Here is a discussion of using quality earplugs AND muffs that explains why doubling up DOES work. This is written by a full-time, professional audio Senior Scientist with MS in acoustical engineering,
OVERVIEW:
For those situations in which noise exceeds 100 dBA, as found in a wide range of industries from on-line aircraft engine maintenance to metal riveting, dual protection may be an appropriate solution.
Although the casual observer of HPD's labeled NRRs might conclude that in almost any noise exposure employees will be adequately protected (because labeled NRRs commonly range from 22 - 33 dB), that is far from the actual case. Many studies in the past 20 years have clearly demonstrated that in real-world environments HPD attenuation falls far short of the EPA-mandated NRRs that have appeared on packaging since the early 1980s (Berger et al., 1996). Thus in critical high-noise instances, where TWAs exceed 100 to 105 dBA, the use of dual hearing protection is warranted.
It is generally recommended that one should consider the use of dual protection when 8-hour time-weighted-average (TWA) exposures exceed 105 dBA as required in the new Mine Safety and Health Administration (MSHA, 1999) noise regulation, which went into effect September, 2000. For an extra margin of safety one may wish to consider implementation of dual protection when exposures exceed 100 dBA as is suggested in the 1998 NIOSH Revised Criteria for Occupational Noise Exposure. Using dual protection at levels or exposures below that is problematic due to interference with speech-based communication. However, once noise levels exceed 100 dBA unaided speech communication (i.e. communication without electronic amplifiers and earphones) is nearly impossible anyway. Therefore, no additional detriment is created by the extra protection.
Elliott H. Berger, MS, INCE Bd. Cert.
After receiving his M.S. in Acoustical Engineering from North Carolina State University, Elliott joined the Aearo Company in 1976. As E•A•R/Aearo's Senior Scientist, Auditory Research, he conducts hearing protector research and development. He has written over 60 articles on hearing protection/conservation, and was the principal editor for the 4th and 5th editions of the AIHA Noise & Hearing Conservation Manual. Elliott chairs ANSI working group S12/WG11 on real-world hearing protector performance, and is also involved with numerous other standards committees. In 1993 he was the recipient of the National Hearing Conservation Association's Outstanding Hearing Conservationist Award, and in both 1993 and 1998 received their Outstanding Lecture Award. Elliott is Past President of the National Hearing Conservation Association (NHCA), Past-Chair of the American Industrial Hygiene Association's (AIHA) Committee on Noise and a Fellow of the Association, a Board Member of the of the Council for Accreditation in Occupational Hearing Conservation (CAOHC), a Fellow of the Acoustical Society of America (ASA), and a member of the American Speech-Language-Hearing Association (ASHA).
