Abstract
For very high-level noise exposures, especially when 8-hour, time-weighted averages are greater than 105 dBA, the attenuation of a single hearing protection device may be inadequate. For such exposures, double hearing protection, i.e. muffs plus plugs, may be called for. It is well recognized that double hearing protection does not simply yield overall attenuation equal to the sum of the individual attenuation of each device due to the bone-conduction flanking paths and the acoustical-mechanical interaction between two such closely spaced devices. The incremental performance to be gained by double protection was investigated experimentally by measuring the real-ear attenuation at threshold for a number of combinations, according to ANSI S3.19-1974. All tests were conducted on 10 subjects, three replications per subject. Devices included foam plugs (partial, standard and deep insertion), fiberglass down, V-51R earplugs, medium and large volume commercially available earmuffs, and an experimental large-volume lead earmuff. All devices were examined singly and in combination. In all cases, plugs-plus-muffs outperformed either device individually, except at 1 kHz for one combination. Although the two pairs of commercially available earmuffs offered significantly different low frequency attenuation (125–1000 Hz), when either earmuff was combined with the V-51R or deeply inserted foam earplug, the improvements were similar. However, when the situation was reversed and the different inserts were examined in combination with a single earmuff, the choice of insert was important below 2 kHz. At and above 2 kHz all plug-plus-muff combinations that were studied provided attenuation approximately equal to the bone-conduction limits of the skull. Based on these laboratory results, it is clear that double hearing protection may provide the additional noise reduction needed in very high noise level environments.