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|Year : 2011
: 13 | Issue : 51 | Page
|Relationship between comfort and attenuation measurements for two types of earplugs
David C Byrne1, Rickie R Davis1, Peter B Shaw1, Burgundy M Specht2, Amy N Holland3
1 National Institute for Occupational Safety and Health, Cincinnati, OH, USA
2 Cincinnati VA Medical Center, Cincinnati, OH, USA
3 Montgomery Ear Nose and Throat Center, Cincinnati, OH, USA
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|Date of Web Publication||1-Mar-2011|
Noise-induced hearing loss is almost always preventable if properly fitted hearing protectors are worn to reduce exposure. Many individuals choose not to wear hearing protection because it may interfere with effective communication in the workplace or it may be uncomfortable. Hearing protector comfort has not received the same amount of attention as noise reduction capability. The present study was conducted to evaluate the comfort level of two different types of insert earplugs as well as the attenuation levels achieved by the earplugs. Attenuation levels were obtained with a commercially available earplug fit-test system, and the comfort ratings were obtained by questionnaire. The primary research objective was to determine whether hearing protector comfort was related to measured attenuation values. A linear mixed effects model provided evidence for an inverse relationship between comfort and attenuation.
Keywords: Comfort, fit-testing, hearing protector
|How to cite this article:|
Byrne DC, Davis RR, Shaw PB, Specht BM, Holland AN. Relationship between comfort and attenuation measurements for two types of earplugs. Noise Health 2011;13:86-92
|How to cite this URL:|
Byrne DC, Davis RR, Shaw PB, Specht BM, Holland AN. Relationship between comfort and attenuation measurements for two types of earplugs. Noise Health [serial online] 2011 [cited 2013 May 18];13:86-92. Available from: http://www.noiseandhealth.org/text.asp?2011/13/51/86/77193
| Introduction|| |
In the United States, employers are required to use all feasible engineering and administrative controls to reduce noise exposures to acceptable levels according to the U.S. Occupational Safety and Health Administration (OSHA) regulations.  If engineering and administrative noise controls fail to sufficiently reduce exposure levels, personal hearing protection (along with training on proper fitting, care, and use) must be provided to workers.
Over 50 manufacturers have developed and sold at least 241 different hearing protection devices in the United States.  Traditional hearing protectors usually take the form of either insert-type earplugs that are placed in the ear and seal against the ear canal walls, muff-type devices that seal against the head around the pinna, or conchaseated protectors that provide an acoustic seal right at the entrance to the external ear canal (so-called canal caps). Other wearing configurations include helmets that provide hearing protection, the use of double hearing protection, and additional variants employing electronic circuitry.  There is no single type of device that is optimal for all individuals or situations. However, some types are better than others for use in specific noise environments, for some work activities, or for some environmental conditions.
Generally speaking, uninformed individuals assume that hearing protectors with the greatest attenuation ratings are the best and, in a typical industrial work environment, hearing protectors are often selected simply according to which device has the highest labeled Noise Reduction Rating (NRR).  This approach overlooks the fact that 90% of the occupational noise exposures do not exceed 95 dB(A).  Most hearing protectors are capable of producing the 10 dB of attenuation necessary to reduce the exposure to 85 dBA or lower.  Therefore, virtually any protector that provides 10 dB of attenuation should suffice for most industrial noise situations. Consequently, an experienced hearing conservationist will recommend whichever suitable protector the worker will consistently and effectively wear as the one that should be chosen.
One barrier to consistent use of hearing protectors is comfort, or lack thereof, while they are worn. A review article by Davis  outlines the existing research on hearing protector comfort and discusses some recent laboratory and field studies that examine issues related to hearing protector comfort. Studies included in this review suggest that hearing protector comfort may be reliably and validly quantified on psychophysical scales, and workers can consistently rate hearing protector comfort on multiple psychological scales. Additionally, hearing protector wearers can effectively rank-order hearing protectors on comfort, ease of use, and desirability.
