Custom-molded Earplugs Research Articles

Roar Aboard: Protecting Service Members' Ears on Aircraft Carriers

Roar Aboard: Protecting Service Members' Ears on Aircraft Carriers

Suitability of commercial systems for earplug individual fit testing

Hearing Protection Device (HPD) rated attenuation is measured using the Real Ear Attenuation at Threshold (REAT) method specified in Standard ISO 4869-1. This statistical method assumes optimal fitting and is applied under laboratory conditions to predict the hearing protector performance for an individual wearer. The rated attenuation is therefore generally higher than that measured in the field. A consequence is the emergence of commercially available systems, which offer the capability of individual fit testing of hearing protectors in the field to control the attenuation actually received by the wearer. The purpose of this paper is to assess the suitability of these systems. Three commercially available systems dedicated to earplugs were used under laboratory conditions to assess the performance of pre-formed, foam or custom-molded earplugs for at least 20 test subjects. Results were compared with REAT attenuations for the same group of subjects. Two of these systems ensure mean attenuations close to benchmark values and individual comparisons are acceptable for these systems, although discrepancies with respect to benchmark values can be wide. These systems can therefore be used to validate a choice of hearing protection as long as a large but acceptable safety margin is considered. They are also quick and easy to use, and can contribute to worker training and motivation.

Intra-subject fit variability using field microphone-in-real-ear attenuation measurement for foam, pre-molded and custom molded earplugs

In recent years, the arrival of several field attenuation estimation systems (FAES) on the industrial marketplace have enabled better assessment of hearing protection in real-life noise environments. FAES measure the individual attenuation of a given hearing protection device (HPD) as fitted by the end-user, but FAES enable predictions based only on measurements taken over a few minutes and do not account for what may occur later in the field over months or years as the earplug may be fitted slightly differently over time. This paper will use the field microphone-in-real-ear (F-MIRE) measurement technique to study in the laboratory how consistently a subject can fit and refit an HPD. A new metric, the intra-subject fit variability, will be introduced and quantified for three different earplugs (roll-down foam, premolded and custom molded), as fitted by two types of test subjects (experienced and inexperienced). This paper will present the experimental process used and statistical calculations performed to quantify intra-subject fit variability. As well, data collected from two different laboratories will be contrasted and reviewed as to the impact of trained versus untrained test subjects.

Attenuation as a function of the canal length of custom-molded earplugs

The fit of a custom-molded earplug (CMEP) and the amount of attenuation it provides can be affected by variables related to the original earmold impression and the subsequent manufacturing process. One variable thought to affect the amount of attenuation is the length of the canal portion of the CMEP. In this pilot study, we systematically examined the relationship between CMEP canal length and attenuation in four human subjects. Two men and two women were fitted with CMEPs extending past the second bend of the ear canal. The attenuation provided by the CMEPs was measured over four visits to the laboratory. Prior to each visit, the canal portion of the subject’s test CMEP was shortened by 2 mm. As expected, attenuation decreased as canal length decreased for all subjects. However, the rate and pattern of decrease varied markedly. Anecdotal reports of comfort as a function of canal length also varied markedly. Results suggest that the critical region/s in the ear canal for maintaining a good acoustic seal may vary from person to person. Implications for future study will be discussed.

Attenuation as a function of the canal length of custom-molded earplugs: A pilot study

Custom-molded earplugs (CMEPs) whose canal segments extend beyond the second bend of the ear canal can provide excellent attenuation but can sometimes be uncomfortable. Attenuation was measured for CMEPs whose canal segments were shortened in 2-mm increments. The within-subjects design permitted illustration of the form of the function relating attenuation to canal segment length for individuals. Reduction of attenuation due to canal segment shortening was generally more pronounced for frequencies ≤1000 Hz. Some regions of the canal segments were more critical than others in maintaining attenuation. The relationship between comfort and canal segment length was not straightforward.

Measurements of earplug attenuation under supra-aural and circumaural headphones

Objective: Supra-aural audiometric headphones are generally not recommended for use in measuring the attenuation of earplugs, because contact between the headphone and pinna and/or earplug could alter the attenuation obtained, and because of concerns of non-comparability between modes of excitation from supra-aural headphones and the sound-field procedure required by the standardized method. In this study, we compared measurements of earplug attenuation obtained under Telephonics TDH-50P supra-aural headphones with measurements obtained under circumaural headphones designed expressly for such testing. Design: The attenuation of three types of earplugs (foam, premolded quadruple-flange, and custom-molded) was measured in a repeated-measures design. Study sample: The study sample comprised 42 normal-hearing adults (21 females, 21 males). Results: With the foam earplugs, nearly all of the attenuation measurements under the supra-aural headphones fell within 10 dB of the measurements under the circumaural headphones. With the flange and custom earplugs, approximately 10% of individuals obtained spuriously high attenuation under the supra-aural headphones. Conclusions: We conclude that standard supra-aural audiometric headphones are suitable for measuring the attenuation provided by foam earplugs. However, supra-aural headphones should not be used to measure the attenuation of flange or custom-molded earplugs. The potential exists for substantial over-estimation of attenuation, especially of custom plugs.

Speech Intelligibility Differences across Sound Classes with in-the-Ear and Free-Field Microphones in Quiet

Speech intelligibility performance with an in-the-ear microphone embedded in a custom-molded deep-insertion earplug was compared with results obtained using a free-field microphone. Intelligibility differences between microphones were further analyzed to assess whether reduced intelligibility was specific to certain sound classes. 36 participants completed the Modified Rhyme Test using recordings made with each microphone. While speech intelligibility for both microphones was highly accurate, intelligibility with the free-field microphone was significantly better than with the in-the-ear microphone. There were significant effects of place and manner of sound production. Significant differences in recognition among specific phonemes were also revealed. Implications included modifying the in-the-ear microphone to transmit more high frequency energy. Use of the in-the-ear microphone was limited by significant loss of high-frequency energy of the speech signal which resulted in reduced intelligibility for some sounds; however, the in-the-ear microphone is a promising technology for effective communication in military environments.

