Investigation of physical and psychophysical methods for the attenuation measurement of circumaural hearing protectors with implication for field use

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Virginia Tech


A field-implementable measure is needed to estimate the attenuation workers are achieving with their hearing protectors in the field. Alternative measures for assessing a hearing protector's effectiveness were evaluated through comparison to the standardized real-ear attenuation at threshold (REAT) method, termed 1I3-REAT, a psychophysical procedure (ANSI S3.19-1984). One alternative deviated from the standardized procedure primarily through utilizing a pure-tone audiometer coupled to an amplifier and loudspeaker for the sound field presentation of pure tones (Pr -REAT). The other alternative. tenned microphone in real-ear (MIRE), used miniature microphones in each concha and just outside of each of the subject's two ears to physically measure the attenuation of the protector using both insertion loss (lL-MIRE) and noise reduction (NR-MIRE) procedures. Comparisons between the alternative measures were made across nine 113 octave bands centered at 125, 250, 500, 1000, 2000, 3150,4000, 6300, and 8000 Hz. The experiment also explored a means for predicting broadband attenuation from data of a single 113 octave band through a regression analysis for both PT -REAT tests and NR-MIRE tests.

Results showed that when comparing the PT -REAT test and the standardized 1/3-REA T method, the standardized method exhibited significantly greater attenuation at most of the 1/3 octave bands tested. The difference, however, may be due to the higher ambient noise levels present in the pure tone condition. The MIRE measures also showed some significant differences with the standardized 1/3-REAT method for the values collapsed across protectors, with the direction of the difference changing with test band. At 125 Hz, the MIRE metrics yielded significantly lower attenuation, while from 500 to 6300 Hz, the 1/3-REAT method generally yielded significantly lower attenuation. These differences may be due in part to the occlusion effect and the bone conduction of sound. In general, however, the size and consistency of the differences across hearing protection devices (HPDs) suggest that PT-REAT and MIRE measures hold promise for providing quick and relatively accurate estimations of an HPD's attenuation in the field. Results of the regression analyses indicated that single test band data obtained at 250 and 500 Hz, from both PT-REAT and NR-MIRE metrics, provided the best predictions for the hearing protectors tested in this study, based on their Pearson product-moment correlation coefficients.