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ARTICLE Table of Contents   
Year : 2009  |  Volume : 11  |  Issue : 45  |  Page : 231-242
Estimates of auditory risk from outdoor impulse noise II: Civilian firearms

Department of Speech Pathology and Audiology, Western Michigan University, USA

Click here for correspondence address and email
Date of Web Publication2-Oct-2009

Firearm impulses are common noise exposures in the United States. This study records, describes and analyzes impulses produced outdoors by civilian firearms with respect to the amount of auditory risk they pose to the unprotected listener under various listening conditions. Risk estimates were obtained using three contemporary damage risk criteria (DRC) including a waveform parameter-based approach (peak SPL and B-duration), an energy-based criterion (A-weighted SEL and equivalent continuous level) and a physiological model (AHAAH). Results from these DRC were converted into a number of maximum permissible unprotected exposures to facilitate interpretation. Acoustic characteristics of firearm impulses differed substantially across guns, ammunition, and microphone location. The type of gun, ammunition and the microphone location all significantly affected estimates of auditory risk from firearms. Vast differences in maximum permissible exposures were observed; the rank order of the differences varied with the source of the impulse. Unprotected exposure to firearm noise is not recommended, but people electing to fire a gun without hearing protection should be advised to minimize auditory risk through careful selection of ammunition and shooting environment. Small-caliber guns with long barrels and guns loaded with the least powerful ammunition tend to be associated with the least auditory risk.

Keywords: Hearing loss, impulse noise, noise/adverse effects, risk factors, weapons

How to cite this article:
Flamme GA, Wong A, Liebe K, Lynd J. Estimates of auditory risk from outdoor impulse noise II: Civilian firearms. Noise Health 2009;11:231-42

How to cite this URL:
Flamme GA, Wong A, Liebe K, Lynd J. Estimates of auditory risk from outdoor impulse noise II: Civilian firearms. Noise Health [serial online] 2009 [cited 2023 Nov 30];11:231-42. Available from: https://www.noiseandhealth.org/text.asp?2009/11/45/231/56217

  Introduction Top

This is the second in a series of two studies reporting the acoustic characteristics and estimates of auditory risk from common sources of impulse noise in the general population. The companion paper [1] includes reviews of some current damage-risk criteria (DRC) for impulse noise and the acoustic data they require. Acoustic characteristics and estimates of auditory risk from a small set of consumer firecrackers were also presented in that paper. The current paper reports the acoustic characteristics and estimates of auditory risk from impulses produced by civilian firearms and reports the relationships between DRC for firecracker and civilian firearm impulses.

Exposure to firearm noise is common among adults in the US. Preliminary analyses of data from the 2007 National Health Interview Survey revealed that approximately 46 % of adult men and 14 % of adult women report having fired a gun. A single exposure to a high level impulse can result in permanent reduction in hearing sensitivity. However, not all firearms produce the same amount of sound and not all types of ammunition are necessarily equally hazardous. For example, a hunting load for a shotgun will contain more explosive material than a target load for the same gun.

In guns, the gas produced via combustion is initially confined to a small volume (i.e., the cartridge in the firing chamber) and the resulting pressure is used to accelerate a projectile (i.e., the bullet or shot). Upon the projectile's exit from the gun barrel, remaining energy is released as a burst of heated gas traveling at supersonic speed. The energy of the ejected gas is greater than the energy imparted to the projectile. [2] Estimates of peak sound levels produced by firearms are in the range of 143.5 to 174.7 dB SPL at the location of the shooter or bystanders in the immediate area. [3],[4]

  Damage Risk Criteria Top

The current damage risk criteria (DRC) have been reviewed in the companion manuscript. [1] They can be categorized briefly according to the information they require to return an estimate of risk. Waveform parameter DRC produce risk estimates based on the structure of the impulse waveform (e.g., peak, duration). Energy-based approaches rely solely on the energy contained in the impulse (e.g., A-weighted Sound Exposure Level). Physiological models can also be used. The estimate risk is based on the amount of activity (e.g., basilar membrane displacement) presumed to result from the impulse. The Coles-CHABA DRC, [5] and approach proposed by Smoorenburg, [6] and the Price-Kalb Auditory Hazard Assessment Algorithm for Humans [7] are contemporary examples of these approaches, respectively. All impulses in this and the companion paper were evaluated by each of these DRC to determine the relationships among risk estimates for these kinds of impulses.

  Study Rationale Top

Impulse noises from firearms are a known risk factor for hearing impairment [8] and hearing protector usage rates are quite low, especially among hunters, [9] except during organized shooting events where hearing protection is required by event organizers. Auditory risk is liable to vary with the combination of firearm, ammunition, and listener location. Measurements of acoustic characteristics and estimates of risk are necessary before practical recommendations can be developed to help firearm users protect their hearing.

