
game animals such as deer. Hearing protection is largely ineffective because a single shot can
produce temporary or permanent hearing loss. Therefore, firearm suppressors that
significantly reduce the muzzle blast by 20 dB or more present a viable solution to effectively
reduce the potential noise exposure. The use of a suppressor coupled with hearing protection
can provide for a more hearing safe experience.
The purpose of this paper was to evaluate the noise reduction of firearm suppressors for high-
and low-velocity ammunition with two different microphone configurations. High-velocity
ammunition was expected to accelerate the projectile to supersonic speed and therefore
produce an N-wave. The first study was conducted with the Michigan Department of Natural
Resources (DNR) at the Rose Lake outdoor firing range (Lansing, MI). Microphones were
positioned to evaluate the effective reduction of the suppressors on four different firearms. In
the second study conducted at a hunting camp in Rudyard, MI, an array of 10 microphones
positioned on a 3-meter radius hemisphere was used to measure the sound power of two
different firearms with and without a suppressor.
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Study 1: Rose Lake Four Microphone
Study 1: Methods
Table 1 lists the different firearms and suppressors used for both the Study 1 (Rose Lake) and
Study 2 (Rudyard). The GEMTECH HVT-QM 7.62 and G5 5.56 suppressors have a proprietary
quick-mount, bi-lock system such that the suppressor slides over the end of the muzzle
providing a repeatable and secure mount. The HVT 7.62 and G5 5.56 suppressors have a series
of v-shaped cones that fit within the suppressor cylinder can and are welded together to form
the baffles that diffuse the muzzle blast energy. The GEMTECH Outback IID threads onto the
end of the muzzle and has a series of six K-shaped baffles. At the Rose Lake study, the low-
velocity ammunition was not used with the Ruger Charger pistol because it could have jammed
the loading mechanism, which relied on a greater volume of gas ejection than was likely to be
produced by the ammunition. The Remington 700 rifles and the Savage MK-11 were bolt action
rifles. A tripod gun stand was used to steady the forearm or barrel of the gun. The firearms
were fired by right-handed shooters from the standing position. Five shots were recorded for
each ammunition type and suppressor condition.
For Study 1, four microphone locations around a right-handed shooter were used to capture
the noise at 0.6 m to the left of the muzzle, at 0.35 m to the right and left of the shooter’s ears
and at approximately 1 m behind the shooter’s head where an instructor might be positioned.
Two microphones were placed at each position, a polarized pressure (200 volt) microphone in
grazing orientation (pointed vertically) and a prepolarized free-field microphone (0 volt)
pointed at the muzzle. The microphones used were a G.R.A.S. Sound and Vibration 40DP 1/8th
inch pressure microphone at 0.6 m to the left of the muzzle. The right and left ear microphones
were Brüel & Kjaer 4136 ¼-inch pressure microphones positioned 0.35 m from the center line
of the firearm and the microphone 1.0 m behind the shooter’s head was a Brüel & Kjaer 4136
microphone as well. The height of the microphones above the ground was 1.63 m and the tip
of the muzzle was 1.53 meters above the ground. When the firearm suppressor was used, the
muzzle microphone was moved along the direction of fire so that it was in the plane of the
muzzle.
Impulse waveforms were acquired from all microphones with a National Instruments PXIe-4499
series 16-channel data acquisition card, sampled at 200 kHz with a 24-bit resolution and a
dynamic range of ±10V. The software controlling the system was a custom-designed NIOSH
Sound Power VI (virtual instrument), which stored results in a MATLAB binary data file for
analysis. The data reported are only from the pressure microphones because the unsuppressed
conditions produced an overload of the prepolarized microphone at the muzzle for some of the
firearms tested.
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Table 1. Firearm, ammunition, caliber, muzzle velocity and suppressor evaluated in Study 1 at Rose Lake. The Savage Mark II
rifle and Ruger 22 Charger pistol were evaluated in Study 2 at Rudyard with the sound power measurements.
