Introduction: This study aimed to analyze and compare four different types of earplugs, divided into premolded plugs and foam plug models, in relation to the level of attenuation, comfort, and the size of the external acoustic meatus (EAM) in an attempt to identify how these variables influence the choice of specific hearing protection devices (HPDs). Materials and Methods: A cross-sectional observational study was performed in a sample of 49 participants, oriented toward the ideal placement of four HPDs, two premolded and two foam plugs (3M™). The procedures included otoscopy, EAM diameter measurement using an otometer, EAM volume measurement via an acoustic impedance test, and the obtainment of the bilateral personal attenuation rating (PAR) for each HPD using the E-A-Rfit™ Validation System (3M™). The Bipolar Comfort Rating Scale (BCRS) instrument was applied twice for each individual: once after the evaluations with the premolded HPDs and again after the evaluations with the foam plug HPDs. Then, each participant was asked which was his/her favorite protector. Results: The volume of the EAM was not directly related to the diameter of the EAM. The attenuation did not interfere with the HPD preference, and the PAR of the foam plug was significantly higher regardless of the preferred HPD. Regarding the BCRS, the variables “Placement,” “Complexity,” and “Occlusion Effect” had higher scores for premolded HPDs and had a direct relationship with the type of preferred HPD. Conclusion: Attention to the use of HPDs should be personalized, taking into account the needs of each individual, considering not only the attenuation, but also the user’s reported well-being.
Keywords: Attenuation, comfort, earplugs, fit test, hearing protection devices
|How to cite this article:|
Samelli AG, Gomes RF, Chammas TV, Silva BG, Moreira RR, Fiorini AC. The study of attenuation levels and the comfort of earplugs. Noise Health 2018;20:112-9
| Introduction|| |
The actions aimed at preserving the hearing of noise exposed workers are mostly based on the performance of audiometry tests and the provision of a hearing protection device (HPD), also called a hearing protector. However, it is important that some variables associated with HPDs are considered, namely, size, placement, actual attenuation, hygiene, and storage.
In addition, the functioning of the HPD depends on not only its intrinsic characteristics but also the physiological and anatomic characteristics of the user, acceptance (comfort), the motivation of the worker to use the HPD, correct placement, the duration of use,,, and working environment (noise level, activity, and environmental conditions).,,
Recent studies clarify that the possible discomfort caused by an HPD is one of the impediments to the continued use of the device.,,,, For example, premolded HPDs may give rise to complaints regarding discomfort, as some people have an irregular external acoustic meatus (EAM), making proper fixation difficult.,
In addition to comfort, it is of utmost importance that the HPD selected, whatever the type, only attenuate what is necessary, avoiding the consequences of overprotection, such as interference with communication, localization and, especially, the perception of alarm signals.,,,
According to Gerges, the noise reduction rating (NRR) reported by the manufacturers of the different types of earplugs on the market are approximately 25 dB. However, the actual attenuation of these devices on a day-to-day basis is influenced by certain conditions, such as the quality of the material used, the working environment, and the placement.,,
Considering the difficulties of acceptance and the various factors that interfere with the proper use of HPDs, this study aimed to analyze and compare four different types of HPDs, divided into premolded plugs and foam plug models, regarding the level of attenuation, comfort, and the size of the EAM, seeking to identify how these variables influence the choice of a specific HPD.
This study is justified by the importance of the attenuation, comfort, and anatomic variables in the selection and use of HPDs and by the need to perform this process in a personalized manner.
| Materials and Methods|| |
This observational and cross-sectional field study was approved by the Research Ethics Committee of the University Hospital (Hospital Universitário − HU) of the University of São Paulo (Universidade de São Paulo − USP) under No. 858/08.
Procedures and measures
A total of 49 individuals of both genders aged over 18 years were evaluated.
The participants signed an informed consent form containing information about the study and the procedures to which they were submitted.
The inclusion criteria of this study were age above 18 years, not having prior knowledge of HPDs, the absence of any impediment in the EAM (i.e., cerumen or foreign body), and/or the alteration of the middle ear.
After signing the informed consent form, all the participants were submitted to otoscopy to identify possible impediments to other procedures, such as excess wax, alterations in the EAM, and/or alterations in the tympanic membrane. When any alteration was observed, the participant was referred for otorhinolaryngological evaluation.