The current study was designed to inform a measurement method and obtain preliminary data on a small set of civilian firearms. Firecracker exposures were considered in the companion paper. [1] The combined objectives of these papers were to (1) report the acoustic characteristics of impulses from these sources and assess the influence of methodological factors (e.g., microphone location, ammunition) on subsequent results, (2) describe the degree of risk to the auditory system posed by these impulse noise exposures according to the three types of damage risk criteria (DRC), and (3) describe the relationships and differences among the recommend maximum number of exposures developed using those DRC.

  Method Top

Firearms and ammunition

Tested firearms included a Savage Model 110 .30-06 hunting rifle, a Marlin Model 60 .22 caliber rifle, a Smith and Wesson Model 686 .357 magnum handgun, a Glock model 17 (9 mm handgun), and a Beretta model Beretta Teknys Gold Model AL391 12 gauge auto loading shotgun. The .30-06 and 12 gauge were selected based on a study that revealed that these were the most common guns used for large game in northern Michigan. [10] The remaining weapons were recommended by one member of the investigative team (JL), a firearm expert and retired professional target shooter and gun salesperson. The .22 caliber rifle was chosen because it is frequently used for target shooting and as a firearm for beginning shooters. The .357 caliber and 9 mm handguns were chosen because they are relatively common, but used different loading and firing mechanisms; the .357 was a revolver, while the 9 mm pistol had an automatic loading mechanism.

In addition to differences in barrel diameter, as represented by the caliber and gauge of the cartridge used, barrel length also differed across firearms. The distance from the cartridge base to the muzzle for the .30-06, .22, .357, 9 mm, and 12 gauges were 560, 508, 146, 145, and 737 mm, respectively.

Two types of ammunition were selected for each firearm. Rifle and handgun cartridges were selected to represent the largest range of bullet mass typically chosen by recreational shooters. Bullet mass is typically reported in grains (100 grains = 6.48 grams). There are differences in the burn rate of the propellants in cartridges with more or less bullet mass. In general, propellants for lighter bullets burn faster. [2] Bullet masses of 150 and 180 grains were used in the .30-06 rifle. Standard velocity and high-velocity cartridges were used in the .22 rifle. The .357 was loaded with 125 and 158 grain .38 caliber ammunition. The .38 caliber ammunition was selected because it is more commonly used than .357 magnum ammunition because it generates less recoil without appreciable loss of accuracy. The 9 mm was loaded with cartridges having 95 and 115 grain bullets.

Target and hunting loads were used in the shotgun. Target loads contain less propellant than hunting loads because a shorter effective range is needed for target shooting. The target and hunting loads contained 12.1 grams and 13 grams of smokeless powder propellant, respectively.

Test conditions

Firearm impulses were recorded at:

1. The location occupied by the shooter's left ear when the weapon was mounted on a gun rest secured to tripods (i.e., shooter-field), 2. A position 150° from the line of fire, at the distance between the muzzle and shooter's left ear (i.e., side-field), and 3. The location occupied by the shooter's left ear when the weapon was mounted on a gun rest on a shooting table (i.e., shooter-table). Recordings of .22 caliber rifle impulses were not obtained at the shooter-table location due to a weapon malfunction.


Firearm impulse recordings were made at a local outdoor firing range. The shooting lanes were covered with loose dirt, and the shooting area had a concrete floor and a tin roof with a 10° slope opening towards the shooting lane. The firing range had a 1.3 m timber wall 4.3 m above the firing lane and soil berms along both sides and behind the range to contain stray shots within the firing area. During firing, the muzzle location was 1.4 m directly above the edge of the concrete floor.

Long guns were secured to a gun rest (Caldwell Lead Sled; ) during firing. The gun rest was secured to tripods for recordings made in the shooter-field and shooter-side conditions, and placed on a plywood firing table for the shooter-table condition. The height of the muzzle was reduced 200 mm when mounted on the table. A small cushioned stand (Hoppe's Bench Rest; ) was used as a hand rest in the shooter-table condition.

Instrumentation and data processing

Recordings were made with the same instrumentation and analysis approach described in the companion paper. [1] Briefly, a 1/4-inch pressure microphone at a grazing incidence to the wave front was used. Signals were digitized at approximately 195 kHz and a 24-bit depth. Data were analyzed using customized Matlab (Mathworks, Inc., Natick, MA) routines prepared by NIOSH [11] and modified for this study. Inferential statistical analyses were conducted using SPSS (SPSS Inc., Chicago, IL).

  Procedure Top


Recordings of at least 10 impulses were made in each measurement condition. Long guns were first moved into place and then the measurement microphone was placed in the desired location. The weapon was then loaded with the selected ammunition, recording was initiated, and the weapon was fired remotely by a person about three meters directly behind the weapon. Handguns were fired with the muzzle in the same location as specified for the rifles and shotgun. The measurement microphone was positioned approximately three cm from the shooter's left ear.

