Dual Protection for Deafening Sounds

April 6, 2007
Audiology and Regulatory Affairs Manager Hearing Safety Group, Bacou-Dalloz

Noise is a fact of life at any airport facility, be it from airplane takeoffs and landings, jetway activities or regular aircraft maintenance. Airports pose a unique noise challenge: high intensity noise lasting for a short duration. There may be times that a single earplug or earmuff cannot adequately protect against unusually high noise levels. The use of combined protection of both an earplug and earmuff, also known as dual or double protection, may be necessary to protect airport workers. But wearing dual protection also seriously isolates the wearer, both from routine conversation or instructions and from alarm or warning signals. So the question becomes when is dual protection better and by how much?

Dual protection is not required in US OSHA regulations for aviation and general industry. However, a guideline document from NIOSH1, the advisory and research body supporting OSHA, recommends dual protection for any exposures over 100 dBA time-weighted average (TWA). While this recommendation may seem overly cautious, it is based on the observation that, in large part, noise-exposed workers do not wear their hearing protectors properly.

Some companies have adopted internal safety policies requiring dual protection in some work locations or for certain noisy tasks. OSHA has even affirmed an employer’s right to require double protection for specific employees, when the audiograms of those employees indicate progressive noise-induced hearing loss despite normal protective measures.

So at what noise level is dual protection advisable? There is no clear answer, mostly because the amount of protection each wearer receives varies according to the individual fit of his/her hearing protection. In fact, some research suggests dual protection is abused. One study found that with properly fitted hearing protectors, there was no need for dual protection even in ambient noise levels of 107 dBA TWA3. 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.

Nor is the amount of attenuation achieved from dual protection simply the combined ratings of the earplug and earmuff. There is a ceiling effect that limits the amount of combined protection. Even when wearing a perfectly fitted earplug and earmuff with ideal attenuation, sound waves can still be conducted through our bodies and bones to the inner ear. For most people, these 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 obtained from wearing earplugs and earmuffs concurrently, OSHA recommends simply adding 5 dB to the higher NRR. But this rule of thumb sacrifices some accuracy. An earmuff typically adds about 4 dB to the NRR of a well-fitted foam earplug and about 7 dB to a well-fitted pre-molded earplug. These are approximate numbers; attenuation in the low frequencies will be a bit more and in the high frequencies a bit less. It is not necessary to use the highest-rated earmuff to achieve maximum attenuation from dual protection. In fact, if the earplug is fit properly, it makes very little difference which earmuff is used, as long as it has decent attenuation in the low frequencies. An earmuff with moderate attenuation for example, has the same effect as a high-attenuation earmuff when either is worn over a well-fitted earplug.

For short-duration exposures, the attenuation of an earmuff should be adequate to protect hearing. But for noise exposures over 105 dB lasting several hours, dual protection is advised. The key to obtaining maximum benefit from dual protection is proper fit, especially the fit of the earplug. When a poorly fitted earplug is worn with an earmuff, the resulting dual protection is little more than the earmuff alone.