Noise and Hearing
The Effect of Fitting Procedure on Hearing Protector Attenuation Carol J. Merry, BS; Curt W. Sizemore; John R. Franks, PhD Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, Division of Biomedical and Behavioral Science, Physical Agents Effects Branch, Cincinnati, Ohio
ABSTRACT Realistically evaluating the effectiveness of hearing protectors continues to be a major problem in hearing conservation. The purpose of this study was to examine a laboratory-based fitting procedure (User Fit) that was designed to yield hearing protector attenuation values similar to that derived from field studies. Ten subjects who were naive to hearing protectors were used in a repeatedmeasures design that measured real ear attenuation at threshold for two types of plugs. Each subject was tested in two fitting conditions that varied based on the type and degree of assistance given to the subjects by the experimenter. The results showed significant differences in attenuation based on the fitting procedure used, with the User Fit best approximating field data. In addition, a generalized learning effect was noted. The results suggest that any experience with earplugs leads to subsequent improvement in attenuation despite the type of earplug used. Further testing is planned with greater numbers of subjects and additional types of hearing protectors (Ear Hear 13 1:ll-
18).
IT IS WELL DOCUMENTED that workers on the job frequently do not obtain the attenuation from their hearing protectors that they would expect based on the manufacturer’s noise reduction rating (NRR) that is printed on the protector packaging. One possible reason for the reduced noise attenuation may be that the overwhelming majority of workers are unable or unwilling to wear their protectors properly. A second possibility is that the NRR is an unrealistic indication of the protective ability of hearing protectors. Berger (1983) summarized 10 studies, including a National Institute for Occupational Safety and Health (NIOSH) work site study conducted by Lempert and Edwards ( 1983), that examined noise attenuation of various hearing protectors as they are worn at the job site in Ear and Hearing, Vol. 13, No. 1,1992
over 50 different industries. These studies showed considerable differences between laboratory-derived hearing protector attenuation (as reflected in the manufacturer’s NRR) and workplace attenuation for a variety of hearing protectors. The laboratory data were found to consistently overestimate the attenuation actually obtained by workers in industry. More recently, Berger (1988) reported that studies by Erlandsson, Hakanson, Ivarsson, and Nilsson (1983), Behar (1985), Mendez, Salazar, and Bontti (1986), and Smoorenburg, ten Ra, and Mimpen (1986), Edwards and Green (1987), and Pekkarinen ( 1987) also indicated substantial discrepancies between the attenuation values measured in the workplace and the standardized laboratory-based estimates of hearing protector attenuation that are currently used to assign an NRR to every hearing protector sold in the United States. The laboratory procedure currently used to assign an NRR to a protector is a carefully controlled, experimenter fit procedure. It results in a “best possible” attenuation rating for hearing protectors that may not be applicable to the work site. It has been suggested that a more realistic laboratory procedure is needed to derive an NRR that is meaningful to workers. Berger (1988) implied that the discrepancy between work site and laboratory attenuation values of hearing protectors may be minimized by altering certain test practices in the laboratory procedures. In particular, he suggested that the type of subjects used (trained versus novice), as well as the characteristics of the laboratory fitting procedure used (i.e., the degree of experimenter intervention, the level of subject training, and the motivation of the subjects) could have a major impact on measured noise attenuation. This article describes a hearing protector attenuation study in which the effects of different fitting procedures were examined. These effects were measured for two types of ear plugs across two fitting procedures: a user fit (UF), in which only the manufacturers’ instructions were available to the subjects, and an informed user fit (IUF), in which substantial fitting assistance was given to the subjects by the experimenter. The UF and IUF procedures used in this study were developed by American National Standards Institute (ANSI) working group S 12/WG 1 1, “Field Effectiveness and Physical Characteristics of Hearing Protectors.” This is the working group that has been established to directly address
0196/0202/92/1301-0011$03.00/0 EARAND HEARING Copyright 0 1992 by Williams & Wilkins
Printed in the U.S.A
Effect of Fitting Procedure
11
the issue that laboratory-based attenuation data consistently overestimate attenuation data collected in field studies. Their charge is to develop a reproducible, laboratory-based testing protocol that will yield results which more realistically approximate the attenuation obtained by typical motivated workers on the job. The study described in this paper is the NIOSH evaluation of the initial ANSI working group test protocol. The data from this study were compared to the NIOSH work site study data (Lempert & Edwards, 1983) as well as to the attenuation data provided by the manufacturer or printed on the hearing protector packages. It was hypothesized that the attenuation values obtained from the two fitting procedures would differ significantly based upon the degree of instruction and fitting assistance given to subjects, with subjects receiving better attenuation in the IUF condition than in the UF condition. Because workers on the job may receive very little instruction/reinstruction or assistance with hearing protector fitting on a day to day basis at the typical work site, the attenuation values measured in the lab by the UF procedure might best approximate the work site data. METHOD
