//Logo Image

Comfort evaluation of hearing protection

Author: Yeh-Liang Hsu, Chung-Cheng Huang, Chin-Yu Yo, Chiou-Jong Chen, Chun-Ming Lien(2003-09-04); recommended: Yeh-Liang Hsu (2004-04-20).
Note: This paper is published in International Journal of Industrial Ergonomics, Volume 33, Issue 6, June 2004, Pages 543-551.

Abstract

Hearing protection is very important for workers in noisy work environments, although the willingness of workers to wear hearing protectors depends heavily on their comfort. This paper evaluates and recommends improvements for the comfort of hearing protection. Workers’ experience and comfort needs for hearing protection were investigated through a questionnaire that established the “comfort indices” for hearing protection. An earmuff “comfort tester” was designed to measure the comfort indices, and an experiment was conducted to measure workers’ perceived comfort into quantitative data. From the data, the range of these comfort indices in which workers will feel comfortable was determined. Finally, guidelines to improve the design of current hearing protection based on these “comfort indices” are proposed, which may help increase workers’ willingness to wear hearing protection.

Relevance to industry

Using these proposed guidelines may help improve the comfort of hearing protection and increase workers’ willingness to wear hearing protection in noisy work environments.

Key words: Hearing protection, earmuff, comfort indices.

1.     Introduction

The influence of noise on humans can be divided into psychological and physiological aspects. Psychologically, sudden loud noise can easily cause shocks. Physiologically, when a person is exposed to noisy surroundings for a period of time, the hearing threshold may be permanently affected. Ahmed et al. (2001) pointed out that gross occupational exposure to noise has been demonstrated to cause hearing loss.

Noise can be controlled by blocking the noise at the source, along its path from the source to the receiver, and at the end of the receiver. Lowering the noise at the source is the most effective way to noise control. An alternative would be blocking the path of noise. Finally, wearing personal hearing protection is the last passive means of self-protection. However, it is often the most cost-effective method of individual exposure control (Sanders and McCormick, 1987; Patton, 2000).

Noise-induced hearing loss can be prevented by avoiding excessive exposure to noise and using hearing protection such as earplugs and earmuffs (Phoon and Lee, 1993; Rabinowitz, 2000). Paakkonen, et al. (2000) found that the attenuation (insertion loss) was 16 to 23 dB for earplugs, 10 to 20 dB for earmuffs, and 24 to 34 dB for the combined use of plugs and muffs.

However, due to the discomfort of wearing hearing protection, workers in noisy work environments often do not wear them. Sergio and Miguel’s (1996) research project on the use of personal hearing protection concluded that education and motivation of the employees, as well as hearing protection comfort, encourage their correct use. Parmentier et al. (2000) used an artificial head made of hard polyurethane for the evaluation of hearing protection, and ISO 1999 has been revised and updated according to the results of this research.

This paper evaluates and provides guidelines to improve the comfort of hearing protection. Workers’ experience and comfort needs for hearing protection were investigated through a questionnaire that established the “comfort indices” for hearing protection. A “comfort tester” for earmuffs also was designed and constructed to measure these indices. Workers’ perceived comfort data was transformed into quantitative data. From this data, ranges for the comfort indices in which workers feel comfortable were developed. Finally, proposed guidelines to improve the design of current hearing protection on these comfort indices is presented. It is hoped that these guidelines will increase the willingness of workers to wear hearing protection.

2.     Finding comfort indices for hearing protection

A questionnaire was designed to identify the critical factors for comfort of hearing protection. The main purpose of the survey was to understand workers’ percetions and actual comfort needs when wearing hearing protection. In particular, the survey aimed at developing “comfort indices” for hearing protection.

There were four parts in the questionnaire. The first part asked for basic information, including gender, age, and whether the respondent had experience wearing earplugs or earmuffs at work. The second part investigated respondent’s work environment, whether it was high decibel, hot and humid, and dusty and dirty. The authors also hoped to find out how long did the worker wear hearing protection at work everyday, and how often did the worker change from normal to high decibel environment everyday. Some of these questions may not be directly related to the comfort of hearing protection. These items were put in the questionnaire by request of Institute of Occupational Safety and Health, Taiwan, to investigate the current situation of high decibel work environments in Taiwan.