Comfort is a subjective perception that is influenced by a number of physiological and psychological factors. For example, in an earlier review paper by Berger and Mitchell,  the authors found that there is no clear relationship between earmuff cushion pressure and perceived comfort. It would be very helpful if hearing protector manufacturers provided a comfort rating in addition to the noise reduction rating for each device. A universally accepted hearing protector comfort rating system would be beneficial for comparing different styles/types and would assist with selecting appropriate protection.  Perhaps the most important aspect of comfort measurement is the potential for relating comfort to specific engineering design parameters of hearing protection devices (HPDs).  Comfort ratings could help designers specify more comfortable design parameters according to pre-established criteria.
One could hypothesize that there would be a higher level of motivation and, therefore, greater compliance when the use of hearing protection is required if HPD comfort could be better assessed and described. Comfort levels of two earplugs along with their corresponding effective attenuation values are the primary focus of the present research. The Comfort Index Rating Method was used in this study. , The purpose of this effort was to determine whether any relationship existed between objective earplug fit-test results and subjective comfort ratings for individuals wearing two different types of earplugs.
| Methods|| |
A convenience sample of 23 subjects (seven male, 16 female) ranging in age from 23 to 50 years was used for this study. Participants were provided with the required assurances of confidentiality and an informed consent form was signed prior to performing any screening/testing procedures. The test protocol was approved by the University of Cincinnati Human Subjects Review Board (IRB #08-12-18-01) and subsequent data analysis was approved by the NIOSH Human Subjects Review Board (approval code: HSRB 09-DART-NR04).
Subjects were not regular hearing protector users, reporting that they had used earplugs/earmuffs on fewer than six occasions in the past 6 months. Subjects were excluded if they were routinely exposed to occupational noise. Subjects were dismissed if the eardrum could not be visualized by otoscopy or if there was excessive cerumen build-up in the earcanal. All subjects had hearing in the normal range (<25 dB HL) from 125 Hz through 8000 Hz, as confirmed by pure-tone air-conduction threshold testing using a Grason-Stadler GSI 61 audiometer with supra-aural earphones.
Earplug fit-testing instrumentation
The commercially available FitCheck system (Michael and Associates Inc., State College, PA, USA) was used to assess hearing protector attenuation. Testing was conducted in a double-walled acoustic examination room that met the maximum permissible ambient noise levels for ears covered testing from 125 to 8000 Hz according to ANSI S3.1.  No extraneous sounds were audible under the FitCheck headset (a pair of large circumaural earphones) with its electronics on and no test signal present.
Instructions were read aloud by the investigator and afterwards the subjects donned the FitCheck headset. Stimuli included pulsed, one-third octave-bands of noise centered at 125, 250, 500, 1000, 2000, 3150, 4000, 6300, and 8000 Hz. Subjects interfaced with the FitCheck system through a conventional audiometer subject response button. Hearing protector attenuation values were obtained by determining the real-ear attenuation measured at the individual's hearing threshold using an automatic threshold tracking paradigm. During the test, the subject depressed the response switch until the stimulus became inaudible. Once the stimulus was no longer heard, the subject was instructed to release the switch, which caused the signal level to increase again (a reversal). The FitCheck system established the subject's threshold from the average of all minimum and maximum reversals, excluding the first reversal, and then the test automatically switched to the next stimulus frequency. Subjects were able to practice taking the test by using FitCheck's built-in demonstration function. This consisted of an abbreviated test using three test frequencies, the results of which were not used in the final threshold determination.
Hearing protector attenuation values were obtained by a subject taking two threshold tests - one without a hearing protector and one with the hearing protector. At each frequency, the difference between thresholds on the two tests indicated how much attenuation was provided by the HPD being examined.
