Attenuation characteristics of fit-compromised ... - ICA 2013 Montreal

Wells et al.

Proceedings of Meetings on Acoustics Volume 19, 2013

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ICA 2013 Montreal Montreal, Canada 2 - 7 June 2013 Noise Session 1aNS: Advanced Hearing Protection and Methods of Measurement I 1aNS3. Attenuation characteristics of fit-compromised earmuffs and various nonstandard hearing protectors Laurie Wells*, Elliott H. Berger and Ron Kieper *Corresponding author's address: Personal Safety Division, 3M, St. Paul, MN 80537, [email protected] Excessive noise exposure can be successfully mitigated by proper use of legitimate hearing protection devices. However, real-life circumstances sometimes drive people to use compromised or alternative means of protection. This paper reports attenuation data measured in the 3M E•A•RCAL facility over several years, in conformance with ANSI real-ear attenuation at threshold test standards (S3.19-1974, S12.61984, and S12.6-2008 Method A) and also provides, for comparison, one dataset from the open literature (fingers/palms). The loss of attenuation was measured for various earmuffs worn in less than ideal conditions, including earmuffs worn in conjunction with various safety glasses, hairnets, head covers, hoods, earmuff cushion covers, and baseball style caps. Data were also obtained for non-standard means of blocking sound, including long hair, cotton balls, and even use of palms and/or fingers to block the ears. Results demonstrated that the effects on earmuff attenuation varied from none at all (suitable cushion cover) to as much as 12 dB (hooded sweatshirt). Realizing that people adapt hearing protectors to meet their needs is one step towards optimizing hearing protection selection and use; knowing the significance of these adaptations is the next step. The authors are employees of 3M and the research was funded by 3M. Published by the Acoustical Society of America through the American Institute of Physics

© 2013 Acoustical Society of America [DOI: 10.1121/1.4799992] Received 22 Jan 2013; published 2 Jun 2013 Proceedings of Meetings on Acoustics, Vol. 19, 040003 (2013)

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INTRODUCTION Excessive noise exposure can be mitigated successfully by proper use of legitimate hearing protection devices (HPDs). Through decades of concerted effort, manufacturers have produced a wide variety of high quality HPDs, making them available to consumers who work and/or play in potentially hazardous noise environments. However, in spite of intentions to deliver devices with predictable attenuation, real-life circumstances often interfere with optimal use of HPDs. People commonly either 1) use HPDs in ways that compromise effectiveness or 2) use alternative, make-shift HPDs (Brueck, 2009). Over the years, studies have been conducted at the 3M E•A•RCAL facility in an attempt to quantify the change in noise reduction when end-users alter the fit or use of HPDs. This paper reports measured loss of attenuation for various earmuffs worn in a number of scenarios chosen to simulate common real-life wearing conditions. These include earmuffs worn in conjunction with safety glasses; earmuff cushion covers, and assorted headgear commonly seen in industrial settings. Included for comparison, are attenuation data for methods people often use as make-shift hearing protectors: using ones hands and/or fingers (Holland, 1967), cotton balls, and even long hair covering the pinnae.

PROCEDURE All results presented in this paper are from tests conducted over a period of many years in either the 3M E•A•RCAL 113-m3 test chamber, in a diffuse field, according to ANSI test standards S3.19-1974, S12.6-1984 or Method A of S12.6-2008, or in other test labs also using an accepted real ear attenuation at threshold (REAT) method for the era. The Noise Reduction Ratings (NRRs; EPA, 1979) computed in this paper differ somewhat from labeled NRRs computed in conformance with the EPA rule since the actual datasets and computational procedures vary slightly. Functionally the values are equivalent and hence to be explicit in the table where all the data are summarized, we refer to them as NRR equivalents. A minimum of 10 subjects was used in each condition. Thresholds were measured either two or three times each for: open earcanals, occluded canals with the earmuff worn in the intended fashion, and occluded canals with the earmuff worn in the “compromised” condition. The compromised fits included: various models and styles of safety glasses with different temple sizes; two types of earmuff cushion covers; head gear: a non-woven hairnet, Dupont Tyvek® hood, sweatshirt hood, and baseball style caps. Earmuff variables included cup size, manufacturer, and cushion styles: gel-filled, liquid-filled, and foam. Subjects were experienced test subjects who were trained and familiar with the test procedures. The subject adjusted the earmuff while listening to a broadband fitting noise to obtain the best fit possible. Also reported in this paper are attenuation data collected for make-shift hearing protectors. Holland (1967) measured threshold shifts of nine subjects for two applications reported here: index fingers blocking the earcanal and palms of the hands clasped over the ears. Two different laboratories conducted REAT measurements on 10 subjects, using dry cotton balls in the earcanals. Lastly, REAT measurements were performed in the 3M E•A•RCAL facility on 10 subjects with hair of sufficient length to completely cover the pinnae.