The third part of the questionnaire surveyed workers’ comfort needs of wearing hearing protection at work. To construct survey items for this purpose, 10 frequent users of hearing protections were interviewed to collect their descriptions of the uncomfortableness of wearing hearing protection at work. These were then summarized into 14 comfort attributes. The questionnaire asked the respondent to select uncomfortable perception when wearing hearing protection at work from these 14 attributes, plus “no discomfort” and “other”. The questionnaire also asked that, after wearing earplugs or earmuffs for how long would the respondent start feeling uncomfortable.

The fourth part of the questionnaire asked the respondent to evaluate the importance of design factors of earplugs and earmuffs that might influence their comfort. To find out the possible factors, 4 hearing protector manufacturers were interviewed to collect their description of important factors for the design of earplugs and earmuffs. Patents of hearing protection were also searched to collect the keywords of hearing protection patents. The descriptions and keywords were summarized into 7 design factors for earplugs and 9 design factors for earmuff. The questionnaire asked the respondent to evaluate the importance of these factors on the comfort of hearing protection using a 5-point scale (very important /important /regular /less important /not important).

This questionnaire was subsequently refined based on consultations with experts of Institute of Occupational Safety and Health, Taiwan, and a limited pretest by a convenience sample of 44 workers from a local factory.

A comprehensive survey was then conducted in 9 selected factories with various types of noisy work environments. The finalized questionnaire (see Appendix) was presented in a booklet style with a cover letter that explained the purpose of the study and provided instructions for its completion. Forty workers were selected by random sampling in each factory for the survey, and a total of 358 questionnaires were distributed (in one factory only 38 workers took the questionnaire). The total response rate of the survey was 77.9%. Among the respondents, 86.7% were male and 13.3% were female. The largest age group was from 35 to 44, and 94.6% of the respondents were between 25 and 44. Most of the respondents (87.5%) had experience wearing earplugs, and 45.2% of them had experience wearing earmuffs.

A large proportion of the respondents were required to wear hearing protection at work for a long time everyday. Among them, 44.8% had to wear hearing protection for more than 8 hours, and 54.8% had to wear them for more than 6 hours. In addition, the percentage of workers who had to frequently change between normal and high-decibel (over 85 dB) work environment (more than 10 times per day) was 41.6%. As for the work environment, 81.7% of the respondents worked in high decibel (over 85 dB) environments, 53.8% worked in hot and humid work environments, and 35.1% worked in dusty and dirty surroundings.

The third part of the questionnaire asked the respondent to choose uncomfortable perceptions when wearing hearing protection at work from the 14 comfort attributes plus “no discomfort” and “other”. As shown in Table 1, 53.4% of the respondents chose “having difficulties in conversations”, followed by “sense of oppression” (39.4%), “feeling itchy” (33.7%), “feeling dirty” (29.7%), and “feeling stuffy” (25.8%). Only 5.0% of the respondents did not feel any discomfort while wearing hearing protection.

The survey also showed that 48.4% of the respondents felt discomfort after wearing earplugs for 1 hour; and only 25.0% did not have any discomfort after wearing them for over 2 hours. The endurable time of wearing earmuffs was much shorter. 67.0% of the respondents felt uncomfortable after wearing earmuffs for 1 hour, and only 12.0% of them did not feel any discomfort after wearing earmuffs for over 2 hours.