Data processing and analysis

After data transfer to the analysis computer, impulse baseline corrections were made by subtracting the mean value during a silent period in the waveform from all points on the recording. Each impulse was then analyzed independently using the Matlab software routines described above. Risk estimates were calculated in terms of Maximum Permissible Exposures (MPE) via each DRC for a listening condition in which the listener was directly facing the sound source (i.e., grazing incidence to the ear). Maximum permissible exposures via the Price-Kalb DRC were calculated using a maximum of 500 Auditory Risk Units under unwarned listening conditions (i.e., no anticipatory middle ear response). We elected to use the unwarned condition based on the results of Bates, et al. [12] which found that anticipatory middle ear reflexes cannot be conditioned in the majority of human listeners.

Descriptive statistics, analyses of variance (ANOVA), and correlations were obtained using SPSS v.14. Except impulses from the .22 caliber rifle, firearm data were evaluated via a single analytic model, wherein the impulse source was represented by the combination of firearm and ammunition. Vast differences in acoustic characteristics and risk estimates from .22 caliber rifle impulses lead us to exclude .22 caliber rifle data from the overall ANOVA model and analyze those data via a separate two-way random effect model. Linear (Pa) rather than logarithmically-transformed (SPL) quantities were used in analyses of sound levels, and estimates of maximum permissible exposures (MPE) were logarithmically transformed to normalize variance.

  Results Top

Acoustic characteristics

The impulse waveforms, spectra [Figure 1] and [Figure 2], and summary acoustic characteristics [Table 1] of firearms were somewhat different from those produced by firecrackers. [1] Firearms produced typical peak levels between 159 and 164 dB SPL at the measurement locations, except the .22 caliber rifle, which produced peak levels around 141 dB SPL. A disordinal interaction between microphone location and the firing condition (i.e., the combination of gun and ammunition) was observed (F 14,213 = 37.7; p < .005), but no interaction was observed for peak levels produced by the .22 caliber rifle (F 1,50 = 10.6; p = .432). Post hoc testing revealed that for impulses from the .30-06 rifle, .357 hand gun loaded with .38 ammunition, and the 9 mm handgun, peak pressures at the side-field location were significantly higher than those measured in the shooter-field or shooter-table locations. Significant differences in peak levels across all microphone locations were observed with the 12 ga. shotgun, but the peak pressures from this firearm were greatest in the shooter-table location, intermediate for the shooter-field location, and least at the side-field location, indicating that the rank-order of measurement locations depended on the firearm.

Unweighted peak pressures were significantly different across many firearms. The .22 caliber rifle produced lower peaks than any other firearm, and the .38 caliber load in the .357 revolver produced the highest peaks. The .30-06 rifle and 9 mm handgun produced peaks that were not significantly different from each other, but that were significantly different from all other weapons. Peaks from the 12 ga. shotgun were significantly higher than those from the .22 caliber rifle and were significantly lower than those produced by any other weapons. The ammunition used in a gun had a significant effect on unweighted peak pressure only for the 12 ga. shotgun, wherein the cartridge with a hunting load produced higher peaks. For all firearms except the .22 caliber rifle (lowest) and the 12 ga. shotgun (next lowest), these differences did not reach statistical significance after A-weighting was applied.

The A-weighted SELs produced by the .22 caliber rifle were 94 and 99 dB SPL with standard and high-velocity loads, respectively, while the SELs of the remaining firearms ranged between 119 and 127 dB SPL. Among weapons producing higher levels, significant main effects were observed for microphone location (F 2,14 = 6.41; p = .011), combination of firearm and ammunition (F 7,14 = 69.9; p < .005), and the interaction between microphone location and firearm/ammunition (F 14,213 = 15.6; p < .005), indicating that the effect of microphone location varied across firearms. Post hoc testing from the .30-06 revealed that SELs from all three locations were significantly different from one another. The greatest SEL was observed at the shooter-table location, the shooter-side location was next greatest, and the shooter-field location was lowest. The SELs were also significantly greater at the shooter-table location for the 12 gauge shotgun and the 9 mm handgun. The SELs for the revolver loaded with .38 caliber ammunition were not significantly different across microphone locations.

Within each firearm, post hoc testing revealed that each kind of ammunition produced impulses with significantly different SEL, and across firearms, all SEL were significantly different, except the lighter .38 caliber and 9 mm bullets (125 grain and 95 grain, respectively) produced similar SEL, and the heavier .38 caliber bullet (158 grain) and the heavier 9 mm bullet (115 grain), which produced SELs that also were not significantly different from one another. SELs produced by the high velocity loads in the .22 caliber rifle were higher than SELs from standard velocity loads (F 1,1 = 419; p = .031), but did not differ across the shooter-field or side-field microphone locations (F 1,1 = 0.055; p = .854). The interaction between microphone location and the combination of microphone location and ammunition was not significant (F 1,50 = 1.23; p = .272).