Firearm
Ammunition
Caliber
Rated
Muzzle
Velocity,
Feet/Sec
Suppressor
Remington 700 Rifle
Winchester 168 grain
0.308
2670
High velocity
GEMTECH
HVT QM 7.62
Remington 700 Rifle
Beck WIN 168 grain
0.308
990
Low velocity
Remington 700 Rifle
Federal FMJBT 55 grain
0.223
3240
High velocity
GEMTECH
G5 5.56
Remington 700 Rifle
Beck REM 52GRJHP
0.223
1000
Low velocity
Savage Mark II Rifle
Winchester 22 Long Rifle
40gr Super-X
0.22
1280
High velocity
GEMTECH
Outback IID
Savage Mark II Rifle
CCI 22LR segmented
hollow point 40 grain
0.22
1050
Low velocity
Ruger 22 Charger Pistol
Winchester 22 Long Rifle
40gr Super-X
0.22
1280
High velocity
GEMTECH
Outback IID
Study 1: Analysis
The impulses were processed for peak SPL, the 8-hour A-weighted equivalent energy levels
(LAeq,8h). The LAeq,8h values were determined as follows:
, = 10 log
()
+10 log
+10 log(), (1)
where t1 was a 20 ms pretrigger and the duration, t2 – t1, was 100 ms. The number of impulses
evaluated for the LAeq,8h was one impulse. The pressure signal was filtered through an A-
weighting filter in MATLAB from Zechmann (2013). The 100-ms duration provided a uniform
time window that minimized influence from other sounds.1
Study 1: Results
The data were analyzed to determine the reductions of the peak impulse levels and A-weighted
equivalent energy levels at each position. In Table 2, the peak level results are described for
each combination of firearm, ammunition type, and suppressor condition. The reductions at
the muzzle microphone and the instructor position were greater than those observed at the
ear-level microphones for the Remington .308 and .223 caliber rifles. For the Savage rifle, the
1 Other equivalent energy metrics could have been used, Sound Exposure Level (SEL) or the new LIAeq,100ms. To
convert LAeq,8h to SELA, add 44.6 dB. To convert LAeq,8h to LIAeq,100ms, add 54.6. All of the unsuppressed A-durations
were less than 0.2 ms, which in turn requires no correction for the A-duration (MIL-STD 1474E, 2015, p 45, Eq. 3a).
Page 10 of 25
peak reductions were comparable across all of the positions, between 20 and 22 dB. For the
firearms and suppressors that were evaluated, the differences in the observed reductions
between the left and right ear microphones tended to be 2 dB or less with the exception of the
Ruger Charger pistol which was 5 dB.
Table 2. Average peak impulse levels for five shots and noise reductions for the firearms tested in Study 1 for each condition of
suppressor and ammunition velocity. The noise reductions are highlighted with gray shading and are the difference in the
averaged peak levels for the unsuppressed minus the suppressed condition.
Firearm Muzzle Velocity Suppressor
Condition
Peak Sound Pressure Levels and Reductions
(shaded) (dB SPL)
Muzzle
Left Ear
Right Ear
Instructor
Ruger Charger
Unsuppressed
160
152
152
135
.22 Caliber
High Velocity
Suppressed
138
128
133
113
Pistol
Reduction
22
24
19
22
Unsuppressed
149
140
140
123
Low Velocity
Suppressed
128
118
120
103
Savage MK-11
Reduction
21
22
20
20
.22 Caliber
Unsuppressed
152
141
141
125
Rifle
High Velocity
Suppressed
131
120
121
105
Reduction
21
21
20
20
Unsuppressed
157
140
140
127
Low Velocity
Suppressed
131
122
120
100
Remington 700
Reduction
26
18
20
27
.223 Caliber
Unsuppressed
176
160
160
148
Rifle
High Velocity
Suppressed
148
134
136
120
Reduction
28
26
24
28
Unsuppressed
164
148
149
134
Low Velocity
Suppressed
137
131
132
111
Remington 700
Reduction
27
17
17
23
.308 Caliber
Unsuppressed
176
161
161
150
Rifle
High Velocity
Suppressed
150
137
136
123
Reduction
26
24
25
27
As shown in Table 3, reductions in LAeq,8h ranged between 9 and 26 dB across all conditions.
The Savage MK-11 exhibited reductions in LAeq,8h of 12 and 9 dB for the left and right ear,
respectively, for the low-velocity condition. The average reductions in LAeq,8h at the instructor
position tended to be slightly greater than the reductions at the right and left ear positions.
The average reductions in LAeq,8h at the left and right ear were not significantly different than
one another. Differences between the left and right ears were not expected for the bolt-action
rifles because the noise primarily emanates from the muzzle or the end of the suppressor. For
the rifles and the pistols, the LAeq,8h suppressed levels tended to be about 1 dB higher at the
right-ear microphone than the left-ear microphone in the suppressed condition. In the
suppressed condition, Lobarinas et al. (2016) observed a larger difference between the left and
right ear microphones which they attributed to a gas ejection port on the semi-automatic rifle.