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

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.

Results of the National Institute for Occupational Safety and Health—U.S. Environmental Protection Agency Interlaboratory Comparison of American National Standards Institute S12.6-1997 Methods A and B

The National Institute for Occupational Safety and Health and the Environmental Protection Agency sponsored the completion of an interlaboratory study to compare two fitting protocols specified by ANSI S12.6-1997 (R2002) [(2002). American National Standard Methods for the Measuring Real-Ear Attenuation of Hearing Protectors, American National Standards Institute, New York]. Six hearing protection devices (two earmuffs, foam, premolded, custom-molded earplugs, and canal-caps) were tested in six laboratories using the experimenter-supervised, Method A, and (naive) subject-fit, Method B, protocols with 24 subjects per laboratory. Within-subject, between-subject, and between-laboratory standard deviations were determined for individual frequencies and A-weighted attenuations. The differences for the within-subject standard deviations were not statistically significant between Methods A and B. Using between-subject standard deviations from Method A, 3-12 subjects would be required to identify 6-dB differences between attenuation distributions. Whereas using between-subject standard deviations from Method B, 5-19 subjects would be required to identify 6-dB differences in attenuation distributions of a product tested within the same laboratory. However, the between-laboratory standard deviations for Method B were -0.1 to 3.0 dB less than the Method A results. These differences resulted in considerably more subjects being required to identify statistically significant differences between laboratories for Method A (12-132 subjects) than for Method B (9-28 subjects).

Custom-molded ear-plug, and process for producing a custom-molded ear-plug device

A hearing device and process for its updating, including a resilient outer shell and one or more internal units, with the outer shell conforming in shape to at least one of the internal units so that the internal unit is held snugly in place. The outer shell can be replaced using either a destructive or non-destructive disassembly method to re-use at least one of the internal units.

Variability of Real-World Hearing Protector Attenuation Measurements

The attenuation provided by a hearing protection device (HPD) in the field is usually estimated by applying a derating factor to the laboratory-determined noise reduction rating (NRR) of the HPD. However, attenuation is highly dependent on individual-specific HPD fit. Prediction of an individual's attenuation depends on the accuracy of the measurement system and the variability of attenuation over time (e.g. after HPD refitting). Variability in attenuation and attenuation test systems has not been adequately characterized to allow for such an assessment. This study compared attenuation measurements made with two systems, Real-Ear-at-Threshold (REAT) and Microphone-in-Real-Ear (MIRE), on 20 workers using two earplugs (foam and custom-molded). Workers' perceptions of the earplugs were also evaluated. Individuals' attenuation results were summarized as personal attenuation ratings (PARs, similar to NRRs). Variability in PARs from between-subject, within-subject and within-day (i.e. repeated tests on a subject without earplug refitting) differences was assessed and used to present the lower confidence limit, or uncertainty factor (UF), of an average individual's attenuation. The custom-molded earplug PARs achieved a higher mean percentage of labeled attenuation than did the foam earplug with both test systems. The custom-molded earplugs also had higher overall acceptance among workers. MIRE PAR levels were lower than REAT levels for both earplugs, but the relationship between the two test systems was highly variable. The MIRE system had lower within-day variability than the REAT system. One individual's MIRE results were highly influential; removal of these results greatly reduced the UF for the custom-molded earplug/MIRE combination. UFs ranged from 8.8 to 13.5 dB. These findings highlight the importance of evaluating variability in individual-specific protection results for personal protective equipment like HPDs, rather than relying on single measurements.

Noise Exposure of Symphony Orchestra Musicians

Abstract Personal noise exposures of classical musicians in the Winnipeg Symphony Orchestra, Centennial Concert Hall, Winnipeg, Manitoba, Canada, were conducted to determine compliance with provincial standards. In Manitoba, a hearing conservation program is required where the equivalent sound exposure level (Lex 8-hour) exceeds 80 dB A-weighted sound pressure level [dB(A)]. In excess of 85 and 90 dB(A), standards require hearing protection and engineering or work practice controls. Approximately 10 percent of the musicians wore conventional or custom-molded earplugs. Dosimetry readings were taken in the rehearsal room, main stage, and orchestra pit during rehearsals and dress rehearsals. Quest model M-8B and Larson Davis model 700 dosimeters were used and the Canadian Standards Association procedure was applied. The mean Lex 8-hour for the rehearsal room surveys were 88 and 90 dB(A), for the pit were 85 and 86 dB(A), and for the main stage were 82, 84, and 88 dB(A). Instantaneous peak exposures were reco...

Custom-Molded Earplug Performance: A Retrospective Study

The purpose of this study was to determine the effectiveness of custom-molded earplugs in preventing hearing threshold shifts over a long period of time. A stepwise regression analysis was employed to accomodate job related variables including workplace noise level, demographic elements including age, and factors related to hearing including ear history. These variables would be expected to exert considerable influence on hearing threshold shifts. However, workplace noise level absorbed unexpected small amounts of variation in hearing threshold shifts for the textile worker subjects (N = 1517) even when noise level was entered as the only variable on the first step for each analysis. And, baseline hearing values absorbed unexpected large amounts even when entered on the last step. An argument based on these findings is presented suggesting that custom-fitted hearing protection was effective in preventing substantial changes in hearing threshold level during the period of this investigation.