Table 1. Statistics of comfort attributes when wearing hearing protection at work

Rank

Comfort attributes

Headcount

Percentage

1

Having difficulties in conversations

149

53.4

2

Sense of oppression

110

39.4

3

Feeling itchy

94

33.7

4

Feeling dirty

83

29.7

5

Feeling stuffy

72

25.8

6

Easy to slide or fall off

66

23.7

7

Feeling of an object in the ear

63

22.6

8

Tinnitus

53

19.0

9

Poor soundproofing

53

19.0

10

Acoustics change

36

12.9

11

Interfere with work

35

12.5

12

Discomfort due to hard contacting parts

28

10.0

13

Protectors deform easily

22

7.9

14

Headache

19

6.8

15

No discomfort

14

5.0

16

Other

4

1.4

The last part of the questionnaire asked the respondent to evaluate the importance of the design factors on the comfort of hearing protection using a 5-point scale (very important /important /regular /less important /not important). The 5-point scale was converted into (2, 1, 0, -1, -2) to calculate the average score and standard deviation of each factor. As shown in Table 2, the most important factor for earplugs was the soundproofing (with the lowest standard deviation), followed by texture and inflation pressure. As for earmuffs, soundproofing was still considered the most important factor, followed by airtightness, weight, heat-sinking ability, texture, and headband force. Although many patents for earplugs or earmuffs emphasize adjustability or wearing convenience, these two factors were rated less important in the responses to the questionnaire. The respondents also considered sweat-absorbing capacity to be less important for both earplugs and earmuffs.

Table 2. Importance of design factors of earplugs and earmuffs that might influence their comfort

Rank

Earplugs

Earmuffs

Factors

Average

Std.

Factors

Average

Std.

1

Soundproofing

1.70

0.54

Soundproofing

1.65

0.56

2

Texture

1.36

0.74

Airtightness

1.48

0.65

3

Inflation pressure

1.18

0.81

Heat-sinking ability

1.36

0.71

4

Wearing convenience

1.16

0.72

Texture

1.31

0.77

5

Heat-sinking ability

1.08

0.82

Weight

1.30

0.72

6

Sweat absorbing ability

0.93

0.91

Headband force

1.21

0.72

7

Weight

0.85

1.22

Adjustability

1.12

0.69

8

 

 

 

Wearing convenience

1.12

0.75

9

 

 

 

Sweat absorbing ability

1.11

0.85

Using hearing protection is the most important means of individual hearing protection. However, the results of this questionnaire showed that most of the workers found it uncomfortable to wear hearing protectors, which obviously would affect their willingness to wear them. The questionnaire results also showed that airtightness, heat-sinking capacity, weight, and headband force are the primary “comfort indices” for earmuffs.

3.     Design of earmuff “comfort tester”

After determining the related comfort indices, a “comfort tester” for earmuffs was designed and constructed to measure these comfort indices. As shown in Figure 1, this earmuff “comfort tester” can simultaneously measure the airtightness, heat-sinking ability, and headband force of an earmuff.

Figure 1. The earmuff “comfort tester”

The headband force is measured conforming to EN352-1 (1993), using a standard head frame with an s-shape load cell in the center of the tester. When an earmuff is mounted on the two side plates, the load cell can measure the pressure applied by the earmuff. The height of the head frame and the distance between the two side plates can be adjusted for measuring the headband forces for different head sizes. The evaluation of the heat-sinking ability of an earmuff imitates the condition when a worker wears an earmuff. A temperature controller is mounted at the back of one of the side plates to maintain the temperature of the side plate at human body temperature. A thermal couple protruding from the same side plate measures the temperature inside the earmuff when the earmuff is in contact with the side plate at human body temperature. A control box supplies power to the electronic devices, and a single chip microprocessor controls the sequence of the test and collects signals from sensors, which are sent to a computer for display and further analysis.

The evaluation of airtightness is similar to the method of evaluating airtightness of breathing protectors. As shown in Figure 2, the control box also contains two airflow sensors and an air pump. When an earmuff is mounted on the tester, the air pump sends air through one of the side plate into the earmuff. The air circulates back through another hole on the side plates. The amount of airflow is controlled at 1 L/min. in our tester. Air tightness of the earmuff can be evaluated by the ratio between exhale and inhale airflow measured by the two airflow sensors.

Figure 2. The evaluation of airtightness

A total of 28 commercial earmuffs were tested using this comfort tester. Table 3 shows the test results of headband force. Headband forces for small head size (the distance between side plates is 130mm), medium head size (150mm), and large head size (170mm) were measured. The headband forces of 6 earmuffs were above the EN standard for maximum headband force 14N. As expected, the headband forces of the earmuffs tend to be larger for the large head size, and the headband adjustability of some earmuffs was limited, which resulted in high headband forces for the large head size.

