Measuring earplug noise attenuation: A comparison of laboratory and field methods

Abstract

Hearing protection device (HPD) fit-testing is a recommended best practice for hearing conservation programs as it yields a metric of the amount of attenuation an individual achieves with an HPD. This metric, the personal attenuation rating (PAR), provides hearing health care, safety, and occupational health personnel the data needed to select the optimal hearing protection for the occupational environment in which the HPD will be worn. Although commercial-off-the-shelf equipment allows the professional to complete HPD fit tests in the field, a standard test methodology does not exist across HPD fit-test systems. The purpose of this study was to compare the amount of attenuation obtained using the “gold standard” laboratory test (i.e., real-ear attenuation at threshold [REAT]) and three commercially available HPD fit-test systems (i.e., Benson Computer Controlled Fit Test System [CCF-200] with narrowband noise stimuli, Benson CCF-200 with pure tone stimuli, and Michael and Associates FitCheck Solo). A total of 57 adults, aged 18 to 63, were enrolled in the study and tested up to seven earplugs each across all fit-test systems. Once fitted by a trained member of the research team, earplugs remained in the ear throughout testing across test systems. Results revealed a statistically significant difference in measured group noise attenuation between the laboratory and field HPD fit-test systems (p < .0001). The mean attenuation was statistically significantly different (Benson CCF-200 narrowband noise was +3.1 dB, Benson CCF-200 pure tone was +2.1 dB, and Michael and Associates FitCheck Solo was +2.5 dB) from the control laboratory method. However, the mean attenuation values across the three experimental HPD fit-test systems did not reach statistical significance and were within 1.0 dB of one another. These findings imply consistency across the evaluated HPD fit-test systems and agree with the control REAT test method. Therefore, the use of each is acceptable for obtaining individual PARs outside of a laboratory environment.

Keywords:

• Fit-test
• hearing conservation
• hearing protection device
• real-ear attenuation at threshold

Introduction

For over two decades, the hearing conservation community has operated under the premise that an individual’s real-world noise attenuation using a hearing protection device (HPD) is significantly less than its manufacturer-published noise reduction rating (NRR), which is a laboratory-measured attenuation (Berger et al. 1998). This has resulted in the widespread application of NRR derating rules to estimate an individual’s effective noise exposure. HPD derating is the use of a correction factor to estimate what a user would be expected to achieve in real-world use.

Within the United States (U.S.), the Occupational Safety and Health Administration (OSHA) and National Institute for Occupational Safety and Health (NIOSH) provide safety and health requirements for employer hearing conservation programs (HCPs) and practices, including HPD derating. OSHA (2022) recommends the use of a 50% correction factor (i.e., reduction) be applied to the NRR of all HPDs regardless of type when determining if the work environment also requires the use of engineering controls in addition to the employee’s use of an HPD. Similarly, NIOSH (1998) still recommends utilizing either a 25, 50, or 70% correction factor to the product NRR depending on the HPD type. Comparable instructional mandates exist within U.S. Department of Defense HCPs (DoD 2019). Generally, derating schemes that apply correction factors result in an assumed attenuation value and are then applied to all users regardless of the actual attenuation obtained.

Three unintended consequences stem from using a derated NRR when determining HPD appropriateness. First, if the actual attenuation is less than the derated NRR, the HPD user can be under-protected from the noise hazard resulting in an increased risk for auditory injury. Second, despite being properly fit with the HPD, it will be assumed that the HPD user is achieving a lower, derated attenuation amount, and therefore would be overprotected from the hazardous noise environment. Overprotection occurs when the actual amount of attenuation obtained by the user exceeds the amount necessary to reduce the noise below the permissible exposure limit. Overprotection can result in slower response times, an inability to hear warning signals, and/or an inability to communicate effectively (Casali et al. 2012). A third consequence of applying a derated NRR is the reduced amount of time an HPD user is permitted to operate within the hazardous occupational environment created by high noise levels. These unintended consequences impact staffing, mission, and occupational effectiveness.