Page 11 of 25
The Ruger Charger pistol was semi-automatic but does not use a large volume of gas to cycle
the action compared to firearms using more propellant.
Table 3. Average 8-hour A-weighted equivalent energy levels, LAeq,8h, for five shots and noise reductions for the firearms tested
in Study 1 for each condition of suppressor and ammunition velocity. The noise reductions are highlighted with gray shading
and are the difference in the averaged peak levels for the unsuppressed minus the suppressed condition.
Firearm Muzzle Velocity Suppressor
Condition
8-hour A-weighted Equivalent Energy levels and
Reductions (shaded) (dB(A))
Muzzle
Left Ear
Right Ear
Instructor
Ruger Charger
Unsuppressed
71
63
63
53
.22 Caliber
High Velocity
Suppressed
47
48
49
33
Pistol
Reduction
24
15
14
20
Unsuppressed
55
49
48
36
Low Velocity
Suppressed
38
37
39
25
Savage MK-11
Reduction
17
12
9
11
.22 Caliber
Unsuppressed
60
50
50
39
Rifle
High Velocity
Suppressed
42
35
36
24
Reduction
18
15
14
15
Unsuppressed
66
54
54
44
Low Velocity
Suppressed
41
34
35
21
Remington 700
Reduction
25
20
19
23
.223 Caliber
Unsuppressed
88
75
75
68
Rifle
High Velocity
Suppressed
62
54
55
42
Reduction
26
21
20
26
Unsuppressed
74
61
61
51
Low Velocity
Suppressed
49
44
45
30
Remington 700
Reduction
25
17
16
21
.308 Caliber
Unsuppressed
90
76
76
69
Rifle
High Velocity
Suppressed
65
57
57
45
Reduction
25
19
19
24
The reductions in LAeq,8h provide an indication of the effect of the ammunition and suppressor
on auditory risk. When LAeq,8h is reduced by 3 dB, the risk is reduced by a factor of 2 and the
allowable number of rounds double because of the last term in Eq. 1, 10 log(). For
example, the 32-dB reduction at the left ear microphone for the .308 Remington 700 rifle (Table
3) from LAeq,8h = 76 dB for the unsuppressed high velocity ammunition to LAeq,8h = 44 dB for the
suppressed low velocity ammunition corresponds to a risk reduction factor of nearly 1,600.
Study 1: Inferential Analyses
Multivariable linear regression models revealed small differences between the signals near the
shooter’s ears for both LAeq,8h (F5,64 = 13.4; p < 0.0005; R2=0.47) and peak (F4,65 = 3.74; p = 0.009;
R2=0.23) levels. Controlling for the other factors in the model, the LAeq,8h values at the right ear
Page 12 of 25
were approximately 1 dB greater (95 % CI: 0.9 to 1.6 dB) than the left in suppressed conditions.
The LAeq,8h values were approximately 0.4 dB lower (95 % CI: -0.82 to -0.05 dB) at the right ear
than the left when high-velocity ammunition was used. Post-hoc comparisons across guns
indicated that this exposure asymmetry was greater for the .22 caliber pistol (0.89 dB) than the
.308 caliber rifle (0.02 dB). A similar pattern of results was observed in analyses of peak levels,
with the exception that the type of ammunition had no significant relationship with the
difference between ears.
The inferential evaluation of the suppressors, ammunition velocity, and guns yielded
statistically significant models at all microphone locations, with each model accounting for at
least 91 % of the variance in the observed LAeq,8h values. Both suppressors and low-velocity
ammunition reduced sound levels at all microphone locations. In addition, the use of low-
velocity ammunition in a gun with a suppressor produced additional reductions in sound levels
beyond the simple combination of the individual effects. At the left ear microphone location,
for example, the use of a suppressor and low-velocity ammunition each had a main effect on
LAeq,8h by approximately 17 dB (16.9 and 17.2 dB, respectively), and in combined (suppressor
and low-velocity ammunition) conditions, the sound levels were, on average, 40 dB lower than
in the unsuppressed high-velocity conditions, indicating an additional 6 dB (95 % CI: 2.5 to 11
dB) of sound reduction. The combined use of a suppressor and low-velocity ammunition
achieved similar benefits at all microphone locations in this study (See Figure 2).
