Will doubling up or wearing dual protection - an earmuff in addition to earplugs - provide added protection against extreme noise levels?
The answer is yes, according to a new Sound Source bulletin recentlyreleased by the Bacou-Dalloz Hearing Safety Group, but perhaps not as much as you thought. The bulletin, Sound Source #11a, "Dual Protection," is authored by noted audiologist Brad Witt, and is available on the company's website at http://www.hearingportal.com/hearingconservation/hc-snd-ame.asp
According to Witt, who is Audiology and Regulatory Affairs Manager for the Hearing Safety Group, dual protection is not required by OSHA regulations for general industry in the U.S., but is required for mining operations governed by the Mine Safety & Health Administration (MSHA) for noise exposures over 105 dBA (8-hour time-weighted average). Similarly, NIOSH recommends dual protection for any exposures over 100 dBA, and some companies require it for employees with progressive noise-induced hearing loss despite normal protective measures.
However, Witt cautions, there are also risks associated with dual protection. "Using earplugs and earmuffs concurrently seriously isolates the wearer," he writes, "so it is warranted only in extreme noise levels." He also suggests dual protection is overused. "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."
So how much protection will doubling up provide? That depends on the fit, says Witt, but it "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." The maximum amount of attenuation that can be attained by most people is 35-50 dB, depending on the frequency of the sound.
As for a rule of thumb for estimating the effects of dual protection, OSHA recommends adding 5 dB to the NRR of the higher rated device. But this, says Witt, "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." He also says that an earmuff with moderate attenuation provides 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," Witt writes, "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."