Derating the NRR per OSHA and NIOSH rules results in an exact estimate that is applied to all users indiscriminately. Conversely, numerous studies have shown that when the real-world attenuation achieved by HPD users is individually measured using an HPD fit-test system, the resulting personal attenuation rating (PAR) values vary widely. The results from these studies challenge the inherent assumption in derating schemes that the everyday attenuation for all HPD users is significantly less than the NRR (Murphy et al. 2006; Federman and Duhon 2016; Sayler et al. 2019; Federman et al. 2021; Gong et al. 2021). Specifically, Federman and Duhon (2016) reported pre-training (baseline) PAR values achieved by 320 U.S. Marine Corps (USMC) training recruits ranged from 0 to 50 decibels (dB) with a foam earplug that had an NRR of 33 decibels (dB). Similarly, Federman et al. (2021) reported pre-training (baseline) PAR values among a cohort of nearly 800 USMC training recruits ranged from 0 to 50 dB when using a foam earplug with an NRR of 33 dB.

HPD fit testing was first identified as a best practice for HCPs by the OSHA Alliance (OSHA/NHCA/NIOSH Alliance 2008). Federal agencies such as the U.S. DoD (2019), the U.S. Navy (2019 2020), and the USMC (Commandant of the Marine Corps 2016) have also recommended the use of HPD fit testing as a hearing conservation best practice.

The American National Standards Institute (ANSI) and American Acoustical Society (ASA) S12.6-2016 “Methods for Measuring the Real Ear Attenuation of Hearing Protectors” stipulates the method for the Real-Ear Attenuation at Threshold (REAT) test. The REAT test is the “gold standard” method for accurately measuring the amount of attenuation provided by a head-worn device or HPD for the individual in the laboratory environment. Specifically, the REAT test is an auditory threshold test completed in a diffuse sound field with and without an HPD donned. Although ANSI/ASA S12.6-2016 describes the detailed method to follow when measuring the attenuation achieved for a given product, methods described in the standard are generally not feasible for widespread implementation.

Conversely, portable, field-based HPD fit-test systems are available that can quantify the amount of attenuation achieved by a user with any donned in-ear HPD (i.e., earplug) in a variety of settings. ANSI/ASA S12.71-2018, “Performance criteria for systems that estimate the attenuation of passive hearing protectors for individual users” details equipment specifications for manufacturers, but a lack of specificity for equipment specifications has resulted in product dissimilarities across manufacturers (i.e., hardware, software). Specifically, competing test systems use different test methods and mathematical formulas to calculate PAR values.

Regarding the PAR measurement method, portable, field-based HPD fit-test systems utilize either a psychophysical (subjective) or physical (objective) measure. And although a comprehensive review of HPD fit-test system methodology and test systems is outside the scope of this paper, several such manuscripts are available in the literature for the interested reader (Berger et al. 2011; Hager 2011; Murphy 2013; Trumpette and Kusy 2013; Murphy et al. 2016; Byrne et al. 2017; Ahroon and Stefanson 2021; Voix et al. 2022). In recent years, new technology utilizing subjective test methods has become commercially available and promoted for use in HCPs. For these newer test systems, the measurement uncertainty (i.e., difference in attenuation from the HPD fit-test system compared to the REAT method) is not yet available. For this newer technology, there is no direct evidence in the literature reporting the relationship of measured individual attenuation (i.e., PAR) across experimental test systems or in contrast to the laboratory REAT method.

For this study, commercially available psychophysical HPD fit-test systems that fulfilled two distinct employee/patient encounter types and test spaces within an HCP were evaluated: multi-person and one-on-one. The HPD fit-test systems used were the CCF-200 Computer Controlled Fit Test System (CCF-200; Benson Medical Instruments, Minneapolis, MN, USA) and the FitCheck Solo (FCS; Michael and Associates, Inc., State College, PA, USA).