In Figure 2, the average reductions of peak levels for each of the firearms are compared to the
reductions for LAeq,8h. Further details are available in Tables 2 & 3. The open symbols indicate
the high velocity ammunition and the closed symbols indicate the low velocity ammunition. The
different symbols indicate the location of the microphone position around the firearm. The
peak level reductions tended to be greater than the reductions observed for LAeq,8h. Except for
the muzzle microphone position for the Ruger Charger .22 pistol, the reductions for the peak
levels were greater than those observed for the LAeq,8h for the high velocity ammunition. For
the low velocity ammunition, the .223 caliber Remington 700 at the left ear was the only case
where the peak level reduction was less than the LAeq,8h reduction. This finding suggests that
reductions expressed as changes in peak level may not completely represent suppressor
effectiveness.
Page 13 of 25
Figure 2. Comparison of suppressors’ noise reduction assessed with change in peak level versus change of 8-hour A-weighted
equivalent energy, LAeq,8h from Study 1. Open symbols denote the high-velocity rounds and closed symbols are low-velocity
rounds. The purple diamonds, blue squares, red circles, and cyan triangles are the reductions measured at the muzzle, left ear,
right ear and instructor positions, respectively.
Study 2: Rudyard Sound Power
Study 2: Methods
As shown in Figure 3, the microphones were placed at the standard locations for sound power
measurements (ANSI/ASA S12.54, 2011). At each location, a ¼-inch prepolarized microphone
and a ½-inch prepolarized free-field microphone pair were located 2.5 cm apart and pointed
toward the center of the hemisphere base. Only the data from the ¼-inch microphones were
used since the ½-inch microphones saturated when measuring the unsuppressed conditions.
The muzzle was 1.55 m above the center of the base of the hemisphere.
Page 14 of 25
Figure 3. The microphone locations and relative positions on the hemisphere for the sound power measurements in Study 2.
The shooter was inside the hemisphere and the muzzle was located over the origin of the hemisphere. The ten microphones are
numbered and their corresponding positions are listed in the inset table.
The firearms used for the sound power study were the .22 caliber Ruger Charger pistol and the
.22 caliber Savage MK-11 bolt action rifle. The firearms were fired from the standing position
by a right-handed shooter for the sound power measurements. Between five and ten shots
were taken in each combination of ammunition type and suppressor condition. The impulse
recordings were reviewed and all impulses were used in the analysis.
The data acquisition system collected about 10 to 15 second recordings that were analyzed in
multiple stages. The location of each impulse was obtained by identifying the peaks
corresponding with the expected number of impulses. A one-third octave-band fifth-order
Butterworth filter was applied at each of the one-third octave band center frequencies from 20
Hz to 20,000 Hz creating 31 bands of data with 10 microphone locations. A time window of 18
ms was applied to the filtered signals centered upon the impulse identified in the first analysis
step. The equivalent energy, LEq,f,i, was computed for each band and microphone location. The
sound power was estimated using equations 22 and 23 from ANSI/ASA S12.54 (2011). Finally,
the sound power levels, LW,f, were averaged across the ten microphone locations assuming
equal areas for each microphone location according to the following formula,
Page 15 of 25
,=1010,,
+10
, , (2)
where LEq,f,i is the equivalent energy over the time interval for the sample in the frequency
band f = {20 Hz, … 20 kHz}, i denotes the i th microphone location, is the area of the
measurement surface 2, is the reference area 1 m2, , is the background noise
correction of the f th band, , is the environmental correction of the f th band. The gun blasts
had a much higher sound pressure than the background noise, so the , were assumed to be
zero. The measurements were made outdoors in an area with sandy-rocky soil at the surface
and thin, mowed grass, so the , were assumed to be zero. No correction for absorption was
made. The total sound power, , is calculated by summing over the 31 frequency bands using
the formula
=1010,
. (3)
The median unweighted sound power levels are reported for each condition and the error bars
were determined as the 25th and 75th quantiles.
Study 2: Results
The sound power level (Table 4) for the high-velocity ammunition was greater than the low-
velocity ammunition. The suppressor reduced the sound power level by 23 dB and 16 dB for
the low-velocity ammunition for the pistol and rifle, respectively. The high velocity ammunition
was reduced by 15 dB and 2 dB for the pistol and rifle, respectively.
