Mitigating hearing loss at the source
Almost all firearms create noise that is over the 140-dB level. A small .22-caliber rifle can produce noise around 140 dB, while big-bore rifles and pistols can produce sound over 175 dB. APD suppressors help mitigate hearing loss at the source

Noise-induced hearing loss (NIHL) and tinnitus which costs tax payers in excess of one billion dollars a year, is the number one issue requiring treatment for persons exiting the military. This impairment is not reversible and follows ex-military personnel into civilian careers. Law enforcement is not immune to this phenomenon. The number one expense for law enforcement agencies has been disability pay outs and at the top of that list is hearing loss. It is important to understand that LE is not simply inheriting this problem, in many cases they are adding to and creating it. Studies show that law enforcement officers shooting on the range are exposed to noise levels in excess of 150 dB peak sound pressure levels. These levels are clearly in excess of accepted OSHA standards. Exposure to impulse sound can cause acute acoustical trauma, which can be followed by symptoms such as tinnitus and hearing impairment. Exposure to these sounds can also cause direct mechanical damage to the middle and inner ear. This is creating not only a quality of life issue for service personnel but an increasingly burdensome cost center.

Noise “surveys” measuring intensity (loudness) and exposure (time) are routinely conducted to determine if workers are at risk for hearing loss. OSHA (Occupation Safety and Health Administration) is part of the U.S. Department of Labor and is responsible for developing and enforcing workplace safety and health regulations. The OSHA standard (29CFR1910.95) carries behind it the force of law and employers in the industrial sector are bound to comply with it. Those employed in mining, railroad, coast guard, military, and construction are bound by their own standards.

NIOSH (National Institute for Occupational Safety and Health) is part of the Centers for Disease Control and Prevention (CDC) in the U.S. Department of Health and Human Services. NIOSH conducts research and provides information, education, training, and recommendations regarding occupational safety and health. As such, NIOSH is in a position to recommend standards and best practices, but it is not in a position to regulate or enforce standards.

The question these agencies tend to ask for determining risk is “how long and how loud can we listen to sound without risking hearing damage”? Damage-risk criteria provide the basis for recommending noise exposure limits based on noise level and exposure time. OSHA and NIOSH criteria are shown in Figure 1.

The differences in OSHA criteria and NIOSH recommendations for exposure limits produce different outcomes: the more lenient OSHA values allow for higher exposures for longer durations and the more conservative NIOSH values recommend lower exposures for shorter durations. The NIOSH values are based on scientific studies relating noise exposure to hearing loss, and are more protective of hearing. It should be noted that both standards are based on the assumption that the noise occurs as part of a work environment, and both assume non-occupational quiet.

The effects of hearing loss as a result of a blast (such as those produced by firearms) aren’t as well documented as occupational hearing loss. Many have suffered permanent damage from shooting, but the effects of single, loud-noise events varies from person to person. The impact on the end user’s hearing is significant and complex. Accurately measuring that impact is difficult and attempts to standardize impact processes and measurements are evolving. Additional factors have to be understood to completely comprehend the event’s impact on hearing. Sound transmission via bone conduction is very real. A shooter’s head placed on the rifle stock allows energy traveling through the rifle stock to be transmitted to the inner ear in addition to the airborne blast. To be effective, sound impact must also be expressed in terms of power, pressure, or sound pressure level (SPL).

Another important consideration regarding risk criteria is the “equivalent energy” theory. The idea that equivalent energy (EE) will result in the same amount of damage for a given person may be insufficient in determining risk for intensities greater than 150dB SPL. A study revealed that 30 impulses (or rounds) of simulated gunfire at 150 dB SPL peak level created a temporary threshold shib (TTS) in the ear whereas 300 impulses of the same pulse shape at 140 dB SPL (to maintain the same total energy) usually produced no TTS. This suggests that 3 impulses at 160 dB SPL would create an even greater TTS than 30 impulses at 150 dB SPL even though the “equivalent energy” is the same. Greater sound pressure levels are at least, if not more, damaging than their lower intensity, albeit longer duration, energy equivalent SPLs.

Conventional sound pressure level (SPL) measurements reference sound or noise that has a duration exceeding several cycles of vibration. Peak pressure levels (PPL) measures the single greatest change in pressure, even though the duration may only be half of a cycle. Using PPL to measure the impact of small arms fire in lieu of SPL is critical and correlates directly to our perception of loudness.

Designing suppression to tame the turbulence and impact of the impulse event produced by a gunshot requires significant time, testing and a thorough understanding of materials science, engineering and physics. Alan Paulson, author of Silencer History and Performance states, “The event is characterized by a broad mix of physical and chemical parameters that literally change microsecond by microsecond and place by place within the silencer.” Understanding this unique scientific event is paramount to new product development but it is no simple feat. Paulson adds, “…the rapidly changing variables inside a suppressor include gas pressure, gas velocity, gas temperature, chemistry of the gases inside the silencer at the time of cartridge ignition, the amount and behavior of unburned particulates remaining in the gas stream, the chemistry of the combustion gases themselves, and the temperature of the silencer materials.” If the science isn’t fully comprehended then it is implausible, if not impossible, to design suppression devices that effectively address the needs of the end user.

In 2007, Asymmetric Product Design (APD) began research and development of a new suppressor technology. The goal was to remove the constraints of traditional machining techniques and materials and leverage the attenuation properties of a broader range of materials in more effective geometries. The design features interior geometries that capitalize on Inters::al Cellular Entrapment (ICE) techniques. Looking beyond the existing science of traditional K type baffles or internal machined parts, the APD design contains a large number of variable dimensioned and breathable side branch resonators. These cavities excel in absorbing the blast energy using a combination of resonator absorption, chamber to chamber phase cancellation, turbulent induced flow, and a large surface area to cool the gas. The cavity shape has been optimized to reduce reflected pressure back toward the chamber while achieving excellent sound attenuation.

The ICE design is proving to be a technically superior suppressor with excellent sound and flash suppression metrics, low/no maintenance, high in-field usability while withstanding the rigors of full auto fire on carbine length barrels. This design process provides for full featured, high-powered magnum suppression in smaller diameters and shorter lengths and weights.

The continual development of our suppressor technology should prove to reduce the injurious effects of small arms fire (NIHL) while providing the benefits of not just dB reduction, but improved stealth and accuracy. Additionally, APD is working directly with Oregon Ballistic Laboratories to have their suppressors fully tested for not only dB, but sound impact, velocity and accuracy.

APD holds in highest regard those whose health and well-being are dependent upon suppression technology. We believe that additional investment in the continuing development of our design will yield a superior product. Where others have cloned, borrowed, or in some instances obtained illegally, suppressor designs, APD has and always will develop and design with integrity and the safety and welfare of the end user in mind.