Subjects Ten subjects were paid to participate in this study. Six females and four males (ages 21-36) were recruited predominantly from two universities in Cincinnati, OH. An effort was made to draw from a broad subject pool by having few selection criteria. The subjects were primarily required to be normal-hearing, novice users of hearing protectors. Subjects responded to posted advertisements for subjects and met the following criteria: 1. Pinnae and ear canals: free of pathology as noted by
otoscopic exam. 2. Little prior experience with hearing protectors. This meant subjects stated they had not worn earplugs for any purpose, including swimming or sleeping more than five times, and they had never received instruction in the use of hearing protectors. 3. Ability to pass a screening audiogram with hearing threshold levels not in excess of 25 dB at 500 to 8000 Hz. 4. Ability to read small print typically found on hearing protector packages. Subjects were accepted into the study as they applied and passed the screening criteria noted above. Three male applicants were rejected because they had too much experience wearing hearing protectors. Otherwise, all applicants were accepted until a test population was obtained that was comprised of approximately equal numbers of males and females (&lo%). Hearing Protectors The E-A-R formable foam plug and the V5 I R premolded, plastic plug have been included in various field and laboratory investigations in the past. They were selected for this study so that comparisons to existing attenuation literature would be possible. The E-A-R is a unisize, expandable foam plug that 12
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must be compressed and rolled into a thin cylinder by the user before insertion into the ear canal. The V5 1R is a roundtipped, single flange, soft plastic plug that is available in various sizes from extra small to extra large. All five sizes were available to the subjects at each fitting. The V5 I R plugs used in this study were not color-coded by size, thus, subjects selected the appropriate size based solely on fit. To use this plug, a user must select the proper size for each ear canal and then insert the device by twisting the plug into the ear canal until a seal is obtained. Apparatus Testing was carried out in a double-walled, double-floored reverberant chamber (Tracoustics RE245) that met the requirements of both ANSI S3.19 (1 974) and ANSI S 12.6 ( 1984) for a diffuse sound field with acceptable background noise levels. Test stimuli for the real ear attenuation at threshold (REAT) measurements were */3 octave bands of noise presented as three pulses within a 200 msec period, with a 20 msec rise and fall interspersed with a 200 msec off time. Pink noise generated by a Grason-Stadler Noise Generator was fed into a Hewlett-Packard Filter Set to yield seven Y3 octave bands of noise centered at 125 Hz, 250 Hz, 500 Hz, 1 kHz, 2 kHz, 4 kHz, and 8 kHz. The I/3 octave bands were fed into a custom control box developed at NIOSH. (The schematics for the prototype control box are available upon request from the authors.) For each threshold measurement, the selected test band was gated by a Wilsonics Cosine Switch and intensity was controlled by a Wilsonics Programmable Attenuator. Output of the programmable attenuator was passed to a Hewlett-Packard 20 dB Gain Preamplifier and then to an Altec Power Amplifier. The power amplifier output was sent through a passive 20 dB T pad attenuator to the loudspeakers in the chamber. This equipment array is diagrammed in Figure 1. When appropriate, a fitting noise was generated by a second Grason-Stadler Noise Generator, sent to a General Radio Multifilter, shaped, and, as needed, switched into the HewlettPackard 20 dB Preamplifier, which fed through the Altec Power Amplifier and then passed through the passive 20 dB T pad attenuator to the loudspeakers in the chamber (see Fig. 1). The low-pass-filtered pink fitting noise was presented at an A-weighted SPL of 60 dBA. The subject responded to the stimulus by triggering a silent optical hand switch, which was developed at NIOSH for this project. The subject’s response was registered by the computer, which determined threshold automatically to the nearest decibel after a modified Hughson-Westlake audiometric procedure (Hughson & Westlake, 1944). Hearing protector attenuation was calculated using REAT procedures and the equipment described above. In REAT testing, a subject’s binaural threshold of hearing is measured with and without hearing protection. The difference between the two thresholds is the measure of the hearing protector’s attenuation. Each day before testing began, the equipment and sound field were calibrated using a Briiel and Kjaer (B & K) 4145 low noise microphone and a 2627 power amplifier. The pickup was displayed on a B & K 2131 digital frequency analyzer. The system was calibrated with a B & K 4220 piston phone. Procedures On the first day of testing, subjects participated in an initial audiometric threshold screening and a minimum of five pracEar and Hearing, Vol. 13, No. 1,1992
Compaq Deskpro Computer
Computer Interface
Hewlett-Packard 8056A Filter Set
General Radio 1382 Noise Generator
Wilsoni~BSIT Cosine Switch
Wilsonics PATT Attenuator
I
- -I
Fitting Noise Switch
I
General Radio
1925 Multifilter
General Radio I381 Noise Generator
Figure 1. Schematic of equipment and sound field.