A minor modification of the Comfort Index developed by Casali, Lam, and Epps  was used in this study. Subjects were asked to identify their perceived comfort of the protector based on a five-point evaluation scale for 14 bipolar word pairs (the original index used a seven-point scale; see [Figure 1]. The word pairs consisted of adjectives such as "painless" and "painful" or "comfortable" and "uncomfortable." Word pairs were randomly varied such that the more desirable words were not always aligned on one side of the page.
|Figure 1: Questionnaire administered to subjects, used to compute the Comfort Index for each earplug type (Adapted from Casali et al. with modifications)|
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The survey was scored by assigning a value of 1 to the most comfortable item in each pair and a value of 5 to the opposite or most uncomfortable descriptor. The Comfort Index was obtained by summing the point values for all 14 word pairs. Comfort Index scores can vary between 14 (most comfortable) to 70 (least comfortable).
Two commercially available styles of earplugs were chosen for this study. The E•A•R® Classic® roll-down foam earplug ( http://www.aearo.com/ ) was used as it is one of the most commonly used and studied hearing protectors in the United States. Joe's Ear Plugs® ( http://www.joesearplugs.org/ ) were selected because they represent a completely different type of insert protector. These earplugs are cylindrically shaped with rounded edges (measuring approximately 20 mm in diameter by 9 mm high) and are easily molded by hand. The manufacturer describes these devices as made primarily from pure beeswax, lanolin, and soft cotton. Joe's Ear Plugs® are inserted by working the earplug between your fingers for 10-15 s to soften the material, shaping the plug into a ball, placing it over the ear canal opening, and flattening to form a complete seal. [Table 1] contains a listing of the published attenuation data for each earplug.
|Table 1: Mean attenuation data (in dB) for each earplug used in this study (according to the package label)|
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Following the screening/qualification procedures, subjects identified whether they were right- or left-handed. The ear that received the specified brand of earplug was alternated; half of the right-handed and half of the left-handed subjects placed an E•A•R® Classic® earplug in the left ear and a Joe's Ear Plug® in the right ear. The other half of the subjects had the opposite ear/earplug condition. Subjects inserted the earplugs according to instructions provided by the manufacturer, which were either printed on each packet of the earplugs or provided on a separate sheet of paper. Subjects were permitted to refer to the instructions as often as necessary; however, they were required to insert the plugs themselves, with no additional guidance or assistance from the investigators. This was intended to mimic a real-world scenario where the earplug wearers are inexperienced.
Each subject wore both types of earplugs - uninterrupted and untouched - for a duration of 1 h. They were told not to chew gum, but could participate in conversation while waiting for the hour to pass. When the hour of wear was completed, all subjects completed two copies of the questionnaire - one for each earplug [Figure 1].
Immediately after the questionnaires were completed, separate earplug attenuation tests were conducted for each ear with the FitCheck system. First, occluded thresholds were obtained for one ear and then the examiner removed the FitCheck circumaural headphones and instructed the subject to remove the earplug from that ear. Threshold testing was then repeated with the ear unoccluded. Finally, the occluded and unoccluded thresholds were obtained for the other ear.
The effect of comfort and other variables on attenuation was modeled with a series of linear mixed-effects models. The independent variables of interest were comfort (sum of comfort variables), type of HPD, and natural logarithm of frequency. Log of the frequency was used following the procedure described in previous research. 
| Results and Discussion|| |
Comfort index scores
Comfort index scores could vary from 14 (most comfortable) to 70 (least comfortable), with a middle score of 42 (if the subject answered all "3's" on the five-point scale). When tabulating the scores, six of the items (nos. 2, 4, 5, 7, 8, and 12) were assigned increasing values from 1 to 5, looking at the check-boxes from left to right [Figure 1]. The remainder of the items (nos. 1, 3, 6, 9, 10, 11, 13, and 14) were scored the opposite way, i.e. assigned decreasing values from 5 to 1 across the page from left to right. This was done to counteract the randomization of the questionnaire items and to assign the lowest score to the word more likely to be associated with the term "comfortable." A paired t-test was used to determine whether the type of earplug affected the total comfort score. The mean of the differences between the two plug types was judged not significantly different from zero (P = 0.375).