Table 3. Test results of headband forces of earmuffs

Headband force

Small head size

Medium head size

Large head size

Maximum (N)

14.4

14.5

16.9

Minimum (N)

3.3

3.9

4.5

Average (N)

8.9

9.5

10.6

Standard deviation (N)

2.8

2.9

3.3

In the heat-sinking ability test, temperatures inside the earmuffs varied from 30°C to 35°C. It was observed that an earmuff with larger interior space tends to have lower inner temperature. In the airtightness test, 18 out of the 28 earmuff tested had airtightness of over 90%, and 6 earmuffs were over 99%. However, the airtightness of 5 earmuffs was lower than 75%, and for 1 earmuff it was lower than 50%.

The weight of the 28 earmuffs ranged from 114 to 323g. The average weight was 203.6g and standard deviation was 58.4g. Weight of the muffs accounts for 68% to 77% of the total weight, which should be the major target if a lightweight design is desired. Table 4 summarizes the test results of earmuff inside temperature, airtightness and weight.

Table 4. Test results of earmuff inside temperature, airtightness and weight

 

Temperature (°C)

Airtightness (%)

Weight (g)

Maximum

35.3

99.7

323.3

Minimum

30.0

49.7

113.8

Average

32.2

89.2

203.6

Standard deviation

1.6

12.6

58.4

The test results of the 28 commercial earmuffs did not show any correlation between headband force, temperature, and airtightness of different earmuffs. For further study, two earmuffs were modified to adjust their headband forces at 6N, 8N, 10N, 12N, and 14N, and then were tested for airtightness. Table 5 shows that for the same earmuff, airtightness has strong correlation with headband force. Airtightness, and therefore soundproofing effect, may be affected when trying to reduce headband force.

Table 5. Correlation between airtightness and headband force for the same earmuff

 

Earmuff A

Earmuff B

Headband force (N)

Airtightness (%)

6

91.8

91.0

8

92.6

92.2

10

92.8

94.2

12

93.3

95.5

14

96.4

98.0

Pearson coefficient

0.88

0.99

4.     Finding the acceptable ranges for the comfort indices

After measuring commercial earmuffs for their comfort indices, an experiment was conducted to determine the acceptable ranges of headband force, earmuff inside temperature, and weight. For this experiment, an earmuff with good comfort indices was used as the base, and it was modified into 3 groups of sample earmuffs. There were 5 identical earmuffs in the first group, although the headband forces were adjusted to 6N, 8N, 10N, 12N, and 14N when measured on the comfort tester on the medium head size. Similarly for the second group of sample earmuffs, small temperature controllers were installed inside the earmuffs of the 4 identical earmuffs in the second group to control the temperature at 30°C, 32°C, 34°C, and 36°C. Finally, weights were added to the 6 identical earmuffs in the third group to adjust their weights to 130g, 170g, 210g, 250g, 290g, and 330g.

The purpose of the experiment was first explained to the test subject. Then the subject randomly picked an earmuff from the group of earmuffs, wore it for 240±10 seconds, and then expressed his/her perceived feeling on the comfort index being examined using a 5-point scale. The subject rested for about 5 minutes, then randomly picked another earmuff from the same group again. This process was repeated for 4 to 6 times in each group depending on the number of sample earmuffs in the group, and it is possible that a subject picked the same earmuff repeatedly. The experiment was conducted in a comfortable lab environment without noise, so that the subjects could concentrate on the comfort index being examined.

A total of 44 subjects finished the experiment for all 3 comfort indices. Their ages were between 18 and 54, and 86.3% of the testers were male. Tables 6 to 8 show the results. Numbers in a column represent the times subjects choose a certain answer after wearing the sample earmuff. Note that since the subjects picked the sample earmuffs randomly, the sum of the numbers in each column may not be the same. Finally the average score (total score/total times) is computed for each sample earmuff.

