The Savannah River Site (SRS) is a key Department of Energy (DOE) industrial complex located near Aiken, S.C., with a national security and environmental management mission. The site is responsible for environmental cleanup, waste management, and disposition of nuclear materials. The mission includes processing and storing nuclear materials in support of national defense and U.S. nuclear nonproliferation efforts. The site also develops and deploys technologies to improve the environment and treat nuclear and hazardous wastes left over from the Cold War.
After the terrorist attacks of Sept. 11, 2001, many facilities like SRS with national security interests revisited protection strategies based on evolving threats. As a result, security contractors responsible for protection of these facilities transitioned to a more tactical response model with advanced weaponry. 
Due to differences in noise standards between OSHA and the U.S. Armed Forces, contractors have struggled to identify proper impulsive noise exposure limits for these military-grade weapons. Companies that implemented the OSHA standards have also struggled to develop adequate controls for safe implementation of these weapons systems.
Centerra, the security contractor at SRS, maintains a large protective force and provides extensive on-site weapons training. The training environment accounts for the greatest impulsive noise exposure risk encountered by protective force personnel, with machine guns and the .50 caliber rifle resulting in the highest impulse noise levels. As a company, we embarked on a mission to ensure adequate controls on use of these weapons without compromising training capabilities and mission readiness. PROBLEM RECOGNITION Legacy sound level meters and noise dosimeters lacked the capability to quantify impulsive noise generated during weapons firing. The search for an affordable instrument capable of quantifying these sounds led to the selection of Larson Davis equipment. Centerra chose and purchased a Larson Davis model 831 integrating sound level meter, a PRM 831 preamplifier, a model 377A12 ¼” pre-polarized microphone, and a Cal200 1000 Hz acoustic calibrator. The model 831 is a Class 1 instrument that satisfies the American National Standards Institute S1.4 requirement for sound level meters used for impulsive noise measurements. This combination supports impulsive measurements up to 190 dB at a measurement speed of 25 microseconds (such impulse noise levels were anticipated with the .50 caliber weapon). By comparison, sounds from small arms can be as high as 180 dB with a typical duration of 3 to 5 milliseconds.
Upon discharge, sound escapes from the muzzle and ejection port of the weapon. Data was collected using worst-case-scenario sampling strategies. This entailed sampling at the shooter’s ear closest to the source of the sound. Day and night measurements were taken to differentiate atmospheric conditions, which proved to be inconsequential for impulsive noise, as did weather conditions.
Through a comprehensive review of current lesson plans, their associated risk assessment documents, and interviews with key personnel, it was determined that the greatest exposure risk was to training instructors and staff at the site’s firing ranges and associated facilities. A comprehensive sampling plan was developed that prioritized sampling events based on frequency of the exposure (how often training occurred, number of rounds per weapon) and perceived loudness of the weapons. Testing began with the larger caliber weapons, descending to the standard M4 rifle and the Glock .40 caliber pistols. Extensive sound level measurements were taken to document the impulsive noise levels of all weapons systems in use and the different environments in which they were used. Sound level tests revealed that all Centerra weapons exceeded 140 dB, with the highest just above 180 dB. DETERMINING EFFECTIVE WORK FORCE PROTECTION Regulations Title 10 of the Code of Federal Regulations (CFR) Part 851 states that DOE contractors must protect their workers using the more conservative exposure limit published by OSHA and ACGIH. In this case, OSHA, which does not make an allowance for unprotected exposures above 140 dB, has the most stringent standards. ACGIH references the U.S. Department of Defense standard MIL-STD-1474C, which does allow for impulsive noises that exceed 140 dB.

Historically, security contractors doing business in the DOE complex utilized MIL-STD-1474C and interpreted it to mean that these exposures were acceptable if double hearing protection was utilized. However, through research and the creation of a formal “technical basis” document for determining noise exposures in use of weapons, Centerra concluded that double hearing protection was acceptable only if the sounds could not be reduced by reasonable means. Centerra-SRS management and our DOE customer agreed that process improvements were needed. Hearing Protection Due to the work performed, it was necessary to default to personal protective equipment (PPE) to maintain the realism of the training. OSHA’s strong recommendation for de-rating the Noise Reduction Rating (NRR) of hearing protection (based on inconsistent use and fit of hearing protectors) was adopted as a conservative measure to provide personnel with the best protection possible. Adhering to OSHA’s recommendation, single hearing protection was required for personnel in close proximity to a weapon system that measured from 140 dB to 156.5 dB (140 dB plus 16.5 dB added for foam earplugs). Weapons that created impulsive noise above 156.5 dB required double hearing protection. Double hearing protection consisted of 16.5 dB for foam earplugs plus 5 dB for the added earmuffs, raising the protected impulsive noise limit to 161.5 dB. Unfortunately, even utilizing double hearing protection, two weapons systems exceeded the 161.5 dB limit that double hearing protection offered. Sound Level Measu​rements Table 1 shows the average impulsive noise measurements of weapons in use by Centerra following extensive testing.
Controlling Impulsive Noise in a Tactical Training Environment

The sound level meter was set to “Z” peak weighting and the detector response at “impulsive.” This provided the fastest response possible with the instrument to capture the quick, sharp, short-duration sounds of gunfire. Measurement speed was most critical during measurements for the Dillon, which cycles at 50 rounds per second.