I
Hewlett-Packard 465A Preamplifier
Altec W A Power Amplifier
Tracoustics RE-245
DoublcWalllDouble Floor Room
tice sound field threshold tests. Subjects were required to demonstrate repeatability of their audiometric thresholds to within 6 dB at each frequency on their last three practice runs. Subjects then came to the laboratory on 4 additional days. Each visit was divided into two 1 h sessions so that repeated measures on both earplugs were obtained each day. The UF protocol was administered during the first and ihird test days and the IUF protocol was administered during the second and fourth test days. Presentation order of the occluded (plugs in) and unoccluded (no plugs) conditions and the presentation order of the two earplugs were counterbalanced across the 10 subjects. In the UF protocol, before the attenuation testing, subjects were simply handed the hearing protectors along with the manufacturer’s instructions as printed on the package and as noted on the dispensing box. They were instructed to practice (up to a maximum of 5 min) with the protectors until they believed them to be properly inserted into their ears. The subjects received no verbal or physical assistance from the experimenter. At the conclusion of the practice period, subjects removed their plugs and were escorted into the test chamber, where they reinserted them for testing. Subjects remained in the chamber for two complete sets of open and occluded thresholds for that plug. Before each insertion of the protectors for threshold testing, subjects could refer to the written instructions; but again, no assistance or feedback was given by the experimenter. Session A ended when the testing of the first plug was complete. Subjects exited the chamber Ear and Hearing, Val. 13, No. 1,1992
for a brief rest period; then session B began with the second plug and the same unassisted fitting process. In the IUF protocol, subjects were given the hearing protector along with the same manufacturer’s written instructions as in the UF condition. In addition, during the practice fitting of the protector, the experimenter facilitated the fitting by offering verbal and physical assistance in adjusting the plugs. In the test chamber before each occluded threshold, subjects refitted hearing protectors by themselves using the written instructions. At this point, the experimenter was not permitted to offer advice or feedback to the subjects. A fitting noise was introduced into the chamber and subjects were instructed to use this noise in order to obtain maximum attenuation from the plugs. (No fitting noise was available in the UF condition because it was designed to yield values approximating work site data. It was felt that a fitting noise in a laboratory setting would permit subjects to attain attenuation values that would be too high to meet this goal.) As in the UF condition, two complete sets of open and occluded thresholds were obtained for each plug. RESULTS
The mean attenuations and standard deviations for the E-A-R plug and the V5 1R plug, as measured in this study, are shown in Table 1. This was a repeated measures design where each subject was tested four Effect of Fitting Procedure
13
times with each plug in each fitting procedure. The events cannot be considered independent; therefore, for statistical accuracy, the standard deviations are computed using the subject means (N = 10) at each frequency. Overall, there was a statistically significant difference between the IUF and UF procedures, with the IUF yielding larger attenuation values than the UF (F[ 1,9] = 7.92, p < 0.02). Additionally, there was a statistically significant difference between the attenuation of the two plugs in general (F[ 1,9] = 41.68, p < 0.0001) and in relation to the two fitting procedures (F [ 1,9] = 11.50, p < 0.008). Figure 2 displays the data for both plugs plotted according to fitting procedure. It can be noted that the E-A-R plug provides more attenuation than the V51R plug in both the IUF and the UF procedures, but there is less of a difference between IUF and UF with the E-A-R than with the V5 1R. A significant “day” effect was noted, in that subjects received more hearing protector attenuation in both fitting conditions the second day they worked with the protectors (F [1,9] = 7.61, p < 0.02). The effect was not differentiated by plug, trial, or fitting technique, and may be attributed to a generic learning effect in which the novice subjects quickly became familiar with earplugs in general. The manufacturers of the hearing protectors used in this study indicate on their packaging that these plugs offer more protection at the higher frequencies tested