Table 4. Study 2 median overall sound power levels and reductions in sound power for each condition of suppressor and
ammunition velocity. The sound power level reductions are highlighted with gray shading and are the difference in the median
sound power levels for the unsuppressed minus the suppressed condition.
Firearm Suppressor Condition
Median Overall Sound Power Level
LW (dB re 10
-12
W)
Low Velocity
High Velocity
Ruger Charger
Unsuppressed
145
150
.22 Caliber
Suppressed
122
135
Pistol
Reduction
23
15
Savage MK-11
Unsuppressed
130
141
.22 Caliber
Suppressed
114
139
Rifle
Reduction
16
2
Page 16 of 25
Figure 4. The effects of suppressor and velocity on sound power of a Ruger Charger .22 caliber pistol in Study 2. The median
sound power levels of the Ruger pistol at each one-third octave frequency band (LW,f) are displayed for the high-velocity/low-
velocity ammunition, and for the suppressed/unsuppressed conditions. The median overall sound power levels (LW) are given in
the legend.
In Figure 4, the median sound power levels (LW,f) and associated ranges for the Ruger Charger
pistol are displayed for each one-third octave frequency band, high-velocity/low-velocity
ammunition, and for the suppressed/unsuppressed conditions are displayed. The spectra for
the unsuppressed conditions are similar to the spectrum for a Friedlander waveform (circles
and squares). The suppressed spectrum tended to exhibit the greatest reduction for the
frequencies below about 2000 Hz. The high-velocity suppressed waveform has increased
spectral energy from 1600 Hz to 6000 Hz (diamonds) and the low-velocity suppressed
waveform (triangles) has an increased spectral energy above 6000 Hz. The overall median
sound power level (LW) for the high-velocity ammunition was 150 dB in the unsuppressed
condition while the suppressed sound power level was 135 dB, yielding about a 15-dB reduction
in sound power level. For the low-velocity ammunition, the unsuppressed median overall
Page 17 of 25
sound power level was 145 dB and the suppressed power level was 122 dB, yielding a 23-dB
reduction in the overall sound power level.
Figure 5. Sound power levels for the 2000 Hz one-third octave band filtered data for a single shot of the Ruger Charger .22
caliber pistol in Study 2. The view is of the microphone array from above and the energy is interpolated over the surface of the
3-meter hemisphere. The pistol was in the center of the hemisphere and the shot was fired towards the left of the hemisphere
(as shown by the arrow). The images in the upper row are the suppressed conditions and the images in the bottom row are the
unsuppressed conditions. The left column images are for low-velocity ammunition and the right column are the high-velocity
conditions. (See supplemental file for the Ruger Pistol to view the hemispherical plots for all of the frequency bands.)
Figure 5 displays the interpolated Leq levels for the Ruger Charger pistol over the hemisphere of
the microphone array viewed from above for the 2000 Hz one-third octave band. The 2000 Hz
band was selected because the frequency dependent effects first begin to differentiate for
frequencies above 2000 Hz. For the suppressed conditions, the energy directed in front of the
shooter is significantly greater than behind the shooter.
Page 18 of 25
Figure 6. The Effects of suppressor and velocity on sound power of Savage MK-11 .22 caliber rifle in Study 2. The median sound
power levels of the Ruger pistol at each one-third octave frequency band (LW,f) are displayed for the high-velocity/low-velocity
ammunition, and for the suppressed/unsuppressed conditions. The median overall sound power levels (LW) are given in the
legend.
In Figure 6, the median sound power levels (LW,f) and associated ranges for the Savage MK-11
rifle are plotted for the four conditions. Above 2000 Hz, the sound power level spectra of the
high-velocity suppressed and the high-velocity unsuppressed conditions are nearly identical.
The median overall sound power levels (LW) for the high-velocity ammunition were 141 dB for
the unsuppressed and 139 dB for the suppressed conditions, yielding a 2-dB reduction in sound
power level. In contrast, the low-velocity ammunition had median overall sound power levels
of 130 dB and 114 dB for the unsuppressed and suppressed conditions. The mean reduction of
overall sound power level for the low-velocity conditions was 16 dB. For the suppressed low-
velocity ammunition, the noise floor below 400 Hz is elevated compared to the other three
conditions. If only the bands at 125 Hz and higher are considered, the suppressor’s effect
increases to a 17-dB reduction in the overall sound power level.
