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

Abstract

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.

Comparison of subjective and objective methods for the measurements of hearing protector devices attenuation and occlusion effect

“…The results illustrate well the differences between the NR * and IL expressed by the TFOE, TF ext and TF canal terms in equation (2). It is expected the TF ext and TF canal terms to depend on the type of HPD [17], [20] as well as ear canal geometry.…”

“…By using the sound pressure locations shown in figure 1, equation (1) (2) where P 0 is the sound pressure at the center of the head in the absence of the subject's head. The last four terms of equation (2) represent what was called above the "correction" factors relating the subjective measurements (REAT) to the objective noise reduction (NR * ).…”

Digital Filters for Accurately Verifying the Performance of Hearing Protectors in Use

“…Previous research has demonstrated that the MIRE method is as uitable wayt om easure the performance of hearing protectors in situ [8]. Further,t he sound pressure at the eardrum can be accurately predicted from the response of the MIRE measurement microphone provided that the transfer function is known.…”

“…Among them, aMicrophone In Real Ear (MIRE)b ased approach appears the most effective manner to conduct field measurements, yielding to the best trade-off between speed, accuracy, repeatability and correspondence with actual practice [8].…”

'Calibrating' the insertion depth of roll-down foam earplugs

“…They still obtain about 15 dB or more protection at all frequencies on average (some are of course doing worse than that), but many clearly would benefit from training and motivation for better performance. Enhanced worker training is now becoming more readily deliverable through the use of various field attenuation estimation systems (colloquially called "fit-test systems") that are being marketed today (Berger et al, 2011). …”