than the lower ones. We found this was true for both plugs regardless of fitting procedure. However, as noted in Figure 3, subjects were able to obtain approximately 50% better attenuation at each frequency with E-A-R earplugs than with the V5 1R. Many second and third order comparisons were not statistically significant and are noted in the analysis of variance summary table, Table 2. In particular, the fit x day interaction was not significant, somewhat alleviating concern about the alternating UF-IUF procedure. Originally, there was interest in knowing if a moderately experienced panel of subjects could be used in the UF procedure instead of totally novice subjects. This was a logistical consideration, as many laboratories maintain a stable panel of listeners for hearing protector testing. Thus, the study was designed to “create” a panel of experienced listeners in a controlled and consistent way. When the subjects returned on the third day for their second UF test session, they were considered “experienced.” The approach was problematic. As expected, there was a significant overall learning effect, but some inconsistencies were seen; perhaps this was based on the different degrees of difficulty inherent in fitting each type of plug. In any revised procedure, only novice subjects will be used and all UF conditions will be tested before subjects are given any instructions for the IUF conditions. DISCUSSION
Table 1. Real ear attenuation at threshold by fitting technique and plug. Mean attenuation in decibels. Standard deviations” based on N = 10.
125 E-A-R UF E-A-R IUF V51R UF V51R IUF
250
500
1000
2000 4000 8000
20(7) 21 (6) 23(7) 24 (5) 28(5) 39 (5)40(7) 22 (6) 24 (7) 26 (7) 25 (6) 30 (5) 40 (4)40 (6) 9 (9) 9 (9) lO(9) 13 (10)19 (10)24 (8)16 (12) 15(11) 15(10) 14(10) 18(9) 23(9) 25 (7)24(10)
Figures in parentheses show standard deviations.
The attenuation values noted from the two conditions used in this study (UF and IUF) were compared to the attenuation values received by workers in the unsupervised self fit of the NIOSH work site study, and the attenuation values derived from the highly supervised experimenter fit of the ANSI S3.19-1974 procedure. The subjects in this study were primarily college graduate students. It is possible that they, and the subject panels used to perform the ANSI S3.19- 1974 procedure, differ from industrial workers on their mo-
Figure 2. Comparison of the attenuation for the E-A-R and V51 R earplugs in both fitting conditions.
125
14
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500 I000 2000 FREQUENCY El H T Z
4000
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Ear and Hearing, Vol. 13, No. 1,1992
E-Afl PLUG
v5l-R PLUG
m
0 45
@ 40 U
*0
35
$30
E 4
9 9
r-l
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15
Ll I5
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Figure 3. Comparison of the overall frequency-specific attenuation for both earplugs collapsed across subjects, fitting conditions, and trials.
II 4000
0 FREQUENCY W HERTZ Table 2. Analysis of variance summary table. Source
df
Sumsof Squares
Mean Squares
F
Pr > F
Repeated measures subject effects 1 40916.19 40916.19 41.68 0.0001 Plug 9 8835.36 981.71 Enor (plug) 1 3150.68 3150.68 7.92 0.0202 Fit technique 9 3578.94 397.66 Error (fit) 1 1863.02 1863.02 7.61 0.0221 Day 9 2202.28 244.70 Error (day) Plug :fit 1 856.63 856.63 11.50 0.0080 74.49 Error (plug * fit) 9 670.42 Nonsignificant effects 0.98 0.3474 Trial (session A/B) Plug :Day 0.10 0.7642 Fit :Day 0.02 0.8960 0.05 0.8232 Plug * Trial Fit * Trial 0.81 0.3925 Day :Trial 0.11 0.7439 0.23 0.6409 Plug :Fit :Day Plug :Fit * Trial 2.63 0.1396 1.11 0.3191 Plug * Day :Trial Fit :Day :Trial 0.90 0.3673 Plug * Fit * Day :Trial 0.08 0.7856
tivation to carefully follow the manufacturer’s instructions. However, because the instrumentation parameters in these studies were acoustically similar, it is reasonable to suggest that to some degree, the differences in attenuation are the result of differences in fitting procedure. The mean attenuations and standard deviations from the manufacturers, the NIOSH work site study, and this study are included in Table 3. Because the manufacturers and NIOSH calculated their standard deviations based on the Environmental Protection AgencyEar and Hearing, Vol. 13, No. 1, 1992
8000