The acoustic impedance test was then performed using an Otoflex 100 Otoflex 100 (GN Otometrics; Tasstrup, Denmark) device to evaluate the mobility of the tympanic–ossicle system and to obtain the EAM volumes from both the ears.
The next step was to perform otometry with the objective of analyzing the ear canal size (XS, S, M, L, or XL) of both the ears of each participant by means of an otometer. This procedure consisted of the insertion of small rods into the EAM according to the sizes described above and provided one of the measurements of the EAM diameter for each of the evaluated ears.
To obtain the personal attenuation rating (PAR), which represents the decibel value of attenuation achieved, the 3M™ E-A-Rfit™ Validation System (Saint Paul, Minnesota) was used, which consists of a speaker, a microphone, with two capture inputs and the 3M.18.104.22.168 (Saint Paul, Minnesota).
The speaker emits a white noise, whose level is automatically calculated and calibrated by the device. The PAR is obtained based on the noise difference that is picked up by the external microphone (positioned outside the EAM) and the internal microphone (positioned inside the EAM).
To perform the tests with the E-A-Rfit™ and to obtain the PAR, the participant was instructed to sit at 0° azimuth, approximately 30 cm from the speaker, which was positioned at face height and facing the participant. Then, the use of protective goggles was requested, which was a part of the equipment, with the single purpose of supporting the microphone mentioned above. Finally, the participants were advised not to move their heads during the tests.
All responses were recorded by the software itself. After measurements were taken in both the ears, the equipment calculated both the uni- and bilateral PAR for each of the HPDs.
For this research, the following four models of insertion HPD of the brand 3M™ were selected: Pomp Plus and Express (grouped as premolded HPDs) and 1100 and Classic (grouped as foam plug HPDs).
The premolded HPD material consists of silicone and plastic, while the foam plug HPD consists of foam. All HPDs for use with the E-A-Rfit™ come with an adapter to connect the microphone of the equipment without losing its characteristics.
To initiate the evaluation with the mentioned HPDs, the participants received all the information regarding the characteristics and appropriate ways for the correct placement of the protectors. The evaluators clarified all the participants’ doubts and then performed the tests to obtain the PARs. If the software found an error regarding the inadequate placement of an HPD, the evaluator provided new training for its insertion. These procedures were repeated for each of the four devices.
After performing the tests with the E-A-Rfit™, the Bipolar Comfort Rating Scale (BCRS) instrument proposed by Casali et al. and translated into Portuguese by Arezes was applied. In the BCRS, the participant was instructed to assign a score between 1 and 7 for each of the following items: pain, comfort, pressure, tolerance, tightness, convenience, weight, embarrassment, stiffness, heat, roughness, a feeling of isolation, placement, complexity, difficulty in moving the head, and a blocked ear sensation. The greater the value assigned to the item, the greater the satisfaction of the individual.
The BCRS was applied twice to each of the participants in the study, once after the evaluations with the premolded HPDs and once after the evaluations with the self-adjusting HPDs. For each participant, the order of the evaluation of the HPD types was alternated to avoid bias.
After completing the BCRS, each participant was asked which was their favorite protector, that is, the one they would choose to use on a day-to-day basis. The responses were registered for further analysis.
Descriptive analyses and the following statistical tests were performed: F-test of the fixed factor analysis of variance (ANOVA) model (the assumptions of normality and homoscedasticity raised for the test application were satisfied); F-test of the two-factors repeated measures ANOVA model; chi-square test of independence and calculation of odds ratios; logistic regression model; and Cronbach’s alpha statistics.
Initially, in the otometry analysis, the two ears evaluated for each individual were grouped according to the diameter of the meatus, constituting three groups (XS + S, M, and L + XL) due to the low number of occurrences in the XS and XL categories. Then, the means of the external auditory canal volumes were compared between the categories.
As for the PARs, the binaural values (given by the software) were chosen for the analysis. Because two models were used for each type of HPD, the lowest PAR values per HPD type were selected as references to compare them with each other.
After grouping individuals regarding preference for each of the types of HPD, the PARs were compared according to the type of HPD evaluated. In addition, the BCRS variables were analyzed between them. To do so, for the score given for each BCRS variable, which can range from 1 to 7, the assigned scores were categorized into “Best” (6–7), “Neutral” (3–4–5), and “Worst” (1–2).