Among firearms producing greater sound levels, pressure envelope (B) durations ranged between 12 and 33 ms, differed by microphone location (F 2,14 = 4.02; p = .042), combination of firearm and ammunition (F 7,14 = 13.0; p < .005), and the interaction between these factors was significant (F 14,213 = 13.7; p < .005), which suggests that changes in B durations across microphone conditions varied by firing condition. Post hoc testing revealed that B durations of handgun impulses were greatest at the shooter-field location, moderate at the shooter-table location, and least at the side-field location. Similar microphone location effects were observed for the .30-06 rifle; B durations recorded at the shooter's ear table and field were significantly greater at the side-field location. Impulse durations from the 12 gauge shotgun were lowest at the shooter-table locations, but B durations measured in the shooter-field and side-field locations were not significantly different from each other. A significant interaction effect of microphone location and ammunition was observed in separate ANOVA for B durations from the .22 caliber rifle (F 1,50 = 6.96; p = .011), but no significant main effects of ammunition or microphone location were observed. Significant differences were not found in post hoc testing (Bonferroni-adjusted p = .114), but examination of means for each combination of microphone location and ammunition suggested that B durations measured the side-field location when the firearm was loaded with standard velocity ammunition were lower than those measured with high velocity ammunition at the shooter-field location or with high velocity ammunition at either microphone location.

Measurements of impulse kurtosis from firearms producing higher levels differed significantly across microphone locations (F 2,14 = 4.17; p = .038) and interacted significantly across microphone location and the firing condition (F 14,213 = 50.34; p < .005). The general trend in kurtosis estimates indicated that impulses from the rifle and handguns had the greatest kurtosis at the side-field location and least at the shooter's ear locations. Kurtosis values from the .30-06 rifle were significantly different across all microphone locations, wherein the kurtosis at the side-field location was greatest and was least at the shooter-table location. Shotgun measurements showed the opposite effect, wherein impulses recorded at the shooter's ear had greater kurtosis than impulses at the side-field location. Kurtosis did not vary across microphone location for the .22 caliber rifle, but was significantly greater when the firearm was loaded with standard velocity ammunition.

Within firearms, ammunition had no significant effect on kurtosis for the .30-06 rifle or the 9 mm handgun, but the revolver loaded with .38 caliber 158 grain ammunition had greater kurtosis than the same firearm loaded with 125 grain ammunition. Impulses from the 12 gauge shotgun had greater kurtosis when the target load was fired.

In summary, some significant differences were observed across firearm impulses, but peaks ranged between 159 and 164 dB SPL for most firing conditions in this study. The .22 caliber rifle produced lower peaks (approximately 141 dB SPL). Measurement factors such as the location of the microphone and the type of ammunition used have measureable effects on the acoustic characteristics of the impulse, and the pattern of these differences depends on the gun.

  Risk Estimates Top


The mean MPE and their associated 95% confidence intervals for recordings made at the most shooter's ear are presented in [Table 2],[Table 3],[Table 4]. The Price-Kalb DRC was often the least conservative, but was the most conservative for the .22 caliber rifle and the 12 gauge shotgun firing a target load. For the other guns, the Price-Kalb DRC permitted one to two unprotected exposures to the impulses produced in the other firing conditions while the other DRC permitted no unprotected exposures. The Price-Kalb model was the most conservative DRC for the .22 caliber rifle, allowing far fewer unprotected exposures than the other DRC. The Smoorenburg-NATO DRC was the least conservative for the 12 gauge shotgun firing a target load, allowing 50 unprotected impulses while the other DRC allowed no more than one.

MPEs by each DRC varied by condition, which suggests that MPE estimates made with one gun model, type of ammunition, and microphone location may not generalize to other conditions. Effects of microphone location and ammunition were evaluated using each DRC, but the evaluation via the Smoorenburg-NATO DRC was limited to .22 caliber rifle impulses because MPEs of 0 or 50 were allowed for all other firearms. Results with the .22 caliber rifle indicated that the MPE via the Smoorenburg-NATO DRC were significantly greater (F 1,1 = 488; p = .029), indicating that more unprotected shots would be permitted for the .22 caliber rifle loaded with standard velocity ammunition.