mandated technique that uses the grand mean (EPA, 1979), the standard deviations for the data in this study were recalculated based on N = 40. Although statistically incorrect, this is the traditionally used method in attenuation testing and it is appropriate in this case for purposes of comparison. Both the IUF and UF groups received substantially less attenuation than the manufacturers’ published data. Additionally, both groups received better attenuation than that reported for these two plugs in the NIOSH work site data (Lempert & Edwards, 1983). Hearing protector manufacturers use a fitting technique (ANSI S3.19-1974) that was developed, in part, to standardize procedures so that psychoacoustic testing of hearing protectors on human subjects could be uniformly performed and accurately replicated with little interlaboratory variability. Thus, the procedure could be used to create a labeling scheme for hearing protectors. Although the ANSI S3.19-1974 standard technically allows for an experimenter fit or an experimentersupervised subject fit, the experimenter fit is the procedure generally used because it is called for by the EPA in the determination of the NRR (EPA, 1979). In experimenter fit, the investigator personally fits the hearing protectors for the subject before each occluded threshold. Three separate trials must be made pairing open and occluded threshold measurements, with each trial requiring a refit of the hearing protector on the subject by the experimenter. The test results are summarized for each hearing protector tested in terms of a grand mean and standard deviation of attenuation at each of the nine test frequencies. This is derived by computing the mean value in decibels of the occluded threshold with the hearing protector in place minus the open threshold for all listeners on all trials under identical test conditions ( N = 30). When reporting the results, the standard permits the experimenter to exEffect of Fitting Procedure
15
Table 3. Real ear attenuation data in decibels: E-A-R plug and V51 R plug (UF/IUF standard deviations' recalculated with N = 40 for Comparison).
V51 R Manufacturer IUF UF NIOSH E-A-R Manufacturer IUF UF NIOSH
125
250
500
1000
2000
4000
8000
NRR
20 (2) 15 (11) 9 (10) 8 (11)
22 (2) 15 (10) 9 (10) 8 (9)
24 (2) 14 (11) 10 (10) 6 (10)
28 (2) 18 (10) 13(11) 10 (10)
34 (2) 23 (10) 19 (11) 21 (12)
37 (3) 25 (8) 24 (10) 19 (9)
37 (3) 24 (11) 16 (13) 15 (10)
23 -3 -9 -9
37 (6) 22 (7) 20 (8) 15 (9)
41 (5) 24 (7) 21 (7) 15 (8)
45 (3) 26 (8) 23 (9) 16 (7)
44 (4) 25 (7) 24 (7) 18 (8)
36 (5) 30 (5) 28 (6) 28 (10)
43 (3) 40 (5) 39 (7) 33 (9)
47 (3) 40 (7) 40 (9) 27 (9)
29 13 10 3
Figures in parentheses show standard deviations.
-
UF ,--+--+
IUF
MANUFACTURER'S DATA
...... +......
NOSH FIELD DATA
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E-A-R Figure 4. E-A-R earplug. Compari-
2 HI
son of attenuation from the four fitting procedures.
Eu
30
a
U w 40
125
250
500
1000
2000
4000
8000
FREQUENCY IN HERTZ
clude data from a subject for whom the experimenter cannot obtain an adequate fit of the hearing protector. The NIOSH study (Lempert & Edwards, 1983) reported on workplace investigations that determined the actual noise reduction achieved by workers using several types of earplugs in various industrial settings. Testing was carried out in a specially instrumented mobile van on the work sites by using circumaural headphone sets that could be worn over earplugs without touching the earplugs or disturbing their fit. Data on the foam plug were provided by 56 subjects, whereas 84 subjects provided data on the V51R plug. The occluded threshold was determined first, followed by the open threshold. Means and standard deviations of attenuation were calculated by pooling the data from all subjects for each plug in order to provide results comparable to ANSI S3.19 summary statistics. When the manufacturers' data (experimenter fit) are compared to the NIOSH work site data (Table 3), it is clearly evident that the attenuation obtained with these two plugs in the workplace is substantially less than 16
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their laboratory-derived, experimenter fit values. Because this experimenter fit procedure does not give a reasonable estimate of hearing protector performance for the average user, hearing conservation managers are advised to routinely derate the published NRRs by 50% in an effort to more realistically compare products for their employees (NHCA, 1990; Royster & Royster, 1990). Unfortunately, this is not a satisfactory solution because an across the board 50% derating ignores the differences between types of protectors. A 50% derating would be too much for an earmuff and perhaps not enough for the V5 1R earplug. When the data from this study, the NIOSH work site data, and the manufacturers' data are plotted for each plug (Figs. 4 and 5), it appears that the UF procedure used in this study with the E-A-R and V51R plugs shows promise as a more realistic indicator of the amount of attenuation that can be expected by average users. The manufacturers' data are derived by testing a minimum of 10 subjects; the carefully controlled exEar and Hearing, Vol. 13, No. 1, 1992
IUF
UF ,--+
t---c.
