Outer ear canal shape and its relation to the effectiveness of sound attenuating earplugs

This investigation was undertaken to explore the relationship between the level of sound attenuation achieved by the wearing of hearing protectors, and the shape of the external ear canal. Bilateral molds of the outer portion of the external ear canal were made in 93 males and females with normal hearing, who had participated in a previous study on the effect of gender on sound attenuation, measured for a variety of insert type devices. The ear canal molds were analyzed in terms of tortuosity, caliber, and degree of funneling. The results showed that individuals with straight canals achieved significantly higher attenuation scores than those with twisted canals. With regard to caliber, the more important of the two parameters reviewed was the cross-sectional area of the canal measured at the cartilaginous-bony junction (IA). The relationship between IA and attenuation depended on both gender and the type of earplug tested. Funneling, measured by the ratio of concheomeatal and cartilaginous-bony cross-sectional areas, had the opposite effect of that for IA. These findings underscore the need for more controlled prescription and fitting procedures for the insert type of hearing protectors commonly used in the industrial setting.