The CCF-200 Computer Controlled Fit Test System is an HPD fit-test system designed to integrate with the company’s existing occupational hearing conservation product line (i.e., audiometer, simulator/sound level meter), and as such it can be deployed in either a single- or multi-person test booth environment like those found in large HCPs. However, recent literature searches failed to reveal any published, publicly available evaluations and comparisons of achieved individual attenuation obtained using the CCF-200 vs. the REAT method.

The FCS is an HPD fit-test system that can be used one-on-one outside of the laboratory and clinic. Like the CCF-200, recent literature searches failed to reveal comparisons of achieved individual attenuation obtained using the FCS vs. the REAT method. However, a seminal report (Byrne et al. 2017) evaluated the predecessor of the FCS, the HPD Well-Fit (NIOSH, Cincinnati, OH, USA). In that article, Byrne et al. (2017) stated that the PAR values obtained with the HPD Well-Fit were in agreement (± 2 dB) with the measured REAT attenuation values.

To be clinically and operationally useful in a large HCP like the U.S. DoD, it is paramount to be able to compare PAR values across test systems for equivalency between systems. This study evaluated and compared three commercially available HPD fit-test systems to the REAT method. The primary aim was to investigate the relationship of attenuation across test systems without changing HPD fit using seven different earplugs (see Table 1). The second aim was to compare the amount of attenuation achieved with the test systems to each product’s NRR, which represents how much attenuation each HPD can provide (ANSI 2008).

Table 1. Tested earplugs.

Earplug	Descriptor	Type	Size	Impression Technique	NRR	n
EP1	Roll-Down Slow Recovery Foam	Non-Custom, Disposable Foam	Universal	NA	29	42
EP2	Roll-Down PVC Foam	Non-Custom, Disposable Foam	Universal	NA	29	46
EP3	Push-to-Fit Foam with Cord	Non-Custom, Hybrid	Universal	NA	32	43
EP4	Polymer Quad Flange with Cord	Non-Custom, Premolded	Universal	NA	27	49
EP5	Full Shell Solid Silicone with Cord	Custom	Custom	Physical	29	44
EP6	Full Shell Solid Silicone with Cord	Custom	Custom	3D Scan of Physical	29	48
EP7*	Canal Silicone Rubber with Cord	Custom	Custom	3D Scan of Ear Canal	26	37

Note: *This earplug is no longer commercially available. EP = earplug, NRR = noise reduction rating, n = sample size for tested product.

Methods

This study (NSMRL.2018.0004) was reviewed and deemed human subject research by the Naval Submarine Medical Research Laboratory (NSMRL) Institutional Review Board and followed all applicable Federal regulations governing the protection of human subjects. Subjects who were nonfederal employees and federal employees in an off-duty status were compensated for their participation.

Subjects

Fifty-seven adults between the ages of 18 and 63 years consented to participate in this study. All 57 met inclusion criteria and completed testing with at least one earplug across the one control and three experimental HPD fit-test systems. Inclusion criteria required both ears to have hearing thresholds better than 25 dB hearing level (HL) at octave frequencies from 125 to 8,000 hertz (Hz) confirmed via air conduction threshold audiometry, normal middle ear function confirmed via tympanometry, and no signs of a preexisting physical condition that would prohibit the making of an ear mold impression (EMI) or use of an earplug. All subjects successfully completed REAT training and testing as defined in ANSI/ASA S12.6-2008, “Methods for Measuring the Real-Ear Attenuation of Hearing Protectors,” section 5.6, “Threshold variability (during qualification).” Subjects were asked to read the manufacturer-provided written instructions for each test system before starting the test, however, they did not complete any training with either non-laboratory based HPD fit test system.

Equipment

Laboratory test system

The laboratory (control method) measure of HPD attenuation utilized the VT HPD Test System (Lee and Casali 2013) to complete the REAT testing in accordance with ANSI/ASA S12.6-2008 “Methods for Measuring the Real-Ear Attenuation of Hearing Protectors” (ASA 2008). For details regarding NSMRL’s REAT facility see Ginsberg and Federman (2019). This method requires the use of a psychophysical (i.e., Békésy) test method that presents white noise filtered into seven one-third octave bands with center frequencies from 125 Hz to 8,000 Hz in a diffuse sound field. The listener is required to complete the test both unoccluded (without earplugs) and occluded (with earplugs). The test system’s program algorithm automatically calculated an attenuation value for each subject based on the measured binaural thresholds for the tested earplug(s).

