John R. Franks Carol J. Merry
Bioacoust. and Occupational Vibration Section, Natl. Inst. for Occupational Safety and Health, 4676 Columbia Pkwy., Mail Stop C-27, Cincinnati, OH 45226-1998
The ANSI standard method for determining real-ear attenuation at threshold requires that no less than ten subjects be used and that each subject be tested no less than three times each with ears occluded and with ears open. Attenuation is calculated as the difference between each paired open and occluded threshold. The grand mean attenuation is calculated for all 30 attenuations. The standard deviation calculation is based on the difference between each attenuation and the grand mean. The ISO standard requires no less than 16 subjects with no less than one calculation of attenuation. From an interlaboratory study using the subject-fit method, it was possible to evaluate the number of subjects and number of repeated measures per subject necessary to reach a desired confidence interval and level of accuracy. The results indicated that the number varies with the type of hearing protector and center frequency of the testing noise. To reach (plus or minus)3 dB for an earmuff, as few as 11 subjects with one attenuation test per subject was adequate, while for one of the earplugs more than 75 subjects with two attenuation tests per subject were necessary. Implications of these results on laboratory testing strategies will be discussed.