Page 19 of 25
Figure 7. Sound power levels for the 2000 Hz one-third octave band filtered data for a single shot of the Savage MK-11 .22
caliber rifle in Study 2. Figure details are similar to Figure 5. Like the pistol, the sound power levels for the high-velocity
conditions are significantly greater than that for the low-velocity conditions. (See supplemental file for the Savage Rifle to view
the hemispherical plots for all of the frequency bands.)
In Figure 7, the 2000 Hz octave band analysis of the four conditions for the Savage MK-11 rifle
are displayed. Comparing the two images in the right column, the sound power levels were
effectively identical and the images were also similar. In contrast, the low-velocity ammunition
(left column) exhibits a significant difference in the sound power levels and the plots indicate
substantial differences with respect to the overall color and levels. All three plots indicate
about a 30-dB difference from front of the shooter to behind the shooter (left to right in the
figure). The suppressed low-velocity condition (upper left) displays some lower levels at the
edge of the plot (dark blue), which could be an interpolation artifact associated with
microphone locations.
Page 20 of 25
Figure 2. Sound power as a function of the one-third octave bands after the N-wave was extracted. The N-wave contributed
substantially to the energy measured at the microphones in the front half of the hemisphere in Study 2.
In Figure 8, the waveforms for the microphones in front of the shooter were edited to eliminate
the N-wave produced by the high velocity projectile as it passed by the microphones. The N-
wave was identified visually and the samples in that time interval were set to zero prior to
repeating the analyses. The spectra still exhibit some convergence above about 2000 Hz as was
observed in Figure 6. The peak band levels are about 12 dB lower when the N-wave is excluded
and the resultant overall sound powers levels were 136 dB and 127 dB for the unsuppressed
and suppressed conditions, respectively. The effect of the N-wave on the sound power
introduces two analytic complications. First the microphones in the front half of the
hemisphere are contaminated with the N-wave if the projectile is supersonic. Second,
substantial sound production by a moving source (i.e., the projectile) is a situation not
anticipated in the sound power measurement equations. The source under evaluation is
expected to be contained within the reference volume. In this case, the bullet becomes a
primary sound source for the frequencies above about 2000 Hz, explaining why the two
conditions merged.
Page 21 of 25
Discussion
Noise Reduction of Suppressors
Two studies were conducted to investigate the benefits of an engineering noise control (i.e., a
firearm noise suppressor) and an administrative control (i.e., low-velocity ammunition). to
reduce noise exposure from firearms. The results of these studies indicated that firearm noise
suppressors tend to reduce peak pressure levels at the shooter’s ears by 17 dB to 26 dB, reduce
equivalent energy levels by 9 to 21 dB, and reduce overall sound power level by 2 dB to 23 dB.
Low velocity ammunition uses less propellant than high-velocity ammunition, thus reducing
both the muzzle blast and eliminating the N-wave. The larger caliber Remington 700 rifles
exhibited greater equivalent energy reductions at the ear microphones for the high velocity
ammunition than for the low velocity ammunition. The Savage MK-11 rifle showed almost no
difference in the reductions between the high and low velocity ammunition. The levels of the
unsuppressed Savage rifle exhibited a 1 dB to 2 dB difference as a function of the ammunition,
whereas the Remington rifles had between a 12 dB to as much as a 20-dB difference between
the two velocities of ammunition. The high velocity ammunition was considerably more
energetic for the low velocity ammunition for .223 and .308 caliber rifles than the ammunition
used in the .22 caliber rifle.
This study did not investigate projectile shapes that might minimize the N-wave nor did it
investigate systematic design features of suppressors. The suppressors evaluated in this study
were a convenience sample of suppressors provided by the Michigan DNR and by one of the
authors (MS). Instead, these results can be used to inform a hearing conservation professional
about how the evaluation of the suppressor effectiveness can be affected by the location of the
microphones as well as the metric used. Typically, suppressor effectiveness has been defined
by the change in the peak impulse level. Peak impulse reduction tended to overestimate the
reduction of the LAeq,8h (see Figure 3), which is more strongly related to auditory hazard. As
well, the peak sound pressure level is no longer used in the most recent MIL-STD 1474E (2015).
