Development and validation of a field microphone-in-real-ear approach for measuring hearing protector attenuation.

Numerous studies have shown that the reliability of using laboratory measurements to predict individual or even group hearing protector attenuation for occupationally exposed workers is quite poor. This makes it difficult to properly assign hearing protectors when one wishes to closely match attenuation to actual exposure. An alternative is the use of field-measurement methods, a number of which have been proposed and are beginning to be implemented. We examine one of those methods, namely the field microphone-in-real-ear (F-MIRE) approach in which a dual-element microphone probe is used to measure noise reduction by quickly sampling the difference in noise levels outside and under an earplug, with appropriate adjustments to predict real-ear attenuation at threshold (REAT). We report on experiments that validate the ability of one commercially available F-MIRE device to predict the REAT of an earplug fitted identically for two tests. Results are reported on a representative roll-down foam earplug, stemmed-style pod plug, and pre-molded earplug, demonstrating that the 95% confidence level of the Personal Attenuation Rating (PAR) as a function of the number of fits varies from ± 4.4 dB to ± 6.3 dB, depending on the plug type, which can be reduced to ± 3.1 dB to ± 4.5 dB with a single repeat measurement. The added measurement improves precision substantially. However, the largest portion of the error is due to the user's fitting variability and not the uncertainty of the measurement system. Further we evaluated the inherent uncertainty of F-MIRE vs. the putative "gold standard" REAT procedures finding, that F-MIRE measurement uncertainty is less than one-half that of REAT at most test frequencies. An American National Standards Institute (ANSI) working group (S12/WG11) is currently involved in developing methods similar to those in this paper so that procedures for evaluating and reporting uncertainty on all types of field attenuation measurement systems can be standardized. We conclude that the hearing conservationist now has available a portable, convenient, quick, and easy-to-use system that can improve training and motivation of employees, assign hearing protection devices based on noise exposures, and address other management and compliance issues.

Development of a new standard laboratory protocol for estimating the field attenuation of hearing protection devices. Part III. The validity of using subject-fit data.

The mandate of ASA Working Group S12/WG11 has been to develop "laboratory and/or field procedure(s) that yield useful estimates of field performance" of hearing protection devices (HPDs). A real-ear attenuation at threshold procedure was selected, devised, tested via an interlaboratory study, and incorporated into a draft standard that was approved in 1997 [J. D. Royster et at., "Development of a new standard laboratory protocol for estimating the field attenuation of hearing protection devices. Part I. Research of Working Group 11, Accredited Standards Committee S12, Noise," J. Acoust. Soc. Am. 99, 1506-1526 (1996); ANSI S12.6-1997, "American National Standard Methods for Measuring Real-Ear Attenuation of Hearing Protectors" (American National Standards Institute, New York, 1997)]. The real-world estimation procedure utilizes a subject-fit methodology with listeners who are audiometrically proficient, but inexperienced in the use of HPDs. A key factor in the decision to utilize the subject-fit method was an evaluation of the representativeness of the laboratory data vis-à-vis attenuation values achieved by workers in practice. Twenty-two field studies were reviewed to develop a data base for comparison purposes. Results indicated that laboratory subject-fit attenuation values were typically equivalent to or greater than the field attenuation values, and yielded a better estimate of those values than did experimenter-fit or experimenter-supervised fit types of results. Recent data which are discussed in the paper, but which were not available at the time of the original analyses, confirm the findings.