Testing identified two weapons, the Barrett .50 caliber rifle and Dillon Minigun, with unacceptable sound levels that exceeded the administrative control limit of 161.5 dB established by Centerra-SRS. The Barrett is a single-barreled semi-automatic rifle that fires .50 BMG rounds, and the Dillon Minigun is a six-barreled electric Gatling Gun that can fire 3,000 7.62 mm NATO rounds per minute. 
Some weapons have muzzle brakes attached to the end of the barrel that deflect gases in a direction that would help control recoil and aid in stabilizing the barrel during shooting; typically, the exhaust gases are deflected back toward the shooter. The barrels of the Dillon do not include muzzle brakes, so the blast from discharge is unobstructed on its path away from the shooter. The Barrett, however, has a large muzzle brake that directs a portion of the exhaust gases rearward to control the rifle’s excessive recoil. This increases the impulsive noise level at the shooter’s position and also blasts the shooter with an uncomfortable wave of gases. EFFECTIVE RISK COMMUNICATION MANAGEMENT​ The Centerra-SRS industrial hygienist worked with company management and our DOE client to further understand impulsive noise hazards and increase the likelihood that engineering controls could reduce these exposures to acceptable levels. It was also necessary to demonstrate that these controls could be used in the training environment without degrading training effectiveness or creating additional safety hazards. The technical basis document was presented to management, along with a recommendation to explore ideas for engineering controls. DEVELOPMENT OF CONTROLS With the approval and enthusiastic support of Centerra-SRS management and DOE, we initiated a plan to develop engineered barriers for use with the Dillon and Barrett weapons systems. Training with these weapons did not require movement of the shooter, which facilitated use of a barrier. The barrier’s design had to incorporate the following considerations:
  • Both the shooter and the instructor needed a clear sight path to the target.
  • The barrels of the weapons would need to protrude through the barrier to obtain maximum attenuation.
  • Sufficient clearance around the barrels was necessary so the shooter could maneuver the weapon both vertically and horizontally.
  • There could be no obstruction to the weapon, instructor, or student.
In one application, the engineered barrier had to be fitted to the top of a structure (see Figure 1). Site engineers were consulted to ensure the structure could support the additional weight. Using a list of materials and their attenuation, barriers were designed and constructed using three-quarter-inch plywood with half-inch Lexan viewing ports. This proved to be practical and cost-effective as these materials were easily purchased and readily available.

Editor’s note: Any mention of specific products or services in this article does not constitute endorsement by AIHA or The Synergist.​

The barriers, constructed by Ceterra engineering and maintenance personnel, were approximately seven feet tall and eight feet wide to allow adequate room for the student and firearms instructors. Enclosing the structure wasn’t practical, so the barrier was designed to block a portion of the sound waves from reaching personnel. The idea was to reduce the sound level to a point to where double hearing protection was sufficiently protective of the workers (shooters and instructors), below 161.5 dB. There was a small opening in the barrier for firing the weapon. The opening was kept as small as possible to block sound without interfering with or altering training.
The procedures of the Centerra Environment, Safety, Health and Quality Assurance Program require that proposed process changes of this nature be formally presented to a cross-functional team that determines whether an item can be utilized safely by employees. The cross-functional team was skeptical, and expressed concerns that the barrier would interfere with both the shooter and the instructor, that it would not be stable, and that the attenuation would not be sufficient. However, the group recognized the exposure issues faced by company personnel and granted permission to construct and test the noise barriers. RESULTS Testing of the engineered barriers validated that the design was successful at attenuating sound levels at the student and instructor positions. A before-and-after comparison of the two weapons systems with unacceptable exposure profiles appears in Table 2.

The variation in attenuation between the barriers of the Barrett and Dillon is attributed to the size of the barrel hole in the barrier. The Dillon requires a much larger opening to allow for the spinning barrels of the weapon and significant open area is needed above the gun to provide the necessary sight picture for target impact.

The only concern raised by firearms instructors after testing was limited to condensation build-up on the Lexan inserts. This was controlled by having a microfiber towel on hand to remove the moisture. A DAY AT THE RANGE The results far exceeded most expectations. The desired noise attenuation was achieved, and feedback was immediate, with one firearms instructor stating that he no longer experiences headaches after a day at the firing range. The project was successful due to a joint commitment by Centerra-SRS management, DOE, firearms instructors and protective force personnel to enhance employee safety. As a result, security contractors at sites throughout the DOE complex have revised the method for measuring impulsive noise and interpreting the data.
NIOSH recently performed an impulsive noise study at the DOE facility in Oak Ridge, Tenn. This study largely substantiated the Centerra-SRS research. STEVE STAMPER is an industrial hygienist with Centerra- SRS. He can be reached at JULIE WILSON is manager, Environmental Protection Department at Centerra-SRS. She can be reached at


ACGIH: Threshold Limit Values for Chemical Substances and Physical Agents (2014).
American National Standards Institute: Specification for Sound Level Meters, S1.4 (1983).
Code of Federal Regulations: 10 CFR 851, Worker Health and Safety Program (2015)
Code of Federal Regulations: 29 CFR 1910.95, Occupational Noise Exposure (2015)
NIOSH: Measurement of Exposure to Impulsive Noise at Indoor and Outdoor Firing Ranges during Tactical Training Exercises (June 2014).
U.S. Department of Defense: Design criteria standard noise limits, MIL-STD-1474C (1991).
A Day at the Range