RESULTS Table 1 contains a summary of the results for all test conditions. Each condition tested is listed with the corresponding NRR and when applicable, the change to the NRR due to the compromising condition. For purposes of identifying statistical differences, the NRR was calculated for each subject and then averaged across subjects following the method described by Williams (2012). Statistical testing utilized a paired t-test evaluated with a onetail criteria, since the presumption was that the compromised attenuation would be less than or equal to the intended attenuation. Results revealed that the NRR differences in the matched pairs was statistically different in all cases except for the application of the 3M™ Peltor™ Clean Hygiene Pads (earmuff cushion covers with thin paper/cellulosic design) on the 3M™ Peltor™ Earmuff H7A and the effect of safety glasses together with dual hearing protection (an earplug plus earmuff). A statistical comparison made between a foam and a gel earmuff cushion when worn over safety glasses was made with a paired t-test evaluated with a two-tail criteria, since it was unknown which cushion type would have a greater attenuation loss when worn over safety glasses. The findings indicated there is a statistical difference between the earmuff cushion styles, with the gel cushions showing less acoustic leakage than the foam for this data set. Real-ear attenuation data as a function of frequency are shown for selected datasets together with photographs of the devices/conditions, and the change in the NRR (referred to in the graphs as NRR).

Proceedings of Meetings on Acoustics, Vol. 19, 040003 (2013)

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TABLE 1. Effect of compromised-fit due to safety glasses, earmuff cushion covers, and headgear on earmuff attenuation for selected hearing protection devices. All Peltor™ and E-A-R™ products referenced below are manufactured by 3M Company.

Human Hair

0

Effect on NRR equivalent (dB) -----

HPD

Cotton Balls (dry) Palm of the hand Human Finger

3 15 19

-------------

Glasses

Peltor H7A Muff

26

-----

Peltor H7A + Safety glass thick temple

15

-11

0.00053

Peltor H7A + Safety glass thin temple

24

-2

0.00309

Medium Cup Muff, foam cushion Medium Cup Muff, foam cushion + Safety glass Medium Cup Muff liquid cushion Medium Cup Muff liquid cushion + Safety glass Peltor H9A

23 19

-----4

0.00003

25 19

-6

0.00120

25

-----

Peltor H9A + glass arc temple PeltorH9A + glass adjustable length Peltor Nordica Pro, Foam cushions Peltor Nordica Pro, Foam + Safety glass adjustable length temple Peltor Nordica Pro, Gel cushions Peltor Nordica Pro, Gel + Safety glasses adjustable length temple E-A-R Model 1000 Model 1000 + Baseball-style cap Peltor H7A

18 17 24 14

-7 -8 -----10

24 18

-----6

23 18 27

------5 -----

22 21

-5 -----

0.00000

Headgear

Peltor H7A + Baseball-style cap E-A-R Model 1000

Headgear

15 25

-6 -----

0.00000

5

E-A-R Model 1000 + Hairnet Peltor H9A

Headgear

Peltor H9A + DuPont Tyvek® Hood E-A-R Model 1000 E-A-R Model 1000 + Sweatshirt Hood Peltor H7A Muff

17 21 14 24

-8 -----7 -----

0.00008

6

Peltor H7A + Sweatshirt Hood Peltor H7A Muff Peltor H7A + Peltor Clean (thin) American Optical 1720 Muff American Optical 1720 + Covers (thick)

12 24 24 21 16

-12 ----0 -----5

0.00002

E-A-R Classic + Peltor H9A

32

-----

Classic + H9A + Safety glass adjustable length temple

32

0

Figure # 1

Condition Category MakeShift

2

None

None

None

3

4

None

Glasses

Glasses

Glasses

Headgear

Cushion Cover

None

Dual HPD

NRR equivalent (dB) Device(s) Tested


Probability of Significance, p 0.05 Does not Apply

0.00037 0.00004 0.00016

0.00000

0.00070

0.00000

0.23051 0.00000 0.41849

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Non-standard Methods of Blocking Sound Figure 1 shows the mean attenuation afforded by make-shift or non-standard means of blocking sound to the ears: human hair, cotton balls, the human palm and human finger plugging the earcanal. Human hair over the ears provides no sound reduction. Two separate laboratory studies showed corroborating results with an NRR of 3 dB for dry cotton balls. NRRs calculated using the attenuation data from Holland (1967) together with a typical standard deviation (SD) value (4 dB) for an earplug (original SD information was not published) are 19 dB and 15 dB for human palms clasped over ears and fingers plugging the earcanals respectively. As a short-term, immediately available option, using ones hands is a reasonable choice for reducing offending sound levels when bona fide hearing protection is not available, whereas using cotton balls provides negligible protection and letting one’s hair down provides none at all.