The internal consistency analysis of the BCRS was evaluated using the Cronbach’s alpha statistics. The value of this test should be positive, ranging from 0 to 1, with ratings of Very Good, Good, Fair, Weak, and Inadmissible. For the two HPDs, the internal consistency was classified as Good (premolded with alpha = 0.8971 and foam plug with alpha = 0.8525).
To select the variables that were most associated with the preferred HPD type, a logistic regression model was fitted (“Preferred HPD Type” as the response variable and “Placement,” “Complexity,” and “Occlusion Effect” as explanatory variables). For this analysis, only the variables that obtained the values of P < 10% in the previous analysis were used. The method for the selection of the variables used was the backward method.
In addition, the odds ratios and respective confidence intervals (CIs) with 95% confidence coefficients were calculated.
To verify the quality of fit of the logistic regression model, the C-statistic of the Hosmer and Lemeshow test (2000) was calculated. The value of C was 0.62. The P-value associated with the C-statistic was 0.960 (>5%). This value indicates that the logistic regression model is well adjusted.
| Results|| |
Comparison between EAM volume and Otometry
In [Table 1] and [Table 2], some descriptive measures are listed for the variable “EAM Volume” by the category of the variable “Otometry.”
|Table 1: Distribution of the measurements of the EAM volume variable by the category of the variable otometry in the right ear|
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|Table 2: Distribution of the measurements of the EAM volume variable by the category of the variable otometry in the left ear|
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The P-values for the analyses of the right and left ears were 0.064 and 0.072, respectively. Therefore, there was no statistically significant difference between the means of the variable “EAM Volume” (P-value > 5%; F-test of the fixed factor ANOVA model).
Comparison between PARs and HPD types (foam plugs and premolded plugs)
[Table 3] presents some descriptive measures for the variable PAR by the type of preferred HPD and by the type of HPD evaluated.
|Table 3: Distribution of PAR variable measures by the type of preferred HPD and HPD evaluated|
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For those who named premolded HPDs as their preferred HPD, the HPD rated with the lowest PAR mean was premolded. The same occurred for those who named foam plug HPDs as their preferred HPD, that is, the protector evaluated with the lower PAR was also the premolded for these participants. The statistical analysis showed a P-value = 0.339, confirming that the behavior of the PAR means for the HPDs evaluated did not present a statistically significant change when the two types of preferred protectors (P-value > 5%) were analyzed. This finding was also verified if the mean PAR values differed between the preferred HPDs and no statistically significant difference was observed (P-value = 0.441 at the significance level of 5%). Regardless of the HPD evaluated, the mean PAR values did not differ statistically between the two groups.
Finally, whether the mean PAR values presented a statistically significant difference between the HPDs evaluated was analyzed. The mean values differed between the HPDs evaluated (P-value < 0.001 at the significance level of 5%) and were higher for the foam plug models, regardless of the preferred HPD.
Analyses of the Bipolar Comfort Rating Scale
[Table 4] shows the joint frequency distribution between the variable “Preferred HPD Type” and the BCRS variables. At the 10% level of significance, there is evidence regarding associations between the variable “Preferred HPD Type” and the following variables.
- Placement (P-value = 0.009): the proportion of individuals who chose the “Best” category was greater among those who preferred premolded HPDs (80.65%) than among those who preferred foam plug HPDs (38.89%). On the other hand, the proportion of individuals who chose the “Worst” and “Neutral” categories was higher among those who preferred foam plug HPDs (61.11%) than among those who preferred premolded HPDs (19.36%).
- Complexity (P-value = 0.016): the proportion of individuals who chose the “Best” category was higher among those who preferred premolded HPDs (90.32%) than among those who preferred foam plug HPDs (55.56%). On the other hand, the proportion of individuals who chose the “Worst” and “Neutral” categories was higher among those who preferred the foam plug HPDs (44.44%) than among those who preferred the premolded HPDs (9.68%).
- Occlusion effect (P-value = 0.083): the proportion of individuals who chose the “Best” and “Neutral” categories was higher among those who preferred the premolded HPDs (96.77%) than among those who preferred the foam plug HPDs (77.78%). On the other hand, the proportion of individuals who chose the “Worst” category was higher among those who preferred the foam plug HPDs (22.22%) than among those who preferred the premolded HPDs (3.23%).
|Table 4: Joint frequency distributions between the variable “Preferred HPD Type” and the BCRS variables|
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To select the variables that were most associated with the variable “Preferred HPD Type,” a logistic regression model was fitted with this variable as the response variable and the variables “Placement,” “Complexity,” and “Occlusion Effect” as explanatory variables (these three variables presented P-values < 10% associated with the chi-square test). The variables “Placement” and “Occlusion Effect” were most associated with the “Preferred HPD Type” variable. It is important to note that the variable “Complexity” was not selected because it was closely correlated with the variable “Placement” (P-value < 0.001).