Overall comfort index results for both earplug types are contained in [Figure 2]. Box-and-whisker plots are used to provide a better visualization of the data distribution. The lower boundary of each box represents the 25 th percentile and the upper boundary represents the 75 th percentile. The vertical length of the box represents the interquartile range, which means that 50% of all data points are within the box. The horizontal line inside the box indicates the median value. Vertical lines (a.k.a. whiskers) are drawn from the edges of the box to the largest and smallest values falling outside of the box that are within 1.5 box lengths. Outliers are plotted as individual points on the graph.
The difference between the two sets of hearing threshold measurements, i.e. the attenuation provided by the earplugs, was calculated and is presented graphically in [Figure 3] and [Figure 4]. As described above, boxplots are used to give a better visualization of the data distribution.
Starting with a full model (with all independent variables and all possible interactions of score, log frequency, log 2 frequency, and type of HPD), a series of model reductions was performed through a series of likelihood ratio tests. One further step was to determine if nesting type of HPD within subject improved model fit. Modeling type of HPD as being nested within subject provided a substantial improvement in fit. Further justification for modeling type of earplug within subject can be seen by observing the graph in [Figure 5]. This graph indicates that levels of attenuation tend to be more similar for the same type of HPD for a given individual (i.e., each subject tended to fit his/her ear plugs with about the same amount of precision). Diagnostic plots showed that the final model did not violate assumptions of normality of error terms and independence with respect to fitted values of attenuation.
|Figure 5: Attenuations by subject and type of hearing protection device (HPD). Note that for a given subject, results from the same type of HPD tend to be more similar|
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In order to determine if use of the preferred hand for insertion of the earplug had an effect, the variable "preferred side" was set equal to zero if the earplug in question was in the right ear of a left-handed person or the left ear of a right-handed person. The variable was set equal to one if the earplug was in the right ear of a right-handed person or in the left ear of a left-handed person. The variable for preferred side was added to the reduced model described above.
Analyses were performed in Stata (version 10.1) using xtmixed. An increase in comfort score (i.e., less overall comfort) was associated with an increase in attenuation. The log of the frequency as well as the log squared and the log cubed were also significant. In terms of a formula, this can be expressed as:
attenuation ijk= β0+β1 *score ij+β2 *log(freq)+β3 *(log(freq)) 2 +β4 *(log(freq)) 3 +u i +u ij +εijk
Where, i = subject number, j = 1 or 2 (type of HPD), k = 1,…,9 (frequency category), u i = random effect due to subject i, u j = random effect due to type of HPD for subject i, and εijk = error term.
[Table 2] contains estimates of the coefficients for the fixed effects of the final model as well as estimates of the variances of the random effects. The fact that the coefficient for score is positive suggests that an increase in attenuation is linearly related to a decrease in comfort. Using these results, we can estimate the attenuation as a function of comfort score and log frequency. Estimated attenuations as a function of log frequency are shown in [Figure 6], where each line represents the estimates for an individual subject. In the plots in [Figure 6], random effects were assumed equal to zero (the mean of the random effects). The impact of the fixed effects of comfort score and frequency on attenuation is shown in [Figure 7]. Using the estimates of the coefficients presented in [Table 2], the overall model is: attenuation = 637.7066 + 0.1934*score - 290.0064*log(frequency) + 43.2580*(log(frequency)) 2 - 2.0635*(log(frequency)) 3 . The graph presents a line for each frequency. The slope of each line is the same (0.1934) and the intercept is 637.7066 - 290.0064*log(frequency) + 43.2580*(log(frequency)) 2 - 2.0635*(log(frequency)) 3; thus, producing a series of parallel lines.