Field HPD fit test systems

Three commercial-off-the-shelf HPD fit-test systems were compared to the REAT fit-test system in this study. The included test systems were two versions of the CCF-200 and one FCS. Due to manufacturer release and software updates of the CCF-200 during data collection, 30 subjects completed testing with the pure tone (PT) stimuli version (software v8.10, Benson Medical Instruments, Minneapolis, MN, USA), and 27 subjects completed testing with the narrowband noise (NBN) stimuli version (software v8.40, Benson Medical Instruments, Minneapolis, MN, USA). Subjects completed testing with only one of the two CCF-200s. All subjects completed testing with the FCS.

The CCF-200 used in this study utilized a manufacturer-modified TDH-39 supra-aural headset and testing was completed in a double-walled, single-person sound booth (Industrial Acoustics Company, Inc., North Aurora, IL, USA). The CCF-200 PT and CCF-200 NBN were only capable of monaural testing from 500 to 8,000 Hz. As such, Federman and Duhon (2016) recommended binaural test protocol (500, 1,000, and 2,000 Hz) was modified to test each ear independently with the Benson CCF-200. Each of the two software versions was run on its own CCF-200 test system, so the two were considered separate test systems for the study. Each software version automatically calculated and provided a left-ear PAR, a right-ear PAR, and an overall PAR.

The CCF-200 utilizes a psychophysical method of adjustment that requires users to complete an auditory threshold search task using the modified Hughson-Westlake test method (Carhart and Jerger 1959) with and without the subjects’ study-issued earplugs. Although measuring an individual’s achieved earplug attenuation via a modified Hughson-Westlake threshold test using an audiometer under headphones has reportedly been done (Stefanson and Ahroon 2016; Zaccardi et al. 2022), the relationship between the individual attenuation as measured with the modified Hughson-Westlake and REAT methods is unknown, and no validity data exist for such a method. Similarly, no such information appears in the literature regarding the CCF-200 with the REAT method or other commercially available HPD fit-test systems.

The FCS is a portable test system that uses a noise-attenuating headset (Telephonics TDH-49P type 296D100-1) to present pulsed NBN stimuli and uses a mouse for subjects to indicate their responses. The FCS can operate in quiet environments outside of the clinic or laboratory as it requires a minimal space footprint (i.e., laptop, headset, and wireless mouse). Testing was completed in a sound-treated room.

Like the CCF-200, the FCS utilizes a method of adjustment that requires the user to complete an auditory threshold test with and without earplugs. Although the system is capable of both monaural and binaural testing from 125 to 8,000 Hz, this study utilized a three-frequency (500, 1,000, and 2,000 Hz) binaural test to measure earplug attenuation. The software algorithm automatically calculated a PAR for each subject based on the binaural frequency-specific thresholds.

Earplugs

A total of seven earplugs (EPs) were used in this study. As shown in Table 1, the tested earplugs were two non-custom disposable foam, one non-custom hybrid, one non-custom pre-molded, and three custom devices. Custom-molded earplugs were created from three-dimensional (3D) scans of the outer ears, physical EMIs, and 3D scans of the physical EMIs. Binaural 3D scans of the ear canal and aperture of the ear canal were acquired first using the AURA 3D Ear Scanning System (Lantos Technologies, Wilmington, MA, USA). Physical EMIs were made next using Silicone Singles (Westone Laboratory, Colorado Springs, CO, USA). The physical molds were then scanned using a 3D scanner (ds Productions 3 Da; Smart Optics Sensortechnik GmbH, Bochum, Germany) to create the scanned EMIs. During data collection, the manufacturer of EP7 halted production and sale of the product, resulting in its early termination from data collection. In accordance with ANSI/ASA S12.71-2018, each earplug was tested by at least 20 subjects.