An energy-based metric and a computational cochlear damage model are used to assess the
noise performance of military materiel. A spectral, energy-based metric for suppressors could
be complementary with spectral methods for characterizing hearing protector performance in
high-level impulse noise (Fackler et al. 2017).
Other Damage Risk Criteria
MIL-STD 1474E includes two metrics that have not been reported in this paper, LIAeq,100ms and
Auditory Hazard Units (AHUs). The LIAeq,100ms includes an adjustment correction based upon the
pressure wave duration (i.e., A-duration) of the impulse and the correction has a lower limit of
0.2 ms. Since the A-durations for the ammunition were less than .2 ms, the A-duration
correction was 0 dB. An additional 54.6 dB must be added to the LAeq,8h to estimate LIAeq,100ms.
Therefore, the differences between the suppressed and unsuppressed conditions yielded the
same reduction as reported above for the LAeq,8h.
AHUs as calculated by the AHAAH model were not reported for several reasons. The AHUs are
not readily related to decibel differences. The middle ear muscle contraction (MEMC) and the
Page 22 of 25
nonlinear annular ligament result in nonlinear growth of the auditory hazard units which
complicates comparisons of measurements at different distances from the muzzle and different
angles relative to the direction of fire. Only the microphone positions for the shooter’s ears
would be relevant. Recent findings have presented additional problems with the assumptions
underlying the AHAAH model. The AHAAH model allows for the middle ear muscle contraction
(MEMC) to be activated depending upon whether the shooter is warned or not. The warned
condition assumes that 100% of persons have a pre-contracted MEMC. Flamme et al (2017)
reported that the prevalence rate of 86% for the acoustic reflex among young 18-31 year olds
normal hearing persons based upon a review of the National Health and Nutrition Examination
Survey data (1999-2012). The prevalence rates decreased with age and increased hearing loss
(Flamme et al, 2017). Zagadou et al (2015) reported that key parameters of the AHAAH model –
namely the stapes dimensions and annular ligament parameters – are not representative of
those found in recent studies of the human.
Sound Power Reduction
Noise in the low frequencies had a small effect in the measurement conditions producing the
lowest sound pressures (e.g., suppressed low-velocity). Windscreens are customarily used
when measuring continuous noise sources outdoors. However, when making the measurement
of high-level impulse noises, the windscreens disrupt the wavefront and dramatically affect the
waveform and subsequent results.
The source directivity was evident in the front of the shooter (left half of the hemispheres) and
exhibited a slight asymmetry to the left front of the shooter (lower left quadrant of the
hemisphere plots). The shooter aimed along the negative y-axis and microphones 3 & 10
(Figure 3) were to the left and in front of the shooter and microphone 6 was to the right and in
front of the shooter. The firearm’s directional nature caused the front/back differences. The
asymmetry likely resulted from the positions of the microphones and the interpolation of the
sound power. The shooter’s body may have produced a shadow effect that was more evident
in the Ruger Charger than was observed for the Savage MK-11. Murphy et al. (2012) observed
an acoustic shadow behind and to the left for a right-handed shooter. Future sound power
measurements should use a symmetric array of 20 microphones.
Sound power does not seem to be an optimal means of characterizing the reduction of typical
auditory risk because the N-wave contributes to the overall power but is only relevant to
exposures of personnel in the front of the firearm. For the low-velocity ammunition, the Ruger
Charger had a reduction of 23 dB and the Savage MK-11 had a reduction of 16 dB. For the high-
velocity ammunition, the reduction in sound power levels were 15 dB and 2 dB for the Ruger
and Savage firearms, respectively.
N-Wave Effects
In recordings containing supersonic projectiles, assessments of suppressor effects may require
waveform modifications to extract the N-wave prior to sound power calculations. However,
the N-wave is an integral part of the noise emission from gunfire, and extraction of the N-wave
should only be undertaken in cases where the sound power from the muzzle blast is the sole
Page 23 of 25
interest of the analyses. For the Savage rifle, removing the N-wave from the waveform
increased the reduction in the sound power level from 2 dB for the unedited waveform to 9 dB
for the edited waveform.