|Figure 6: Estimated attenuation as a function of log frequency for each individual. Differences between individuals are due to the effect of the comfort score|
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|Figure 7: Effect of comfort score and frequency on attenuation (see text and Table 2). Note that an increase in the Comfort Index (a less-comfortable rating) implies an increase in attenuation|
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|Table 2: Maximum likelihood estimates of parameters for the fixed effects and of the variances for the random effects of the final model|
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Only the written instructions provided by the earplug manufacturers were used in this study. Sixteen of the 23 participants requested further clarification; however, the investigators intentionally did not give any guidance or assistance to the subjects as they inserted their earplugs. It is quite possible that the comfort level ratings could have been affected by the improper or incomplete insertion of the HPDs. Previous research with slow-recovery foam earplugs found that untrained subjects obtained less attenuation but higher comfort scores, while trained subjects obtained more attenuation and lower comfort scores.  To thoroughly assess a particular HPD in future studies, comfort ratings should be obtained from subjects that have received training and demonstrated proficiency in using the device.
Another limitation is the amount of time the participants wore the earplugs before assessing their comfort. Typically, industrial workers would be wearing HPDs for extended periods of time throughout the workday (e.g., 8-10 h) instead of only 1 h, as was done for this study. Prior research with foam earplugs revealed slightly lower comfort scores after a 2-h wearing period.  It is unknown whether wearing the wax-type HPDs for longer than the 1-h duration would affect the comfort ratings. Future investigations should administer the comfort questionnaire and conduct earplug attenuation measurements after participants wear these HPDs for a longer period of time.
It should also be noted that the two earplugs used in this study represent only two examples of a large assortment of available HPDs, and were not representative of HPDs in general. Future studies are needed to determine how other earplugs, earmuffs, and concha-seated devices yield comfort versus attenuation predictions. Additionally, when completing the comfort questionnaires, it was impossible to determine whether the participants were truly rating the given earplug independently of the other plug, as they were instructed. Finally, each subject filled out the same version of the Comfort Index questionnaire sequentially for each hearing protector, for which there was no control in the study design.
| Conclusions|| |
Subjects were administered questionnaires that were comprised of 14 items, which assessed the perceived comfort of two different types of earplugs. Individual fit-testing of each subject indicated that different amounts of attenuation were obtained at different frequencies. Overall, the results suggest that comfort and attenuation are inversely related for the naïve subjects (and the two types of earplugs) evaluated in this study. The present study found no statistically significant difference in comfort between a foam roll-down earplug and a formable wax earplug. In addition, we did not observe a statistically significant difference in attenuation between the two devices. As anticipated, the subject's ability to correctly seat the earplug was the source of much of the variability noted both in attenuation and in comfort.
There is evidence to suggest that comfort is an important consideration regarding HPD use for noise-exposed workers.  Thirty years ago, Nilsson and Lindgren concluded that attenuation values of hearing protectors are of secondary importance, with comfort of the protector being of primary importance.  However, most discussions of hearing protection still seem to focus on issues related to noise attenuation, with less attention directed toward comfort. A few different approaches to assessing hearing protector comfort have been proposed, but none have gained widespread acceptance. ,,, A standardized, valid, reliable, and easy-to-use multi-dimensional comfort assessment tool such as the one used in this study would benefit the hearing conservation community. Data provided by studies similar to this one could then be used to evaluate important aspects of hearing protector design/manufacture, in addition to proper fit/usage, which may be currently overlooked.
| Acknowledgments|| |
This project was the Capstone project for BMS and ANH for their Doctor of Audiology degrees. We would like to thank John G. Clark, Ph.D. and Peter M. Scheifele, Ph.D. for serving as advisors on this project. Funding for supplies was provided by the U.S. Environmental Protection Agency through a cooperative agreement with the NIOSH.
| References|| |
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David C Byrne
NIOSH - Taft Laboratories, 4676 Columbia Parkway, Cincinnati, OH 45226
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]
[Table 1], [Table 2]