Exposure to Recreational/Occupational Shooting Range Noise vs. Industrial Impulsive Noise
Marlund E. Hale
Advanced Engineering Acoustics
Simi Valley, CA
Popular version of paper 2aNCb10
Presented Tuesday morning, April 20, 2010
159th ASA Meeting, Baltimore, MD
Recreation and occupational noise exposure comes in many forms. But the most sever and potentially damaging is exposure to high intensity impulsive noise such as is experienced at a gun firing range or from impact hammering, power chipping or pneumatic riveting operations. High-intensity impact noises, such as those found along the firing line of a busy gun range and along a production line with pneumatic riveting, are noises of short duration but with such high intensity as may damage unprotected hearing. At public firing ranges an observer or shooter usually is not the only one on the range. They are frequently exposed to gunfire noise from other shooters as well. The same situation often occurs in industry where a worker can be exposed to the high intensity impulse noise from multiple pneumatic chippers or riveting guns. For both the busy firing line and industrial production line, this situation can change noise exposure from “impulsive” (i.e., high intensity sound lasting less than one second) to “continuous” (i.e., high intensity sound lasting more than one second to several minutes).
The federal Occupational Safety and Health Administration (OSHA) has established decibel (dB) levels for hearing protection that include limits on “continuous” and “impulsive” noise exposure. A worker’s cumulative “continuous” noise exposure is established by the varying noise levels and his exposure times to each level. It is also based on a prescribed method of modifying the noise level at each audio frequency (pitch) using an adjustment called “A-weighting” that reduces the contribution of lower frequency noises. This A-weighting adjustment has been found to correspond well to human hearing sensitivity for moderate and moderately loud sounds. A worker’s “impulsive” peak noise exposure is determined from unweighted impulsive sound (also referred to as “flat” or “Z-weighted”) and is sometimes approximated by the nearly flat “C-weighting” frequency adjustment. The flat or C-weighted sound level better corresponds to human hearing sensitivity for very loud sounds.
The OSHA criteria for unprotected occupational noise exposure to “continuous”, time-weighted average (TWA), A-weighted sounds ranges from 90 to 115 dB(A), depending on the worker’s cumulative noise exposure over a standard 8-hour work day. The current standard threshold criterion for unprotected occupational “impulsive” noise is the “peak” (not time-averaged) unweighted sound level of 140 dB, but the peak 140 dB(C) limit is often used for practical and technical instrumentation reasons. Under current OSHA procedures, one evaluates impulsive and continuous noise exposure separately, each according to its own exposure limit. Excessive noise exposure above either permissible limit requires worker noise exposure reduction.
There are many different sources of impulsive noise associated with non-occupational activities. Examples include door slams, hammering, percussive music, dropped objects, fireworks, and firearms. An impulsive source of noise experienced by many sportsmen, law enforcement and military personnel is gunfire. The discharge of firearms produces impulse noise levels that often exceed the OSHA 140 dB peak unprotected sound pressure level limit. In fact, peak sound levels generated by large caliber handguns, small caliber rifles and shotguns can range from 132 dB up to more than 172 dB for high-powered firearms. A single unprotected exposure to loud gunfire can result in a temporary hearing loss. However, repeated exposure to impulsive firearm noise can result in permanent noise-induced hearing loss.
It is reported that the acoustic energy in a single shot from a high-powered rifle or shotgun is equivalent to nearly 40 hours of continuous occupational exposure to a TWA of 90 dB(A). This means that the single firing of a high-powered rifle or handgun can essentially equal one week of hazardous occupational noise exposure. Thus, any target shooter can be exposed to an entire year's worth of hazardous occupational noise in just a few minutes of shooting.
There are also several different sources of impulsive noise in industry. One impulsive noise source is fastening and assembly line pneumatic riveting. The primary riveting noise is caused by the impact of the riveting gun hammer on the rivet and associated metal work piece. The size of the work piece also influences the generated noise. Based on the results of riveting noise exposure measurements, the noise generated by their own riveting causes the greatest exposure for each riveter. Impulsive peak riveting noise exposure greater that 150 dB(C) has been measured by the author. Audiometric test data show that even riveters in lower noise production areas who never exceed the OSHA 90 dB(A) 8-hour exposure limit still suffered permanent hearing loss. Riveting noise monitoring showed that these workers experienced infrequent (less than 10 minutes per day) but repeated impact noise exposure to peak sounds greater than 140 dB(C). In another study, permanent hearing loss is documented in unprotected workers exposed to intermittent, repeated impulsive peak noise levels as low as 120 dB.
Properly fitting single hearing protection devices can provide about 25 - 30 dB(A) noise reduction. A conservative recommendation for shooters (whether casual or frequent) and shooting observers, is to wear properly fitting double hearing protection (ear plugs and muff-type) at all times near any active shooting location. Hearing protection should also be worn by all employees and firing range observers along an active firing line. A conservative recommendation for employees whose impulsive C-weighted peak noise exposure exceeds 140 dB(C) for more than 2 minutes per day, is to wear properly fitting double hearing protection, whether or not their 8-hour TWA exceeds 90 dBA. While these conservative recommendations are not required under current OSHA regulations, they are based on current and prior findings and observations of personnel exposed to high impact noise. It is important to note that effective double hearing protection only provides about 5-7 dB increased protection beyond that of the highest rated single hearing protection device and is NOT the summation of the noise reduction rating (NRR) of each hearing protection device.
The days of believing one has “iron” ears must be put behind us. No one is immune from permanent hearing loss due to nearby gunfire, muzzle blast or high intensity riveting noise. Whether they are a shooter, an observer, a riveter or a bucker, every person exposed to intense impulsive noise must diligently protect their own hearing.