Procedure

After meeting the study inclusion criteria, all qualified participants were assigned a randomized test schedule. To minimize test-related fatigue, participants were limited to a maximum of two test sessions (i.e., earplugs) per day. To prevent a test order effect within each test session, the factors HPD fit-test system, fit condition (occluded/unoccluded), and ear sequence for monaural testing (right/left; CCF-200 only) were randomized and counterbalanced.

To reduce the variability of fit from subject inexperience, a modification from ANSI S12.6-2008 was imposed so that all earplugs were fitted by a trained member of the research team before the completion of the occluded fit test condition for each test session. To ensure consistency of fit across fit-test systems during each test session, once fit, the study earplugs remained in the subject’s ear until the testing across all fit-test systems was completed. Per ANSI S12.6-2008, REAT attenuation values require two test trials. Therefore, after completing the randomized occluded test order of fit-tests, the earplug was researcher-fitted for a second time, and a second occluded REAT trial was completed.

Data analysis

Descriptive analysis of attenuation values by subject across earplugs (mean and standard deviation [SD]) by test systems (group means using each subject’s average attenuation) was performed. The attenuation values used in this analysis were automatically calculated by each test system’s unique software. To compare the attenuation values from the CCF-200 with those acquired from the REAT method and FCS (which were measured binaurally), the ear that had the lower PAR value (indicating poorer fit) was used for data analysis.

To determine whether a statistically significant difference existed between test systems, a one-way repeated measures analysis of variance (ANOVA) was completed using the pooled subject means. A post-hoc pairwise contrast was completed to determine the relationship between the test systems.

To determine whether earplug type (i.e., custom, non-custom) affected attained attenuation separately from the product NRR one-sample t-tests were completed using a pooled average difference value (i.e., REAT attenuation value minus product NRR). A difference value of zero indicated there was no difference between the achieved attenuation and the product NRR, a positive difference indicated the study attenuation value was greater than the NRR, and a negative difference indicated the study attenuation value was less than the NRR. A paired t-test was also used to compare whether the amount of deviation from product NRRs was significantly different between custom and non-custom earplugs.

To compare the effect of EMI techniques on attenuation, the achieved REAT attenuation of the three custom earplugs was compared using repeated measures ANOVA. A significant result from the repeated measures ANOVA was followed by pair-wise contrasts of the PAR results of the three custom devices.

Results

The group means and standard deviations (SDs) of the four test systems were calculated by pooling the individual attenuation values from across the tested earplugs (see Figure 1). By test system, the mean (± SD) attenuation values were 28.2 ± 4.3 dB with the REAT test system, 30.7 ± 5.0 dB with the FCS test system, 30.3 ± 3.5 dB with the CCF-200 PT test system, and 31.3 ± 6.4 dB with the CCF-200 NBN test system.

Figure 1. Mean group attenuation (in dB) by test system. Note. Sample size (n) differed by test-by-test system (REAT n = 57, FCS n = 57, CCF-200 PT n = 30, CCF-200 NBN n = 27). error bars are ± 1 SD. ^‡ significant difference between the REAT and the FCS, the REAT and CCF-200 PT, and the REAT and CCF-200 NBN at p < .0001. REAT = Real ear attenuation at threshold, FCS = FitCheck Solo, PT = pure tone, NBN = narrowband noise, dB = decibel.

The repeated measures ANOVA results revealed that the mean attenuation values for all tested earplugs were significantly different across the test systems (p < .0001). To identify which test systems differed, we did a pairwise contrast between each of the test systems. As shown in Figure 1, the mean attenuation value obtained with the REAT test system was significantly lower than the attenuation values from all three other test systems (p < .0001). The analysis failed to reveal a statistically significant difference between the measured mean attenuation values of the FCS and CCF-200 PT (p = .636), the FCS and CCF-200 NBN (p = .811), and the CCF-200 PT and CCF-200 NBN (p = .503).