[Table 5] presents odds ratios and respective CIs. According to the table, for the variable “Placement,” premolded HPDs were 8.81 times more likely to obtain a “Best” response than a “Neutral” response compared with foam plug HPDs; 22.84 times more likely to obtain a “Better” response than a “Worse” response compared with foam plug HPDs; and 22.84 times more likely to obtain a “Best” response than a “Worst” response compared with foam plug HPDs. There was no difference between the HPDs regarding the odds of obtaining a “Worst” response compared to a “Neutral” response. For the “Occlusion Effect” variable, premolded HPDs were 14.47 times more likely to obtain a “Best” response than a “Worst” response compared with foam plug HPDs and 36.06 times more likely to obtain a “Neutral” response than a “Worst” response compared with foam plug HPDs. There was no difference between the HPDs regarding the odds of obtaining a “Best” response compared to a “Neutral” response.
|Table 5: Odds ratios and 95% confidence intervals for odds ratios relative to logistic regression parameters|
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| Discussion|| |
The HPDs have changed considerably in the last decades with respect to durability, cost, esthetics, communication, comfort, and technology to achieve the best attenuation. However, the greater attenuation ends up being prioritized, to the detriment of comfort.
When workers were asked why they did not use HPDs in environments where they were necessary, the following two reasons were commonly presented: difficulty in communicating and discomfort.,
In 2003, the National Institute for Occupational Safety and Health conducted a survey to analyze the physical factors regarding the comfort of HPDs. One of the factors evaluated was the pressure exerted by the insertion of HPDs in the EAM wall. As a result, they found that foam plug HPDs exert the same pressure on any diameter of EAM. In turn, the premolded models seemed to be more suitable for EAMs with larger diameters. No HPDs used in this study were found to be appropriate for small EAMs, such as those of women and children.
Yu et al. conducted an extensive study evaluating the EAMs of Taiwanese workers by means of image examination. The opening of the EAM, the length up to the first curve, the length up to the second curve, and the total length of the EAM were measured. They found differences in the measurements of men and women and suggested that manufacturers should change the design of insertion HPDs. Another study, also with Taiwanese adults, used imaging to obtain the measurements they deemed necessary for adaptation to HPD, such as the ear shell length, width, and depth; the ear opening length and width; the canal isthmus; and the lengths of the ear opening to the first and second curves. Differences were also found between men and women, suggesting that these measurements should be considered in the manufacture of HPDs.
Thus, considering the importance of the EAM anatomy in the use of insertion HPDs, this study compared the participants’ EAM volume with the otometry results and found that the volume was not directly related to otometry, possibly because a narrow, small diameter, which would be considered an EAM with S otometry, can be deep and have a larger volume. The same is true for an EAM with M or L otometry, which may have a larger diameter and be shorter. Therefore, the otometer cannot be the only tool to select the HPD to be used, but rather a complementary procedure in the selection process.
These anatomical issues are important not only for the comfort of the HPD, but also for it to achieve the necessary attenuation and effectively protect the worker exposed to noise. The ability of the user to place the HPD correctly, thus achieving the necessary attenuation, is fundamental for effective protection and depends intrinsically on the user’s manual dexterity, the EAM’s morphology, and the HPD’s shape., Thus, the influence of the ear anatomy on the correct use of HPDs cannot be overlooked and should be taken into account during the selection of the most adequate HPD for the user.
In addition, it is critical that the HPD selected, regardless of type, does not cause overprotection, generating difficulties in communication, localization, and, especially, the perception of alarm signals.,,,
It is known that the NRR reported by the manufacturers of the different types of HPDs on the market are approximately 25 dB. However, the real attenuation of these devices on a day-to-day basis is influenced by certain conditions, such as the quality of the material used, the working environment, and the HPD’s placement.,,
The discrepancy between the performance of HPDs in the laboratory and the workplace is evident. Berger compared the results of laboratory and field HPD tests and found no correspondence, because the field attenuation was well below the laboratory values. Even when using laboratory test protocols with untrained participants, the results generally exceeded the values of field tests. Although several methods have been developed to express HPD performance as a single attenuation value, none of these methods have been shown to be effective in predicting real-world performance.