Significant main effects of the combination of firearm and ammunition (F 7,14 = 6.57; p = .001) and an interaction between the combination of firearm and ammunition and microphone location (F 14,213 = 27.3; p < .001) were observed within the Coles-CHABA DRC. There was no significant main effect of microphone location (F 2,14 = 0.86; p = .444). Post hoc testing was conducted for each firearm separately. The post hoc results with the .30-06 rifle revealed that MPE via the Coles-CHABA DRC were greater at the shooter's ear than at the side-field location, regardless of ammunition or whether the firearm was fired from a standing position or mounted on a table. Post hoc results from the .357 revolver loaded with the .38 caliber ammunition revealed that the MPE were greater with the 158 grain bullet when recordings were made at the shooter's ear, but that the same bullet resulted in a lower MPE in the side-field location. Post hoc results from the 9 mm handgun revealed that the MPEs were greater for the 115 grain bullet than the 95 grain bullet at the side-field and the shooter-table locations, but that the impulses produced by the ammunition with the 95 grain bullet resulted in greater MPEs when measured at the ear of the standing shooter. The 12 gauge shotgun had greater MPEs when fired with a target load. MPEs were also greater at the side-field location and least at the shooter-table location. A separate two-way ANOVA conducted with the .22 caliber rifle revealed a significant interaction between microphone location and ammunition (F 1,50 = 4.21; p = .045), but no contrasts reached statistical significance on follow-up testing. Greater MPEs were observed with standard velocity ammunition than high velocity ammunition at the side-field location, but standard velocity ammunition had lower MPEs than high velocity ammunition at the shooter-field location.

Evaluations of MPE using the Price-Kalb DRC revealed a significant main effect for the combination of firearm and ammunition (F 7,14 = 11.2; p < .001) and a significant interaction between the combination of firearm and ammunition and microphone location (F 14,213 = 6.57; p = .001). Follow-up testing of individual firearms revealed no significant main effects of ammunition or microphone location, or their interaction, for the .30-06 rifle or revolver loaded with .38 caliber ammunition. Post hoc MPE testing with the 9 mm handgun revealed that significantly different MPEs were observed across microphone locations. Maximum permissible exposures were greater at the shooter-field location and lowest at the shooter-table location. Post hoc testing with the 12 gauge shotgun impulses generated by the target load revealed that MPEs were greater at the side-field location and lower at the shooter's ear, regardless of whether the weapon was fired from a standing position or from a table. Impulses from the same firearm firing a hunting load revealed that the MPEs were significantly different from one another across all microphone locations. The greatest MPEs were observed at the side-field location and least at the shooter-table location. Evaluations of MPE for the .22 caliber rifle suggested significantly lower MPEs when the gun fired high velocity ammunition.

Relationships between DRC

Scatterplots of the relationships between estimates revealed that the logarithms of the estimates were linearly related to each other [Figure 3]. All impulses recorded in this study were included in this analysis (N = 953), and impulses having zero safe unprotected exposures via the Smoorenburg-NATO criteria were replaced with values below one to permit representation on the plot. The data represented in [Figure 3] reiterate the vast differences in MPEs reported above [Table 2],[Table 3],[Table 4] and extend them to the level of individual impulses.

The dashed line in each plot represents the locus of agreement between the DRC. Data above this line indicate that the DRC plotted along the vertical axis was less conservative. The greatest differences among MPE were observed at the upper end of the MPE range, but substantial disagreements were also present at the lower end. Relative to the Coles-CHABA DRC, the Price-Kalb DRC was less conservative at lower MPEs but much more conservative for higher MPEs (e.g., permitting approximately 100 unprotected exposures while the Coles-CHABA DRC permitted more than 1,000,000). The relationship with the Smoorenburg-NATO DRC is not as simple. Only in rare cases were the MPEs greater via the Smoorenburg-NATO DRC when MPEs via Coles-CHABA were less than one. This suggests that the two DRC often agree about the impulses that are categorically unsafe. In the range of one to 50 MPEs via Coles-CHABA, the Smoorenburg-NATO DRC was typically less conservative, but progressively more conservative when the MPEs via Coles-CHABA were greater than 50. Turning to the relationship between the Price-Kalb and Smoorenburg-NATO DRC, the Price-Kalb DRC was uniformly more conservative in cases where the MPEs via Smoorenburg-NATO were greater than one. It was less conservative in cases where the Smoorenburg-NATO DRC permitted no unprotected exposures.

  Discussion Top

The results indicate that there are significant differences in acoustic characteristics and risk estimates across firearms, ammunition and microphone location. These differences do not always follow a consistent rank-order. This suggests that results obtained for one model of gun, type of ammunition, and microphone location cannot be generalized with confidence to other guns, ammunition, or locations.

Vastly different estimates of risk can be returned by different DRC. The estimates can differ by many orders of magnitude, implying that DRC are not exchangeable. The preferred DRC can only be identified through comparative study with animal models and human volunteers.

Auditory risk from firearms

The acoustic characteristics of impulses recorded the side-field location were frequently different from those made at the shooter's left ear. This finding is consistent with Plomp's [13] representation of the sound radiation characteristics of the FAL rifle. Furthermore, the findings of the current study suggest that these radiation characteristics are not consistent across firearms. Recordings made the side-field location led to overestimates of exposure at the shooter's ear for most firearms evaluated in the current study, but underestimated the exposure for the shooter of the shotgun evaluated in the current study. Such differences make it difficult to extrapolate the acoustic characteristics of impulses recorded at other locations to the shooter's ear.

