Hearing Protection: It's Not Just About Noise Reduction

A shoe store that only carried one size shoe won't be in business long. Like your feet, ear canals come in different sizes. Unlike foam earplugs that can form to almost any size ear canal, preformed earplugs have a limited range of compression. If sized and fitted properly, preformed earplugs can approach a foam level of comfort. A recommended strategy is to fit the smallest size that will seal the ear canal while avoiding insertion beyond the last flange of the earplug.

EASE OF USE

A close cousin of comfort is ease of use. If users have to work too hard to insert and seat an earplug properly, their ear canals could become tender and sore. They could become reluctant to insert the earplugs any further than the entrance to the ear canal or they may give up on earplugs entirely. This is more likely to occur with poorly fitted preformed earplugs or with deep-insertion, custom-molded earplugs.

The K.I.S.S. principle (Keep It Simple Stupid) should apply for all types of hearing protection, but keeping it simple may be a challenge if deep insertion custom-molded earplugs are the protector of choice. A lubricant is recommended that has to be available every time the earplug is inserted. There is also the logistical challenge of replacing deep insertion custom-molded earplugs if they are damaged or lost. Technology is being developed to make replacements more practical.

Foam earplugs have their own challenges. Expertise and effort are required to roll down and insert foam earplugs properly. Foam earplug technology is evolving with designs that are not as challenging to insert properly. Your hands don't have to be clean to prepare them for insertion. You don't have to roll them down. You just “screw” them in. Minimal expertise and effort are required to achieve maximum protection.

Cords can be a mixed blessing. It is easier to keep track of tethered earplugs. They aren't lost or dropped in the mud or on a tarmac as readily (or in the soup, if used the food industry). When cords at ear level have to go below the neck area for connections to communication devices, they easily can be snagged during rigorous physical activity. Cord management issues can be minimized by reducing the number of connecting cords that go across the body and channeling the cords down one vertical path to radio(s) and push-to-talk (PTT) functions.

COMMUNICATION ENHANCEMENT

Communication requirements in noise-hazardous operations and areas can vary from marginal to essential. A raised voice with conventional hearing protectors could be sufficient in some cases. In other situations, hearing protectors that enhance verbal communication could be essential to the operation and vital to preventing injury or even death.

As a minimum, consider the level of the noise, the frequency and content of the communication, the distance between the speaker and the listener and the hearing acuity of the listeners. Distance obviously influences voice communication, but also impacts speaker visual cues. Standard phrases used repeatedly on the job are more predictable and resistant to misinterpretation. If enhanced communication is required, a variety of devices are available from simple “push-to-listen” models to high-end communication headsets.

During periods of quiet, conventional hearing protectors make a quiet situation that much quieter, which can interfere with verbal communication and detection of sounds vital to maintaining situational awareness. The answer to this conundrum is a level-dependent hearing protector that lets lower level sounds like speech pass relatively unimpeded, but progressively impedes hazardous noise.

Researchers at the French-German Institute in Saint Louis exploited the acoustic principle of impedance (acoustic friction) to develop the technology for a passive level dependent earplug.² The heart of this technology is a small cylinder with precision sized holes (0.012" ± 0.0004") at either end. “The calibrated holes dampen (sic) the more hazardous high frequency component of the impulse noise signature. Noise reduction capability increases with the noise level of the weapons fire, hence the term, nonlinear (level dependent).”³ Of six level dependent devices evaluated, this technology has shown “the greatest level dependency over the range of impulse levels tested” (110-190 dBP).⁴

There was one caveat to these test results that has implications for all earplug applications, especially when maximum protection is required. In this study, human use protocols required that the insertion and seating of the earplugs had to be under experimenter supervision. Earplugs must be inserted and seated properly for an effective level of protection.

Passive, level-dependent hearing protectors are low tech and require no batteries. Ergonomic design of an earplug also should meet requirements for durability, a low profile in the ear and comfort for long wear times. The user also should be able to switch modes between level dependent (for impulse noise) and linear (for steady state noise) while the earplug is in the ear. Changing modes should be as easy as opening and closing a bottle of shampoo. Although protective in both modes for impulse noise, only the linear (conventional) earplug mode is protective for steady state noise.

Possible explanations for poor hearing protection performance can include overprotection, shortcomings in hearing protection training and lack of user motivation, which can be related to unresolved ergonomic issues. User acceptance and effective use could be less influenced by a higher NRR and more determined by comfort, ease of use, compatibility with other equipment and communication enhancement.

References

• 1E.H.Berger, “The Naked Truth About NRR's 1993; Earlog 20.”

• 2A. Dancer and P. Hamery, “Nonlinear Hearing Protective Devices,” Proceedings of the 27nd Annual Conference of the National Hearing Conservation Association, Albuquerque, New Mexico, 19-21 Feb 1998.

• 3USACHPPM Fact Sheet, “The Combat Arms Earplug,” 51-004-0204.

• 4E.H.Berger, and P. Hamery, “Empirical Evaluation using Impulse Noise of the Level-Dependency of Various Passive Earplug Designs,” Presentation of Acoustics '08 Paris, June 29-July 24. 2008.

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Doug Ohlin, Ph.D., is an audiologist who served as a consultant to the U.S. Army Hearing Conservation Program for 35 years. In his consultant capacity, he was program manager for hearing conservation at the U.S. Army Center for Health Promotion and Preventive Medicine, chair of a Department of Defense Tri-Service Working Group for 13 years and a member of NORA, the National Occupational Research Agenda, for NIOSH. He is past president of the National Hearing Conservation Association and the Military Audiology Society. He presently is a member of the American National Standards Institute Working Group S12/WG 11, a member of the Advisory Board to the National Research Center for Rehabilitative Auditory Research for Veterans Affairs and a contract consultant to 3M. He can be contacted at doug.ohlin@mmm.com.