To determine whether the achieved attenuation for custom and non-custom earplugs significantly differed from the manufacturer's published NRRs, the mean difference between these values was calculated for each earplug. Then, the difference in these values was averaged by type (i.e., non-custom, custom) to estimate the overall mean difference between the calculated REAT attenuation and NRR (see Table 2). On average, the difference between the achieved attenuation and published NRR with custom earplugs differed to a larger degree than that of the non-custom earplugs (-3.8 vs. 1.5 dB, p < .0001). Although the one-sample t-tests showed that both earplug types were significantly different from their product NRRs (non-custom p = .0353; custom p < .0001), differences trended in opposite directions. On average, the calculated mean REAT attenuation for non-custom earplugs was higher (i.e., greater attenuation) than their product NRRs and custom earplug mean PAR results were lower than their product NRRs.

Table 2. Average difference in attenuation (in dB) from NRR by earplug type.

Earplug Type	n	Mean	SD
Non-Custom	55	1.5*	5.3
Custom	51	−3.8^‡	4.5

Note. Reported attenuation values were obtained with the REAT test system. *Significant at p < .05, ^‡significant at p < .0001; Non-custom Group includes EP1, EP2, EP3, and EP4; Custom group includes EP5, EP6, and EP7. N = sample size, SD = standard deviation.

To determine whether there was a significant difference between the calculated group mean REAT attenuation value and the manufacturer-provided NRR for each earplug, one-sample t-tests were calculated for each earplug (see Figure 2). Among the attained attenuation for non-custom earplugs, the mean values for EP1 (roll-down slow recovery foam) and EP2 (roll-down PVC foam) were significantly higher than their respective manufacturer-provided NRR (p < .0001), while the mean attenuation for EP3 (push-to-fit foam with cord) and EP4 (polymer quad flange with cord) did not significantly differ from their respective NRRs (p = .1795 and p = .2959, respectively). Among the custom devices, EP6 (full shell solid silicone with cord, 3D scan of physical impression) and EP7 (canal silicone rubber with cord, 3D scan of ear canal) were significantly lower than their respective manufacturer-provided NRR (p < .05), while the mean attenuation for EP5 (full shell solid silicone with cord, physical impression) did not significantly differ from the product NRR (p = .0711) (see Figure 2).

Figure 2. Measured REAT attenuation (in dB) by earplug. Note. Sample size (n) differed by tested earplug. The solid horizontal line is the manufacturer-provided noise reduction rating (NRR) for the tested earplug. ^‡significant at p < .05, *significant at p < .0001. The tested earplug (EP) types were EP1 = disposable foam, EP2 = disposable foam, EP3 = hybrid push-to-fit foam with cord, EP4 = premolded quad flange with cord, EP5 = custom full shell with cord manufactured from a physical impression of the outer ears, EP6 = custom full shell with cord manufactured from a 3D scan of the physical impression, and EP7 = custom canal with cord manufactured from a 3D scan of the ear canals.

Discussion

This study was designed to answer questions concerning newer commercial HPD fit-test system methods and their calculated noise attenuation levels for the individual (i.e., PAR) in comparison to the REAT method. Results revealed that the average measured attenuation for all earplugs was statistically different when measured with any of the three experimental HPD fit-test systems compared to the control REAT method. All three HPD fit-test systems resulted in measured attenuation greater than (2 to 3 dB) the REAT method mean attenuation. This finding implies that there is relatively good agreement between all measurement methods. Regarding the relationship across experimental HPD fit-test systems used in this study, statistical analyses failed to reveal a significant difference between the group mean PAR values. Further, the calculated mean attenuation was within 1 dB across the three field test systems.

As mentioned previously, there are few reports in the literature relating to the investigation and documentation of HPD fit test attenuation measures and their relationship to the laboratory REAT method following either ANSI S12.6 (Franks et al. 2003; Byrne et al. 2017; Liu and Wells 2020) or the International Organization for Standardization (ISO) 4869-1 (Dyrba et al. 2014; Trompette et al. 2015). To date, there is no known published evidence evaluating the HPD fit-test systems included in the current study, although Byrne et al. (2017) did report results for the NIOSH HPD Well Fit (predecessor of the FCS) and Michael and Associates FitCheck. Specifically, Byrne et al. (2017) determined that the measured attenuation values were in good agreement (± 2 dB) across field and laboratory test systems. As the FCS is procedurally like the HPD Well-Fit, the agreement of measured attenuation between the FCS and REAT method was anticipated.

