Intra-Subject Fit Variability for Field Microphone- In-Real-Ear Attenuation Measurement for Custom Molded Earplugs

Abstract

Over the last few years, several field attenuation measurement systems (FAMS) have been introduced to the industrial marketplace to measure the individual end-user attenuation for some hearing protection devices (HPDs). Although individual measurement is necessary to determine whether a given user is properly protected in his or her real-life noise environment (assuming that the exposure level is known), one unknown remains with a FAMS measurement: how reliable are the predictions made from the instantaneous measurement (over a few minutes), for determining the attenuation that will be achieved later in the field (over months or years) by the end-user who may fit them slightly differently every time? This paper will address that question for one FAMS, the field microphonein-real-ear (F-MIRE) measurement technique, and we will study, in the laboratory, how consistently subjects can fit and refit HPDs without assistance. A new metric, the intra-subject fit variability, will be introduced and will be quantified for custom-molded earplugs as fitted by inexperienced test subjects. This paper will present the experimental process used and statistical calculations performed to quantify the intra-subject fit variability. The number of successive refits required for a given prediction accuracy will also be presented, as well as the uncertainty component associated with the intra-subject fit variability when using an F-MIRE field attenuation measurement system.

Consistency of Attenuation across Multiple Fittings of Custom and Non-custom Earplugs

“…Values are shown for each of the four test frequencies and for the PAR. Voix, C. L. Cocq, E. Berger, unpublished data;Voix and Cocq, 2010;Berger et al, 2011). For example, using a MIRE procedure to evaluate a different brand of custom plug in a trained-user condition, J. Voix, C. L. Cocq, E. Berger (unpublished data) obtained a fitting uncertainty value of 3.1 dB for the PAR versus a value of 3.6 in the current study.…”

“…Consistency of earplug fitting was assessed in two ways. First, following Voix and Cocq (2010) and Berger et al (2011), consistency of earplug fitting was assessed with the statistic s ΔFIT , the standard deviation of the 'distance to ear mean attenuation'. This measure of fitting uncertainty is used in a new draft ANSI standard on performance criteria for FAES (American National Standards Institute, n.d.).…”

“…A user is unlikely to fit an HPD in exactly the same way each time, day after day. A single fit-test may not provide an adequate representation of the attenuation that an individual will attain across repeated fittings of the HPD (Neitzel et al, 2006;Voix and Cocq, 2010;Berger et al, 2011). In fact, the largest source of fit-test measurement variability is typically the fitting uncertainty, i.e.…”

“…the uncertainty introduced by repeated fitting of the HPD (Berger et al, 2011). For this reason, retesting the individual multiple times, with the HPD removed and refit between tests, is recommended (Neitzel et al, 2006;Voix and Cocq, 2010;Berger et al, 2011).…”

“…While retesting allows a more accurate estimate of the fitting uncertainty for an individual, it does not reduce the fitting uncertainty itself. Fitting uncertainty is affected by characteristics of the HPD (Voix and Cocq, 2010;Berger et al, 2011) and by the individual's skill in fitting the HPD (Berger et al, 2011). Thus, fitting uncertainty could be reduced by using an HPD that is easy to fit consistently, or, presumably, by training the user to wear the HPD properly.…”