Thanks to the broader band noise reduction offered in the ear canal, the active portion adds more than three times the allowable ambient noise exposure time compared to double hearing protection.

New ANR Technology in Triple Hearing Protection Safely Extends Work Time

There is a certain class of occupational environments where engineering controls are not feasible and double hearing protection is inadequate.

Have you ever wished that your employees could spend more time safely doing their job in noisy work environments? Maybe you've let go of that dream because the performance limitations of even double hearing protection won’t keep them safe from long-term hearing loss. If your team members are part of that select group of workers who are exposed to an eight-hour time weighted average of 115 dB(A) or more of continuous noise, one solution to this problem may be "triple hearing protection."

Before introducing what triple hearing protection means and the new active noise reduction (ANR) technology behind it, let's quickly review single and double hearing protection.

Single and Double Hearing Protection Review
The U.S. Occupational Safety and Health Administration (OSHA) states that hearing protectors must reduce an employee's eight-hour time weighted average noise exposure to at least 90 dB(A) or less (85 dB(A) if a standard threshold shift has been identified). The majority of workers in noisy environments need less than 10 dB of attenuation from their hearing protectors in order to meet this requirement. Most commodity hearing protectors (ear plugs or ear muffs) can deliver at least 10 dB of attenuation when worn properly in single hearing protection configurations.

Double hearing protection, the use of two different hearing protectors simultaneously (ear muffs over ear plugs), may be advised or even required in certain workplace scenarios. OSHA guidance for double protection indicates that if an employee has progressive noise induced hearing loss, the employer may require the use of double hearing protection. The Mine Safety and Health Administration (MSHA) goes a step further and mandates that double hearing protection is required when an employee's eight-hour time weighted average noise exposure is at or above 105 dB(A). The National Institute for Occupational Safety and Health recommends the threshold for using double hearing protection should be at or above 100 dB(A). These regulatory agencies explicitly or implicitly have one recommendation in common: when the continuous noise level is over 105 dB(A), double up on the hearing protection, especially if the person is already losing their hearing.

The combined attenuation of an ear plug and an ear muff is not simply the algebraic sum of the performance of each individual protector. This is due to an acoustic and vibratory interaction between the ear muff and the ear plug that causes them to behave together as a system rather than as independent hearing protectors. Generally speaking, when you combine two hearing protectors, ear muffs over ear plugs, you can expect an increase of between 3 and 10 dB over the higher performing hearing protector. OSHA recommends using 5 dB as the benefit offered by combining hearing protectors. Even with the prescribed derating methods, this will often provide enough attenuation to satisfy your employee's safety requirements.

Nevertheless, there is certain class of occupational environments where engineering controls are not feasible and double hearing protection is inadequate.

Double Hearing Protection Plus New ANR Technology = Triple Hearing Protection
Triple hearing protection can provide the extra attenuation unattainable by the combination of two passive hearing protectors. Triple hearing protection combines the conventional double hearing protection offered by two passive hearing protectors (ear muffs over ear plugs) with an ANR hearing protection component enclosed in the tip of the ear plugs. The latest ANSI measurement standards now recognize this new technology and make provisions for measuring and reporting the attenuation in accordance with national standards.

Active noise reduction has been prevalent as a technology in circumaural headsets for several decades, but the advent of high-speed processing capabilities has matured the technology to a point that it can be miniaturized and placed deep in the ear canal. A typical active noise reduction circuit contains a microphone, a controller, and a speaker. The microphone measures the sound present in the occluded space between the tip of the ear plug and the eardrum. The microphone signal is then processed by the controller which generates an "anti-sound" that is delivered to the occluded space by the speaker. The in-ear microphone measures the residual noise that is left after the combined ear muff and ear plug have done all they can do, and the active noise reduction attenuates this noise further offering a third layer of hearing protection.

This miniaturized approach to in-the-ear active noise reduction provides the benefit of a wider frequency range of attenuation, up to 2 kHz in some cases. You may have read about active noise reduction only attenuating at frequencies below about 600 Hz. This is a physical limitation experienced by circumaural headset implementations of active noise reduction technology. The combination of microphone proximity to the eardrum and the smaller acoustic volume between the eardrum and the ear plug means higher frequencies can now be attenuated. Thanks to the broader band noise reduction offered in the ear canal, the active portion adds more than three times the allowable ambient noise exposure time compared to double hearing protection. Triple hearing protection keeps your employees safer for longer while increasing productivity, preventing permanent hearing loss, and improving morale in the workplace.

Evaluating New Technologies
There are always important limitations that should be considered when evaluating hearing protection options incorporating new technologies. The triple hearing protection approach described above cannot be duplicated with an ANR circumaural headset combined with an ear plug. The ANR effectiveness becomes isolated to the external ear when the ear plug is inserted into the canal, offering no added benefit to the overall hearing protection performance.

Additionally, environmental situational awareness can be compromised by overprotection. A balance must be struck between providing adequate hearing protection without diminishing other aspects of occupational safety.

When to Use Triple Hearing Protection
So when should you consider fitting your employees with triple hearing protection? A general rule of thumb is this: When your employee is exposed to an eight-hour time weighted average noise level of 115 dB(A) or more, you should consider triple hearing protection. Although OSHA continues to recommend an exchange rate of 5 dB (an increase of 5 dB results in a halving of allowable exposure time), much of the rest of the hearing conservation community recognizes that 3 dB is more appropriate. Triple hearing protection becomes even more relevant in the context of a 3 dB exchange rate because allowable exposure times drop more rapidly as noise levels increase.

Many manufacturers and consultants will offer services to help you determine the most appropriate hearing protection for your specific occupational environment. The first step will be to carefully evaluate the noise environment at relevant occupational locations. The most sophisticated and accurate methods will include evaluation of the spectral content of the noise field, levels, and durations of exposures. These noise measurements can then be evaluated against candidate hearing protectors to predict their effectiveness. The NIOSH long method is the most accurate, and affords the benefit of accounting for and evaluating new active noise reduction technologies. Because the ANR benefit is more frequency dependent than passive noise reduction, it’s important to account for the spectral content of the noise field as well as the complementary passive attenuation. In-the-ear active noise reduction and new triple hearing protection products offer yet another tool in the toolbox to keep workers safe in extreme noise environments.

References
1. OSHA Noise Regulation, 29 CFR 1910.95.
2. NIOSH Publication No. 98-126:1998, "Criteria for a recommended standard: Occupational noise exposure: Revised criteria, 1998."
3. MSHA, 30 CFR Part 62—Health Standards for Occupational Noise Exposure.

This article originally appeared in the September 2017 issue of Occupational Health & Safety.

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