Propellant combustion is the source of impulses from firearms, so it is not surprising that significant differences in acoustic characteristics [Table 1] and maximum permissible exposures [Table 2],[Table 3],[Table 4] were observed across types of ammunition. Weighted SELs from the .22 caliber rifle and the 12 gauge shotgun differed by 5 and 3 dB, respectively, and kurtosis differed by more than a factor of 2 across the two types of ammunition loaded into these firearms. Maximum permissible exposures were also influenced by ammunition in many cases.

The acoustic characteristics of the firearms evaluated in the current study were somewhat different than those found in other studies. The peak levels observed in the current study tend to be 3 to 7 dB higher than those reported by Kramer [3] and 10 to 13 dB lower than those reported by Odess. [4] The results reported by Kramer can be expected to underestimate peak levels due to the use of a precision sound level meter. Sound level meter circuitry was not designed to capture the rapid rise time present in a firearm impulse and will therefore underestimate peak levels due to their relatively sluggish temporal characteristics. In addition, some differences were expected because the data reported were averages across a large number of firearms and types of ammunition. Differences with the results reported in Odess could be expected based on the use of a larger (15 mm) microphone diaphragm, which was covered with plastic to prevent mechanical overload.

Measurements of peak levels and A durations from .22 caliber rifle impulses were generally consistent with those cited in Coles et al. [5] The study by Acton and Forrest reproduced by Coles et al. found open field peak levels of 138 dB SPL and A-durations less than 0.045 ms. However the B-durations observed in the current study were approximately one order of magnitude greater than those reported by Coles et al.

Different estimates of acoustic characteristics of impulses are obtained at different microphone locations because firearms do not produce spherical blast waves. Peak sound levels at 1 m 180° from the line of fire have been observed to be roughly equivalent to the levels observed at 5 m 90° from the line of fire. [13] The results of this study suggest that the shape of the shock wave surface varies with the firearm. With a few exceptions, greater peak levels, envelope (B) durations, and kurtosis were observed at the side-field location relative to measurements made at the shooter's ear when impulses came from rifles and handguns. However, lower values on these acoustic parameters were observed at the side-field location when impulses from the 12 gauge shotgun were evaluated. These results imply that if the acoustic characteristics of impulses from firearms are to be evaluated at only one location, the location should be the shooter's ear, with reflective surfaces (e.g., shooting table, walls) present if they will often be present during shooting. For example, measurements in the open field cannot be expected to represent the auditory risk for a shooter or any bystanders in a hunting blind. It should also be noted that results obtained from recordings at the shooter's ear may not generalize to nearby shooters, shooting instructors, or other observers, and that one cannot assume that these people would be at less auditory risk simply because they are farther away from the firearm.

The ammunition used in the firearm can also have an impact on the acoustic characteristics of the impulse produced by the firearm. Although no significant ammunition effects were observed with the. 30-06 rifle, large differences were observed between the target and hunting loads for the 12 gauge shotgun and high- versus standard-velocity rounds for the .22 caliber rifle.

Further research is needed to determine whether the differences between the shotgun and the rifles and handguns is characteristic of all shotgun impulses or if it was a consequence of the specific design of the Beretta shotgun used in this study, which has a gas pressure-driven auto loading mechanism and side ejection of the spent ammunition casing.

The DRC uniformly recommended limiting unprotected exposures to zero (Coles-CHABA and Smoorenburg-NATO) or one (Price-Kalb) across firing conditions and microphone locations for most guns [Table 2],[Table 3],[Table 4]. It is natural to wonder about the importance of significant differences across estimates covering such a restricted range of permissible exposures. Although these differences do not change the overall recommendation (i.e., never more than one unprotected exposure), they provide crucial information to those who consult with recreational hunters and shooters. There are nonauditory concerns that advise against the attenuation of environmental sounds, such as the need for localization and identification of other people and game. Recreational hunters and shooters who make an informed decision to forego hearing protection should not be considered eager to subject their hearing to unnecessary risk. Therefore they need instruction on how to minimize auditory risks via careful selection of firearm, ammunition, and shooting environment. For example, if one is guided by the Coles-CHABA DRC, auditory risk can be reduced by 75% if the unprotected shooter of the Beretta Teknys Gold 12 gauge shotgun stands in an open area firing a target load instead of firing a hunting load from a table. A similar concern exists for those in the immediate vicinity (e.g., shooting coaches, hunting partners, and children). From the perspective of the Coles-CHABA DRC, the bystander to the shooter's left will have half the auditory risk (relative to the shooter) when the shooter fires this shotgun, but would have two to five times the risk if the .30-06 or .357 revolver was fired.