As new HPD fit testing systems become commercially available for everyday use in clinics, offices, and HCPs, newer test systems’ methods and functionality must be understood. The current wording of ANSI/ASA S12.71-2018 (ANSI 2018) allows manufacturers to obtain compliance without requiring the use of the Békésy test method defined by the REAT method, which has resulted in a diverse product market where test systems employ a variety of test procedures, hardware, and software. Additionally, since HCPs may utilize more than one type of HPD fit-test system, future studies should compare measurement techniques and investigate the relationship between test methods and PAR values.

Limitations

The field HPD fit-test systems used in this study were evaluated in acoustically treated laboratory spaces like those found in hospitals and clinics, possibly limiting the generalizability of the findings for use outside such spaces. Further testing should be conducted in spaces that include additional real-world test environments to determine whether auditory or visual stimuli affect subject response or calculated PAR values.

Another possible limiting factor was the change in study design mid-study due to manufacturer release and software updates of the CCF-200 during data collection. Two separate groups were created for the CCF-200 test systems which led to unequal subject cohorts, where 30 subjects tested with the PT version (software v8.10) and 27 subjects tested with the NBN version (software v8.40). The result of this was three field test system groups with divided usage on the CCF-200 systems, instead of all 57 subjects being tested on the same two field test systems. However, no significant difference in mean attenuation was found between the two CCF-200 versions, and both had the same findings when comparing attenuation to the other test systems, indicating the software update mid-study likely did not affect the results.

Conclusions

In a variety of ways, HPD fit-testing can be incorporated into an organization’s HCP. HPD fit-testing can be used as a training tool (Smith et al. 2014, Voix et al. 2022), to provide supporting evidence guiding the selection of an individual’s HPD (Hager 2011), to verify and validate the HPD fit-training method provided (Federman and Duhon 2016; Federman et al. 2021), to identify those who would benefit from additional HPD fit-training (Murphy et al. 2022), and to verify measures of attenuation achieved with the issued or dispensed earplug (Federman and Duhon 2016; Federman et al. 2021).

The results from this study show that mean attenuation from each of the three experimental HPD fit-test systems was statistically different from that of the laboratory REAT method (p < .0001). The calculated mean PARs from these three test systems were approximately +3 dB greater than the mean attenuation as measured by the REAT method for all earplugs tested. Although the mean PAR values across the three experimental HPD fit-test systems were in excellent agreement, with reported mean values being within 1 dB. These results suggest that each of these test systems is suitable for use outside of the laboratory environment. However, if measurement or concern of small changes is warranted, comparison across test systems is not advised.

Disclaimer

The views expressed in this article are those of the authors and do not necessarily reflect the official policy or position of the Department of the Navy, Department of Defense, nor the U.S. Government. Reference in this paper to any specific commercial product, process, or service or the use of any trade, firm, or corporation name is for the information and convenience of the public, and does not constitute endorsement, recommendation, or favoring by the Department of the Navy. This work was prepared by employees of the U.S. Government as part of their official duties. Title 17 U.S.C. §105 provides that ‘Copyright protection under this title is not available for any work of the United States Government.’ Title 17 U.S.C. §101 defines a U.S. Government work as a work prepared by a military Service member or employee of the U.S. Government as part of that person’s official duties.

Acknowledgments

The authors of this report would like to thank the participants who made this project possible.

Data availability statement

The datasets generated during and/or analyzed during the current study are not publicly available due to government restrictions regarding data sharing but are available from the corresponding author on reasonable request and when requirements are met.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the U.S. Navy Bureau of Medicine and Surgery funding work unit F1016.