FIGURE 1. Attenuation data for four make-shift hearing protectors: hair, cotton balls, palm and finger in earcanal.

Earmuff and Compromised Wearing Conditions The conditions tested are described below by category. Figures 2 and 4 - 7 show the attenuation curves of selected paired tests listed in Table 1, including photographs of the earmuff(s) and the fit-compromising device(s) tested. The text in each graph lists the decrease in the NRR that each device produced when tested with the respective earmuff. Figures 3(a) and 3(b) illustrate an example of foam and gel-style earmuff cushions.

Earmuffs and Safety Glasses Multiple studies have been conducted to quantify the acoustic leak on earmuff attenuation caused by eyeglass temples (Anderson & Garinther, 1996; Brueck, 2009; Lemstad & Kluge, 2004; Nixon and Knoblach, 1974). In a Health and Safety Laboratory Research Report, Brueck measured multiple fit-compromised scenarios commonly observed in the workforce including earmuffs worn over safety glasses. However, in contrast to this current paper, she used microphone-in-real-ear (MIRE) measurements on a small number of subjects (n = 4) and reported attenuation as a Single Number Rating (SNR) as per European convention. Figure 2 shows data for the 3M™ Peltor™ Earmuffs H7A alone and in combination with two representative safety glasses: one with a thin and one with a thick temple-style. The NRR of 26 dB for the earmuff alone was reduced by 2 dB (thin temple), and 11 dB (thick temple). Another combination, the 3M™ Peltor™ Earmuff H9A, NRR 25 dB, was reduced by 7 and 8 dB for two alternate models of safety glasses, one with an arc-shaped temple and the other with an adjustable length temple, respectively. The effect of earmuff cushion style was examined with a medium-cup earmuff in two conditions: foam and liquid-filled cushions in combination with safety glasses. Five of the subjects who normally wore glasses used their own frames for this test; those who did not, wore a standard pair of safety glasses. The glasses decreased the NRR from 23 dB, obtained by the foam-cushion earmuffs, and 25 dB by the liquid-filledcushion earmuffs, to 19 dB, by 4 and 6 dB, respectively.


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An additional earmuff cushion comparison was made between foam and gel earmuff cushions, which differ in four ways: filler substance, bladder thickness, cushion cover (skin) thickness, and contour (see Figure 3 for example of the foam and gel cushions.). The 3M™ Peltor™ Nordica Pro Model MT22H53A Earmuff with both foam and gel-filled cushions was tested in combination with safety glasses with a -10 dB affect on foam and -6 dB affect for the gel-filled cushions.

FIGURE 2 - Effect of thick and thin temples on attenuation of the 3M™ Peltor™ Earmuffs H7A.

FIGURE 3 (a) left photo and (b) right photo - Example of foam and gel earmuff cushions (a) front view and (b) side view.

Because the data collected on the 3M™ Peltor™ Nordica Pro Earmuff were obtained over a short time period using the same subjects, a paired comparison can be made to assess the difference between the foam and gel earmuff cushions when worn over safety glasses. A paired t-test using a two-tailed analysis yielded a significant difference (P = 0.02) between the two cushion types. The gel cushion sustained less acoustic leakage when worn over the safety glass temple than did the foam cushion. Regarding dual hearing protection (earplug plus earmuff), introducing the safety glasses caused no appreciable change to the NRR. Presumably, the attenuation of the well-fitted earplug dominated the combined attenuation of the two devices sufficiently to eliminate any negative effect of the glasses. The eyeglass/earmuff combination findings and as well as other 3M E•A•RCAL measurements not included here are generally consistent with the literature on acoustic leakage. There is an expected loss of attenuation across all frequencies with variable reduction dependent on factors such as eyeglass temple thickness and shape. We are unaware of any other data in the literature on the leakage under dual protection.

Earmuff Cushion Covers The application of an absorbent cover to the earmuff cushion, sometimes worn to improve comfort in hot conditions, was tested. The 3M™ Peltor™ Clean Hygiene Pads (earmuff cushion cover with thin paper design) had


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no affect on the NRR of the Peltor earmuff H7A, whereas a thicker cotton cushion cover (American Optical™ HP61) applied to an American Optical™ 1720 earmuff, decreased the NRR by 5 dB, from 21 to 16 dB.

Earmuffs and Headgear Noise-exposed employees frequently wear earmuffs over the top of some type of headgear, either for sanitation, personal protection or comfort. All variations experienced reduced NRRs ranging from 5 to 12 dB. Two different earmuff models, 3M™ E-A-R™ Earmuff Model 1000 and Peltor Earmuff H7A, both showed NRR reductions of 5 dB when worn over the top of a baseball cap (see Figure 4).