Furthermore, MPE values less than one can inform the selection of hearing protectors for use with a given gun, ammunition, and listener location. Hearing protectors alter the acoustic characteristics of the impulse and consequently alter one's auditory risk. Knowledge of unprotected risk is crucial for selection adequate hearing protectors for a listener's exposure profile.

Consider for example a trap shooter who typically completes one or two competition or practice rounds (125 or 250 shots) daily using the shotgun and target load used in this study. The Coles-CHABA DRC was the most conservative for this combination of firearm and ammunition, and the low boundary of the interquartile range was 0.657 [Table 2]. A hearing protector that reduces the DRC by a factor of 100 would result in up to 65 protected exposures. Although this amount might be adequate for the average shooter, it would be inadequate in such a case. A protector reducing the DRC by a factor of 400 or more would be needed.

Accuracy of risk estimates

Early DRC [5],[14],[15] tended to adopt a greater acceptable amount or risk of hearing impairment while more recent DRC have restricted exposures to the No Observed Effect Level, or NOEL. Coles et al. [5] proposed to protect the least susceptible 75% of the population from a permanent threshold shift of 30 dB or greater at frequencies above 2 kHz and their recommendations were adjusted in CHABA [15] to protect the least susceptible 95%. Starting with the DRC proposed by Pfander [16] and including the Smoorenburg-NATO [6] and Price-Kalb [7],[17],[18] DRC., TTS at any frequency in excess of 25 dB for more than five per cent of participants was considered an indication of an unacceptable risk of noise-induced PTS greater than 0 dB. Some differences in MPEs across DRC are consistent with a greater acceptable amount of hearing impairment, e.g., less conservative MEP estimates via Coles-CHABA for lower-level impulses. However, the differences between the Coles-CHABA and the Price-Kalb DRC for high-level impulses do not follow such a pattern. In this range, the MPEs via Coles-CHABA can be 10% of those estimated via Price-Kalb, which was intended to keep exposures at or below the NOEL.

Perhaps the most unsatisfactory implication of this work is that the vast differences in risk estimates leave the user of any impulse noise DRC in an unacceptable position. A given impulse can produce mutually exclusive risk estimates across DRC. No more than one of the methods can be best, and one must recognize the possibility that all existing approaches might be fundamentally flawed. Prospective studies with animal models and human volunteers must be conducted to resolve these differences.

The DRC of Smoorenburg [6] might underestimate permanent threshold shift. The Smoorenburg DRC depends on only the A-weighted energy in the impulse. However, kurtosis is a factor beyond A-weighted energy that predicts PTS. [19] As kurtosis grows from a value of 3, which is the kurtosis of white noise, to a kurtosis of approximately 50, an increase in PTS has been observed in chinchilla models, after controlling total energy in the signal and temporal variation in amplitudes. [20] The minimum kurtosis among the impulses in the current study was well above this range [Table 1] and [Table 2]. A joint exposure criterion based on sound energy and kurtosis has been hypothesized as necessary and sufficient, [21] but this approach has not been evaluated with respect to impulses produced by firearms and firecrackers.

Relationships among DRC

A consistent trend emerges when the results of this study and its companion [1] are combined. There was near agreement across the DRC that there is a reasonable chance of significant hearing impairment for the unprotected listener who fires more than a single shot from a .30-06 rifle and .357 caliber handgun loaded with 125 grain .38 caliber ammunition; excessive hazard may result from even this one shot [Table 2],[Table 3],[Table 4]. In addition, DRC were in general agreement that the number of unprotected exposures to impulses from the 12 gauge shotgun firing a hunting load should be two or less.

There were vast differences in MPE for the .22 caliber rifle and for firecracker impulses at most distances included in the companion paper. [1] MPEs via the Coles-CHABA DRC were considerably greater than either of the other DRC, which may result from an invalid extrapolation to shorter-duration impulses to results obtained from impulses of longer duration or the inclusion of TTS data from listeners with what would currently be described as abnormal hearing thresholds.

The slope of the contour of constant risk (peak level v. duration) employed in the Coles-CHABA DRC was primarily determined via interpolation and extrapolation of a slope between two points (Coles et al., [5] p. 13). The higher point was determined by a series of studies conducted in open environments with firearms using 7.62 Χ 51 mm NATO ammunition. The lower point was determined by studies with .303 and .22 caliber rifles fired in reverberant environments. It is possible that the risk contour for these conditions may not generalize to other weapons and/or firing conditions.

