The viability of hearing protection device fit-testing at navy and marine corps accession points

Abstract

Introduction: The viability of hearing protection device (HPD) verification (i.e., fit-testing) on a large scale was investigated to address this gap in a military accession environment.

Materials and methods: Personal Attenuation Ratings (PARs) following self-fitted (SELF-Fit) HPDs were acquired from 320 US Marine Corps training recruits (87.5% male, 12.5% female) across four test protocols (1-, 3-, 5-, and 7- frequency). SELF-Fit failures received follow-up to assess potential causes. Follow-up PARs were acquired (Experimenter fit [EXP-Fit], followed by Subject re-fit [SUB Re-Fit]). EXP-Fit was intended to provide a perception (dubbed "ear canal muscle memory") of what a correctly fitted HPD should feel like. SUB Re-Fit was completed following EXP-Fit to determine whether a training recruit could duplicate EXP-Fit on her/his own without assistance.

Results: A one-way analysis of variance (ANOVA) (N = 320) showed that SELF-Fit means differed significantly between protocols (P < 0.001). Post-hoc analyses showed that the 1-freq SELF-Fit mean was significantly lower than all other protocols (P < 0.03) by 5.6 dB or more. No difference was found between the multi-frequency protocols. For recruits who were followed up with EXP-Fit (n = 79), across all protocols, a significant (P < 0.001) mean improvement of 25.68 dB (10.99) was found, but PARs did not differ (P = 0.99) between EXP-Fit protocols. For recruits in the 3-freq and 5-freq protocol groups who experienced all three PAR test methods (n = 33), PAR methods differed (P < 0.001) but no method by protocol interaction was found (P = 0.46). Post hoc tests showed that both EXP-Fit and SUB Re-Fit had significantly better attenuation than SELF-Fit (P < 0.001), but no difference was found between EXPFit and SUB Re-Fit (P = 0.59). For SELF-Fit, the 1-freq protocol resulted in a 35% pass rate, whereas the 3-, 5-, and 7-freq protocols resulted in >60% pass rates. Results showed that once recruits experienced how HPDs should feel when inserted correctly, they were able to properly replicate the procedure with similar results to the expert fit suggesting "ear canal muscle memory" may be a viable training strategy concomitant with HPD verification. Fit-test duration was also measured to examine the tradeoff between results accuracy and time required to complete each protocol.

Discussion: Results from this study showed the critical importance of initial selection and fitting of HPDs followed by verification (i.e., fit-testing) at Navy and Marine Corps accession points. Achieving adequate protection from an HPD is fundamentally dependent on obtaining proper fit of the issued HPD as well as the quality of training recruits receive regarding HPD use.

Figure 1

Mean (SD) for Personal Attenuation Rating (PAR). Group size by fit method and protocol are indicated on x-axis. PAR indicates the amount of sound attenuation provided by a hearing protection device (HPD). A larger value indicates greater sound attenuation. Dark gray columns (SELF-Fit) represent results when training recruits fit their own HPDs. Cross-hatch columns represent the PAR results when HPDs were fit by the experimenter (EXP-Fit). Light gray columns show results when training recruits fit their own HPDs following an experimenter fit (SUB Re-Fit). *SDs could not be calculated with one recruit and should be viewed with caution with three recruits; means are included to show results for follow-up subjects

Figure 2

PAR pass rates. A PAR of 25 dB or better was pass criterion level. Dark gray bars represent initial fit-test results for each test procedure (1-, 3-, 5-, and 7-freq). Cross-hatched bars represent experimenter fit, and light gray bars represent subject re-fit following experimenter fit

Figure 3

Referral Personal Attenuation Ratings. 79 of the 320 recruits (25%) were referred for follow-up testing. They are rank-ordered and presented here based on their initial PAR results. Each column presents initial PAR results (SELF-Fit), experimenter fit (EXP-Fit), and, when available, a subject re-fit (SUB Re-Fit) for an individual subject, SELF-Fit displays the original rank-ordered PAR results. EXP-Fit shows PAR results when the HPD was fit by the experimenter. SUB Re-Fit shows the results for participants who were asked to re-fit their hearing protection following the experimenter fit. Each column is representative of an individual's results. The dashed horizontal line shows the pass/fail criterion level. All those with PARs above the line are considered representative of attaining adequate hearing protection for basic training at USMC MCRD, Parris Island. Note: PARs displayed as PAR < 0 should be interpreted as zero attenuation (i.e., suggesting minor variability in the repetition of threshold measurement). A negative PAR was obtained when the occluded thresholds were less than un-occluded thresholds. This suggests no protection from the HPD.

Figure 4

Test durations for Self-Fit. Each protocol differed in the number of frequencies tested. Gray bars represent average test time for each group of 80 participants. Mean duration values are displayed for each test protocol. Error bars represent one standard deviation from the mean