FIGURE 4 - Effect of baseball-type cap on the attenuation of 3M™ E-A-R™ Earmuff Model 1000 and 3M™ Peltor ™ Earmuff H7A.

Figure 5 shows a white, non-woven, bouffant-style hairnet, commonly used in the food industry that reduced the NRR of the Model 1000 earmuff by 7 dB, and a Dupont Tyvek® hood, completely covering the head and ears, reduced the NRR of the Peltor earmuff H9A from 25 to 18 dB (Figure 5).

FIGURE 5 - Effect of hairnet on the attenuation of the 3 M™ E-A-R™ Earmuff Models 1000


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FIGURE 6 - Effect of Tyvek® hood on the attenuation of the 3M™ Peltor™ Earmuff H9A.

The largest effect on NRR was from wearing a hooded sweatshirt, with the hood covering both ears, and the earmuff worn over the hood (see Figure 7). This condition is common among workers in cold environments, such as refrigerated warehouses, and outdoor enthusiasts, such as target shooters, who spend time in cold climates. The earmuff alone obtained an NRR of 21 dB (Model 1000) and 24 dB (H9A). The sweatshirt hood caused a 7 dB and 12 dB reduction in the NRR respectively.

FIGURE 7 - Effect of sweatshirt hood on attenuation of 3M™ E-A-R™ Earmuff Model 1000 and 3M™ Peltor ™ Earmuff H7A.

DISCUSSION Key to managing an effective hearing conservation program is knowing whether or not noise-exposed employees are adequately protected. This can be challenging, given the reality that HPD users will modify the intended fit of a given device to accommodate their environmental needs and personal preferences. Understanding the ramifications of using nonstandard HPDs or compromised fits of earmuffs is a step towards proper selection of HPDs and training


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people to use them correctly. In this series of HPD studies, all modifications to the intended fit of earmuffs had a negative effect on attenuation, with the exceptions of: a) thin paper style earmuff cushion cover and b) safety glasses used with dual hearing protection (earplug plus earmuff). Regarding foam and gel earmuff cushions, there appears to be an advantage, for the earmuff model tested in this study, to choosing gel cushions over foam cushions only when safety glasses and earmuffs are worn simultaneously. Although the cushion style was irrelevant without safety glasses present, the gel cushion out-performed the foam cushion when safety glasses were introduced. Additional data are needed to make a definitive generalization concerning performance of different earmuff cushions styles when worn with safety glasses. Our measurements underscore the value of employee awareness, training, and of employer enforcement of proper use of hearing protection devices. Whether or not the degree of attenuation loss is statistically significant is a separate question from the practicality of small differences in NRR estimations in the real-world applications. The hearing conservationist must understand the relative effect of the attenuation loss when making and enforcing policies about HPD use in the workplace as well as in selecting appropriate HPD for the work environment. In addition to the noise attenuating characteristics of the HPD, considerations should include the noise exposure in the working environment, the other demands of the work tasks for additional personal safety equipment, and of course the employee’s personal preferences for comfort and style.

REFERENCES Anderson, B.W. and Garinther, G.R.., (1996). “Effects of active noise reduction in armour crew headsets,” paper presented at AMP Symposium, Copenhagen, Denmark. ANSI (1984). “Method for the measurement of the real-ear attenuation of hearing protectors at threshold,” American National Standards Inst. S12.6-1984, New York, NY. ANSI/ASA (2008). “Methods for measuring the real-ear attenuation of hearing protectors,” American National Standards Inst./Acoustical Soc. Am., S12.6-2008, New York, NY). ASA (1975). “Method for the measurement of real-ear protection of hearing protectors and physical attenuation of earmuffs," Acoustical Society of America, ASA STD1-1975 (ANSI S3.19-1974), New York, NY. EPA (1979). “Noise labeling requirements for hearing protectors,” Environmental Protection Agency, Fed. Regist. 44(190), 40CFR Part 211, 56130-56147. Brueck, L. (2009). “Real world use and performance of hearing protectors,” Health and Safety Laboratory, RR720 Research Report, Derbyshire. Holland, H.H. Jr. (1967). "Attenuation provided by fingers, palms, tragi and V-51R ear plugs," letter to editor, J. Acoust. Soc. Am. 41(6), 1545. Lemstad, F., and Kluge, R., (2004). “’Real-world‘ attenuation of muff-type hearing protectors: the effect of spectacles,” Joint Baltic-Nordic Acoustics Meeting, Mariehamm, Aland. Nixon, C.W., and Knoblach, W.C., (1974). “Hearing protection of earmuffs worn over eyeglasses,” report for Air force Aerospace Medical Research Lab Wright-Patterson AFB, OH. Williams, W., (2012). “A proposal for a more refined single number rating system of hearing protector attenuation specification,” Noise & Health, 14, 210-214.


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