The separation of the N-wave and muzzle blast depends upon the speed of the projectile. The
velocity of the .22 caliber high-velocity ammunition was not as great as that for the .223 and
.308 caliber ammunition. The N-waves were not clearly evident in the Ruger Charger sound
power recordings, whereas the N-waves were evident in the Savage MK-11 recordings. In the
NATO AES 4875 standard, the microphones are located 5 m from the muzzle which increases
the separation between the N-wave and the muzzle blast. Changing hemisphere’s radius from
3 m to 5 m would have increased the separation of the N-wave 1.1 ms to 1.9 ms, assuming the
bullet’s velocity was 390 m/s (1280 fps) and speed of sound was 340 m/s. The amplitude of the
N-wave obeys a power law relationship of b-3/4 where b is the distance from the trajectory to
the nearest microphone (Stoughton, 1997). A 5-m versus a 3-m hemisphere would increase the
distance to the nearest microphone and result in about a 3.3 dB reduction in the amplitude of
the N-wave.
Biases and Limitations
Although the use of a suppressor and/or low-velocity ammunition yielded significant reductions
in auditory risk to the unprotected shooter in Study 1, hearing protection is still recommended
while firing any gun. The guns selected for Study 1 do not necessarily represent many guns
commonly used in recreational or occupational settings. All guns but the Ruger Charger pistol
used a bolt-action loading mechanism, which forces all combusted gas out of the muzzle. Semi-
automatic loading mechanisms capture the energy in the combusted gas to eject the spent
cartridge and load a new cartridge into the chamber. This gas is then released through ports
located midway down the gun. Prior work (e.g., Lobarinas et al., 2015) have shown substantial
declines in suppressor effectiveness at the right ear position of a right-handed shooter firing
from the shoulder, and it is probable that these declines are due to sound energy
accompanying the gas ejection. Analyses with a larger number of guns and suppressors are
necessary before definitive statements about the elimination of auditory risk can be made.
The recordings for both studies were made in open field conditions and cannot be generalized
to conditions where reflective surfaces are nearby. The reduction of the peak impulse levels
would be expected to be the same whether measured indoors or outdoors. For some police
and military tactics where personnel progress in single file toward an objective, persons may be
positioned in front of personnel firing from behind to provide cover fire. In those cases, the
persons in front of the weapon may experience both N-waves and muzzle blast as was
observed in recordings from microphones in Study 2. Blast waves and N-waves from a gun fired
in an enclosure (e.g., a firing range with walls or other surfaces, a hunting blind, or a tactical
training facility) would reflect from nearby surfaces and reach the ear by multiple paths. The
additional reflected energy reaching the listener’s ears via these paths will increase the hazard
relative to the open field recordings obtained in this study. Suppressors and low-velocity
ammunition can be expected to reduce the sound exposure in these settings, but the overall
amount of reduction is likely to differ from the results obtained in this study.
Page 24 of 25
Conclusions
The measurements described in this paper are in general agreement with those reported
previously. The suppressors reduced exposures at the ear between 17 and 26 dB peak sound
pressure level and reduced the 8-hour equivalent A-weighted energy between 9 dB and 21 dB
depending upon the firearm and ammunition. Noise reductions observed for the instructor’s
position about a meter behind the shooter were between 20 dB and 28 dB peak sound pressure
level and between 11 dB and 26 dB LAeq,8h. Although these results imply a substantial risk
reduction, the limited numbers of firearms, suppressors, and environmental conditions
evaluated in this study are insufficient to consider any combination of gun and suppressor
“hearing safe”. Thus, hearing protection should be worn whenever firing a gun. For more
energetic firearms when target shooting double protection is warranted for the unsuppressed
gun and single protection is needed for a suppressed gun.
Acknowledgements
The findings from this paper were presented at the 171st meeting of the Acoustical Society of
America on May 24, 2016 in Salt Lake City, UT and at the National Hearing Conservation
Association meeting on February 21, 2017 in San Antonio, TX. The authors thank Lieutenant
Thomas R. Wanless with the Michigan Department of Natural Resources, Law Enforcement
Division, Recreational Safety, Education and Enforcement Section for his support and
cooperation with measurements in Study 1 collected at the Rose Lake firing range. Other
Michigan DNR officers assisting were Lieutenant Andrew Turner and Sergeant Steve Orange.
We thank the NIOSH reviewers Rauno Pääkönen and Robert Maher and the three anonymous
reviewers for the International Journal of Audiology to improve the readability and organization
of the manuscript.
Disclaimer
The findings and conclusions in this report are those of the authors and do not represent any
official policy of the Centers for Disease Control and Prevention or the National Institute for
Occupational Safety and Health. Mention of company names and products does not constitute
endorsement by the CDC or NIOSH.
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