These findings highlight the importance of determining the performance of the HPD for each user. One method to accomplish this objective is by performing individual assessment and generating the user’s PAR. There is no standardized test for the PAR to be obtained, because there are several methods that can be used.
In this study, the equipment used to obtain each user’s PAR was the E-A-Rfit™. The PAR values were compared by the types of HPD (foam plugs and premolded plugs). The results indicated that the attenuation did not interfere with HPD preference, disagreeing with other studies in the literature.,
Byrne et al. evaluated 23 participants regarding the comfort and attenuation of two insertion HPDs, and their results suggested that the variables were inversely proportional, that is, the smaller the PAR of the protector is, the higher the preference for it in the questionnaire regarding comfort. In contrast, in the study by Arezes and Miguel, the data suggested that the insertion HPD with the best result in the comfort questionnaire was the one that reached greater attenuation.
Nilsson and Lindgren argued that attenuation is of secondary importance because comfort determines consistency in the use of HPDs, which is the first aspect to be considered in choosing the protector. Davis noted that the most effective HPD is the one that is used consistently and correctly. Thus, we can say that our findings agree with the statement of Nilsson and Lingrend because, in this study, the attenuation values did not interfere with the HPD preference.
When the BCRS analysis was performed, it was observed that the placement of the premolded HPD was selected as the easiest, and that the complexity of this HPD was considered the lowest. Schulz et al. evaluated the comfort levels of five types of HPDs, including a premolded and a foam plug HPD, through a questionnaire with seven questions. The premolded HPD was considered the one with the least need for repositioning and the easiest to use. Park and Casali observed, as in this study, that the foam plug was considered the most difficult to place. Toivonen et al. evaluated the efficacy of training for the placement of an S- or L-size foam plug HPD and observed that, although the training was effective, some participants continued to have difficulty achieving an appropriate insertion of the HPD.
When using an HPD or any other insertion device in the canal, the participant can perceive a difference in her/his own voice and can also listen to her/his own body sounds, such as heart beats and breathing, because an amplification of the lower frequencies occurs., This amplification is called the occlusion effect. Although this effect is not a direct threat to hearing, it should be noted, because people may become more resistant to HPD use because they are bothered by these amplified sounds.
In the BCRS, the occlusion effect, when comparing the two types of HPDs, was lower in the premolded model than in the foam plug one. Lee and Casali evaluated the occlusion effects of two earplugs, where one of them was a foam plug HPD and the other was a balloon-based prototype. The insertion depth was the same for the two HPDs, and the occlusion effect was higher with the foam plug HPD, as in this study.
The analysis of odds ratios performed by the authors of this study for the variables “Placement” and “Occlusion Effect” corroborates the studies cited previously,,, that is, the placement and the occlusion effect interfere in the choice of HPD and are usually more unfavorable with foam plug HPDs.
Looking at the data from the literature and the present results, the importance of considering the comfort, the characteristics of the HPD, and the anatomy of the user when selecting these devices is evident. For comfort, placement, complexity, and occlusion effect were relevant in this study as variables that most influenced the preference of the protector, while the attenuation levels did not interfere in the choice.
The fact that users in this study used each of the HPDs for a short period was a limitation that may have influenced the choice of preferred protector and the perception of comfort. However, previous studies have suggested that the tests of HPD used for short periods may predict the results obtained in tests performed for longer periods. Future studies evaluating aspects related to comfort should allow users to use HPDs for longer periods before responding to such questions.,
It should be noted, in addition to the variables studied (EAM, attenuation, and comfort), that other factors may interfere with the use of HPDs, although they have not been investigated in this study. Among them, the importance of user risk perception and self-perception about auditory status, along with interpersonal influences (standards, institutional support, and peer influence), stand out.,,
| Conclusion|| |
The attention to the use of HPDs should be personalized, taking into account the needs of each individual. Therefore, the influencing factors to be considered include not only attenuation but also well-being, because both the PAR and the comfort sensation vary greatly for each individual based on the characteristics of each HPD and the anatomy of the user.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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Alessandra G Samelli
Rua Cipotânea, 51, Cidade Universitária, 05360-160 São Paulo, SP
Source of Support: None, Conflict of Interest: None
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5]