The Smoorenburg-NATO [6] and Price-Kalb [7] DRC presume that exposures producing no more than 25 dB of temporary threshold shift, normalized to 2 minutes following the end of the exposure, i.e., TTS 2 , [22] result in no permanent threshold shift. Coles et al., and CHABA adopted the definition proposed by CHABA, [23] in which damage was considered likely with TTS 2 exceeding 10 dB at 1000 Hz and below, 15 dB at 2000 Hz, and 20 dB at 3000 Hz or above was observed in 50% of an otologically normal sample having pure tone thresholds within 15 dB of 0 dB re: ASA-1951. 1 TTS magnitude is reduced among those with poorer hearing thresholds (Coles et al., [5] p. 27), and it is likely listeners whom we would now judge to have abnormal hearing participated in the TTS studies that form the basis of that DRC. All listeners in those studies were evaluated using the same TTS criteria, and participants with slight or mild high frequency hearing loss could be expected to bias average TTS values downward, which would lead to a less conservative DRC.

The logarithms of all three risk estimates were strongly interrelated [Figure 3]. This means that within measurement conditions represented in this study, exposures determined as having great risk of auditory damage via one method will also be similarly ranked via another method. Absolute agreement on the maximum permissible exposures was better for the most intense exposures included in this study. However, poor agreement was observed for the least intense impulses, where differences of many orders of magnitude were common. Better agreement for the .30-06 rifle could be expected due to the similarity in the ammunition used for this rifle and the 7.62 Χ 51 mm NATO cartridge that was used in many military small arms.

Some differences between DRC were expected. The Coles-CHABA DRC tends to overestimate auditory risk from exposure to impulses from large-caliber weapons, [6] and the Price-Kalb model was developed in part to explain and correct this error. [7] So relative to Coles-CHABA, the Price-Kalb model was expected to permit more exposures than the Coles-CHABA model. However, the Price-Kalb model showed the opposite effect for many impulses in this study, indicating greater auditory risk than Coles-CHABA, often by many orders of magnitude [Figure 3].

The vast differences among risk estimates for the low intensity impulses present a much-needed opportunity for comparative validation of the DRC. These DRC could be evaluated based on the degree to which the growth of TTS 2 conforms to the expectations of each DRC. The large differences in estimated maximum permissible exposures suggest that the number of participants in such a study could be reasonably small, and could follow the basic paradigm of the Blast Overpressure Project, [25] in which participants were sequentially exposed to incrementally increasing numbers of exposures until a criterion threshold shift was observed.

Biases and limitations

The study provides data for a limited set of firearms loaded with ammunition that would be typically used by the recreational hunter and shooter. We made the impulse recordings at an outdoor shooting range having no large reflective surfaces nearby except a concrete floor and an angled tin roof. The results presented here cannot be expected to generalize to conditions where the impulse source is located near large reflective surfaces. This recognition is most important in cases of impulses propagated within enclosures (e.g., indoor shooting facilities and hunting blinds). Analyses of impulses recorded in these environments are required to determine the reduction in the maximum numbers of permissible exposures in such environments, but it is reasonable to expect that, among the stimuli included in this study, only the .22 caliber rifle could potentially have one or more permissible unprotected exposures via the most conservative approach to risk estimation.

Recordings of impulses from the handguns were made near the shooter's ear while the shooter fired the gun. We cannot rule out the possibility that the shooter's head caused an artifactual increase in observed sound pressure at the shooter's ear locations. However, substantial reflections were not apparent in the time waveforms when examined informally.

The estimates of auditory risk detailed in this study represent dominant current approaches to risk estimation and we have attempted to provide the necessary information for interested readers to assess these exposures via alternate DRC.

Finally, MPEs reported in this study could underestimate auditory risk for some listeners, as noted by Ward [26] and mentioned in the companion paper. [1]

  Conclusion Top

The results from firearms indicate that only a complicated set of practical recommendations can be made for civilian firearm users. Auditory risk estimates differ with the combination of firearm and ammunition to be used. In addition, estimates based on recordings obtained at locations other than the shooter's head cannot be generalized with confidence to the shooter's head location. Hence, recommendations for shooters must be made based on recordings taken at the location of the shooter's head. These recommendations are likely to differ from those made to bystanders at other locations.

Practical recommendations are complicated further by the differences in estimates of risk returned by the DRC. Substantial relationships were observed between the DRC, but the rank ordering of risks occasionally differed across measurement conditions, and tremendous differences in MPEs were observed. Until an optimal DRC is identified, those developing practical recommendations must base those recommendations on the most conservative DRC and recognize the uncertainty inherent in such recommendations.

  Acknowledgements Top

The authors thank William Murphy, Chucri Kardous, and Edward Zechmann (all from CDC/NIOSH, Taft Laboratories, Cincinnati, OH) for the Matlab software used to analyze the impulse data. We also thank the Southern Michigan Gun Club for access to the outdoor firing range used for recordings.

  References Top

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Correspondence Address:
Gregory A Flamme
Western Michigan University, 1903 W. Michigan Ave, Kalamazoo, MI 49008
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/1463-1741.56217

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