Table 6. Subjects’ perceptions for earmuff headband force

 

Subjects’ qualitative feeling

Score

Earmuff headband force

6N

8N

10N

12N

14N

1

No particular feeling

0

37

19

13

3

1

2

A little tight

-1

5

22

21

19

8

3

Tight

-2

0

0

4

14

13

4

Very tight

-3

0

0

1

3

20

5

Unbearable

-4

0

0

0

0

6

Total score

-5

-22

-32

-56

-118

Average score (total score/total times)

-0.12

-0.54

-0.82

-1.44

-2.46

Table 7. Subjects’ perceptions for earmuff temperature

 

Subjects’ qualitative feeling

Score

Earmuff temperature

30°C

32°C

34°C

36°C

1

No particular feeling

0

22

7

26

7

2

A little hot

-1

18

26

23

15

3

Hot

-2

7

9

1

7

4

Very hot

-3

1

1

2

0

5

Unbearable

-4

0

0

0

0

Total score

-35

-47

-31

-29

Average score (total score/total times)

-0.73

-1.09

-0.60

-1.00

Table 8. Subjects’ perceptions for earmuff weight

 

Subjects’ qualitative feeling

Score

Earmuff weight

130g

170g

210g

250g

290g

330g

1

No particular feeling

0

29

36

27

10

4

3

2

A little heavy

-1

2

1

21

27

17

12

3

Heavy

-2

0

0

4

6

12

20

4

Very heavy

-3

0

0

0

1

7

7

5

Unbearable

-4

0

0

0

0

0

4

Total score

-2

-1

-29

-42

-62

-89

Average score (total score/total times)

-0.06

-0.03

-0.56

-0.95

-1.55

-1.93

The results in headband force and weight showed that tighter and heavier earmuffs were more uncomfortable, as expected. The results of subjects’ reaction to temperature did not show any trend, perhaps because the subjects did not feel significant differences while temperature change was within 30°C to 36°C.

Using the data in Table 6 and 8, Figure 3 shows continuous contours of the relation of subjects’ perceived feelings and magnitudes of earmuff headband force and weight. Further interpolating the data, it was found that a headband force under 10.5N would be described by 80% of the wearers as “no particular feeling” or “ a little tight”, while an earmuff weight under 245 g would be described by 80% of the wearers as “no particular feeling” or “a little heavy”. 12 out of the 28 commercial earmuffs tested in this project (43%) had average headband forces higher than 10.5N, and 7 (25%) were heavier than 245g.

Figure 3. Relation of subjects’ perceptions and magnitudes of earmuff headband force and weight

5.     Conclusions

From this study, guidelines to improve the design of current hearing protection can be summarized as follows:

(1)       The important design factors that influence the comfort of earmuffs are airtightness, weight, heat-sinking ability, texture, and headband force. Adjustability and wearing convenience, which are emphasized in many hearing protection patents, were rated less important.

(2)       An earmuff weight under 245 g would be described by 80% of the wearers as “no particular feeling” or “a little heavy”. Weight of the muffs accounts for 68% to 77% of the total weight, which should be the major target if a lightweight design is desired.

(3)       An earmuff with larger interior space tends to have lower inner temperature.

(4)       A headband force under 10.5N would be described by 80% of the wearers as “no particular feeling” or “ a little tight”. “Sense of oppression” is the number 2 reason of discomfort of wearing hearing protection, but 43% of the commercial earmuffs tested in this project had average headband forces higher than 10.5N. More attention should be paid reduce the headband force, while keeping airtight ness at an acceptable level.

(5)       “Having difficulties in conversations” is the number one reason why workers perceive that hearing protection are uncomfortable, so future research could emphasize the development of electronic earmuffs that enables conversation but filters out the noise.

Acknowledgement

This research is supported by Institute of Occupational Safety and Health, Taiwan, R.O.C. This support is gratefully acknowledged.

