The attenuation of combinations of ear muffs and ear plugs

Abstract

In a number of situations, a single hearing protector (earplug or earmuff) may not provide sufficient hearing protection. This may concern, for example, shooting noise in indoor firing ranges, shooting instructors also outdoors, and jet-engine testing sites. Combinations of earplugs and earmuffs may then be considered. The protection provided by these combinations cannot simply be obtained by adding the attenuation figures provided for the plug and muff individually. A principal limit is set by bone conduction of the sound through the skull to the inner ear. In addition, the total attenuation may be affected by acoustic coupling of the two protectors. It will be demonstrated how to estimate the total attenuation using attenuation measurements of combinations of three earmuffs (Bilsom Comfort, Clark 19A, Peltor H7A) and three earplugs (Bilsom Form, Bilsom Soft, Willson EP100) for 16 subjects and data on bone conduction. The experiment was carefully designed, aiming at a direct comparison of the attenuation for the protectors individually and together, within one subject and with a minimum of replacement of the protectors. The evaluation was based primarily on subjective measurements (REAT method) but the earmuffs have also been measured using miniature microphones in the outer ear canal.