Sound Source: Dual Protection

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Sound Source: Dual Protection
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How much reduction in noise level can we expect from using both earplugs and earmuffs simultaneously?

ss11a_plugsWhen earplugs and an earmuff are used together simultaneously, we call this “dual protection” or “double protection.” Dual protection is often the only available method to achieve maximum protection from hazardous noise. Using earplugs and earmuffs concurrently seriously isolates the wearer, so it is warranted only in extreme noise levels.

In the United States, though dual protection is not required in OSHA regulations for general industry, it is required in mining operations governed by the Mine Safety & Health Administration* (MHSA) for noise exposures over 105 dBA (8-hour time-weighted average). Similarly, a guideline document from NIOSH (the advisory and research body supporting OSHA) recommends dual protection for any exposures over 100 dBA (8-hour time-weighted average). This recommendation may seem overly cautious, but it is based on the observation that, in large part, noise-exposed workers do not wear their hearing protectors properly.

Dual protection is not required in European Union† or Australian†† noise standards for general industry, however some companies have adopted internal safety policies requiring dual protection in some work locations or for certain noisy tasks. Dual protection may even be required for a particular employee, specifically when that employee’s audiogram indicates progressive noise-induced hearing loss despite normal protective measures.

OSHA has even affirmed an employer’s right to require double protection for a particular employee‡,  specifically when that employee’s audiogram indicates progressive noise-induced hearing loss despite normal protective measures.

ss11a_muffsAt what noise level is dual protection advisable? There is no clear answer, mostly due to the varying amounts of protection each wearer receives from the individual fit of his/her hearing protection. But some research suggests dual protection is overused. One study using continuously monitoring dosimetry (noise measurements taken under the earplug or earmuff) among coal miners found that with properly fitted hearing protectors, there was no need for dual protection even in ambient noise levels of 107 dBA Time Weighted Average. ‡ ‡  When a high attenuation earplug or earmuff is properly fitted and the user is motivated to use it correctly, some hearing professionals say the need for dual protection is rare. The effort expended by a safety manager to enforce dual protection is often better-spent in ensuring best fit of single hearing protection.

The amount of attenuation achieved from dual protection is not simply the combined ratings of the earplug and earmuff. There is a ceiling effect that limits the amount of combined protection. Even if wearing a perfectly-fitted earplug and earmuff with ideal attenuation, we would still hear sound transmitted through our bodies and bones to the inner ear. For most people, these bone conduction pathways limit the maximum amount of attenuation obtainable at the ear to 35—50 dB, depending on the frequency of the sound.

In terms of estimating the amount of protection while wearing earplugs and earmuffs concurrently, the most commonly used practice is to add 5 dB to the higher published attenuation. But this rule of thumb sacrifices some accuracy. An earmuff typically adds about 4 dB to the published attenuation of a well-fitted foam earplug, and about 7 dB to a well-fitted pre-molded earplug. These are overall numbers; attenuation in the low frequencies will be a bit more, and in the high frequencies a bit less. As noted in the graph below, it is not necessary to use the highest-rated earmuff to achieve maximum attenuation from dual protection.
 
ss11a_chart1ss11a_chart2

In fact, as long as the earplug is fit properly, it makes very little difference which earmuff is used, so long as it has decent attenuation in the low frequencies. An earmuff with moderate attenuation, for example, has just the same effect as a high-attenuation earmuff when either is worn over a well-fitted earplug.

The key to obtaining maximum benefit from dual protection is proper fit, especially the fit of the earplug. When a poorlyfitted earplug is worn with an earmuff, the resulting dual protection is little more than the earmuff alone.

* Mine Safety & Health Administration, 30 CFR Part 62. “Health standards for occupational noise exposure: Final rule.”
†EU Directive 2003/10/EC of the European Parliament on the minimum health and safety requirements regarding the exposure of workers to the
risks arising from physical agents (noise), 6 February 2003.
††Australian/New Zealand Standard 1269:2005 (Parts 0 — 4), Occupational Noise Management.

‡ National Institute for Occupational Safety & Health, Publication No. 98-126:1998. “Criteria for a recommended standard:
Occupational noise exposure: Revised criteria, 1998.”
‡ ‡  OSHA Interpretations, posted at www.osha/gov. Interpretation letter dated 5/8/84, “Can the employer mandate a combination of a plug and a muff?”
4 Study by Dr. Kevin Michael, presented at AIHce 2002. “Upstream Prevention of Occupational NIHL Via Individual Exposure Management.” Posted at
www.dosebusters.com