References

Ahmed H. O., Dennis J. H., Badran O., Ismail M., Ballal S. G., and Jerwood D., 2001. Occupational noise exposure and hearing loss of workers in two plants in eastern Saudi Arabia. Annals of Occupational Hygiene, 45(5): 371-380.

EN 352-1, 1993. Hearing Protectors – General requirement – Part 1: Earmuffs.

Paakkonen R., Lehtomaki K., Savolainen S., Myllyniemi J., and Hamalainen E., 2000. Noise attenuation of hearing protectors against heavy weapon noise. Military Medicine, 165(9): 678-682.

Parmentier, G., Dancer, A., Buck K., Kronenberger G., and Beck C., 2000. Artificial head (ATF) for evaluation of hearing protectors. Acta Acustica (Stuttgart), 86(5): 847-852.

Patton, E., 2000. Ergonomic Hearing Protectors. Ergonomics in Design, 8(4): 25-28.

Phoon, W. H., and Lee, H. S., 1993. Hearing protection plans require proper ear plug selection, usage. Occupational Health & Safety, 62(5): 98-100.

Rabinowitz, P. M., 2000. Noise-induced hearing loss. American Family Physician, 61(9): 2749-2756.

Sanders, M. S. and McCormick, E. J., 1987. Human Factors in Engineering and Design (6th edition). McGRAW-HILL, New York, pp.478-479.

Sergio, A. and Miguel, S.R., 1996. Personal hearing protection. Safety Science, 23(2-3): 183-184.

Appendix

1.      Basic information

l          Gender: (Male /Female)

l          Age: (15 to 24 /25 to 34 /35 to 44 /45 to 54 /over 55)

l          Do you have experience wearing earmuffs at work? (Yes /No)

l          Do you have experience wearing earplugs at work? (Yes /No)

2.      Work environment

l          Do you work in a high decibel (over 85 dBA) environment? (Yes /No)

l          How many hours do you have to wear hearing protection at work everyday? (Less than 1 hour /1 hour to 2 hours /2 hours to 3 hours /3 hours to 4 hours /4 hours to 6 hours /6 hours to 8 hours /over 8 hours)

l          How many times do you have to change between normal and high-decibel environment at work everyday? (1 to 3 times /4 to 6 times /7 to 10 times /over 10 times)

l          Do you work in a hot and humid environment? (Yes /No)

l          Do you work in a dusty and dirty environment? (Yes /No)

3.      Uncomfortable feelings of wearing hearing protection at work

l          Select uncomfortable feelings when you wear hearing protection at work. (No discomfort /Feeling dirty /Feeling itchy /Feeling of an object in the ear /Sense of oppression /Feeling stuffy /Easy to slide or fall off /Headache /Protectors deform easily /Having difficulties in conversations /Poor soundproofing /Interfere with work /Tinnitus /Acoustics change /Discomfort due to hard contacting parts /Other__________)

l          (Skip this question if you do not have experience wearing earplugs at work.) When you wear earplugs at work, you start to feel uncomfortable after wearing them for (within 15 minutes /15 to 30 minutes /30 minutes to 1 hour /1 to 1.5 hours/1.5 to 2 hours /over 2 hours)

l          (Skip this question if you do not have experience wearing earmuffs at work.) When you wear an earmuff at work, you start to feel uncomfortable after wearing it for (within 15 minutes /15 to 30 minutes /30 minutes to 1 hour /1 to 1.5 hours /1.5 to 2 hours /over 2 hours)

4.      Evaluate the importance of design factors of earplugs and earmuffs that might influence their comfort.

l          (Skip this question if you do not have experience wearing earplugs at work.) Please evaluate the importance of following factors to the comfort of earplugs on a 5-point scale (very important /important /average /less important /not important): weight, texture, inflation pressure, heat-sinking ability, sweat absorbing ability, wearing convenience, soundproofing.

l          (Skip this question if you do not have experience wearing earmuffs at work.) Please evaluate the importance of the following factors to the comfort of earmuffs on a 5-point scale (very important /important /average /less important /not important): weight, texture, headband force, adjustability, heat-sinking ability, sweat absorbing ability, wearing convenience, soundproofing, airtightness.