:
NIOSH FIELD DATA
MANUFACTURERS DATA
---,
0
..... .....0
* . + a +
20
T
e
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.................+................. ..................*.................. .................................... .*
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Figure 5. V51 R earplug. Compari-
son of attenuation from the four fitting procedures.
T
125
250
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......+.................*
500 1000 2000 FREQUENCY IN HERTZ
perimenter fit procedure in that protocol results in less variability than that of the UF and IUF procedures of this study. Further testing of the revised UF and IUF procedures with more than 10 subjects is planned to see if a larger N will result in decreased variability. Also to be considered is the learning effect: any laboratory experience with earplugs seems to affect subsequent attenuation. Perhaps the laboratory procedure best approximating real world attenuation values will be a modified UF procedure in which many novice subjects test the plugs only once. Unfortunately, the logistic and economic difficulty with such a protocol would be great. Gasaway ( 1988) lists approximately 280 different types of hearing protectors that are currently on the market. Should a new standard such as the UF protocol be adopted, manufacturers may have difficulty finding enough novice subjects for testing. Certainly, it is important for hearing conservation professionals to continue to emphasize better supervision and education of the workers in their programs. Unfortunately, studies at NIOSH and elsewhere seem to indicate that consistent, effective use of hearing protectors by workers on a day to day basis does not always occur. The factors affecting a hearing protector's effectiveness include both the physical characteristics and fitting (demands) of the protector as well as the physical characteristics and motivation of the user. One way to potentially improve protection for users is to assure that hearing protectors on the market are rated by a method that takes the human motivation component into consideration. Thus, the significance of developing a laboratory methodology that will accurately predict hearing protector effectiveness in the workplace is profound. Workers and hearing conservation managers who select hearing protection based on current NRR labeling may Ear and Hearing, Val. 13, No. 1,1992
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8000
be disappointed to find that the devices, in the hands of the average person, do not perform nearly as well as the packaging indicates. Until a laboratory procedure has been validated that correlates well with workplace data, perhaps it would be prudent to derate the NRR and select hearing protectors based on an analysis of the type of noise encountered and on the attenuation values that have been published thus far in work site studies. It would be helpful if an updated compendium was published of those hearing protectors for which actual work site attenuation data are available. A hearing conservation manager could consult such a reference and take the workplace attenuation values into consideration as an additional factor influencing the selection of hearing protectors for employees. REFERENCES American National Standards Institute. Method of the measurement of real-ear protection of hearing protectors and physical attenuation ofearmuffs. ANSI S3.19-1974. New York: ANSI, 1975. American National Standards Institute. Method for measurement of the real-ear attenuation of hearing protectors. ANSI S12.6-1984. New York ANSI, 1984. Behar A. Field evaluation of hearing protectors. Noise Control Eng 1985;24( 1): 13-1 8. Berger EH. Using the NRR to estimate the real world performance of hearing protectors. Sound Vib 1983;17( 1):12-18. Berger EH. Can real-world hearing protector attenuation be estimated using laboratory data? Sound Vib 1988;22(12):26-31. Edwards RG and Green WW. Effect of an improved hearing conservation program on earplug performance in the workplace. Noise Control Eng 1987;28(2):55-65. Environmental Protection Agency. Noise labeling requirements for hearing protectors. Federal Register 1979;44( 190), 40CFR Part 21:56 120-56 147.
Erlandsson B, Hakanson H, lvarsson A, and Nilsson P. Noise dose measurements inside and outside earmuffs on shipyard workers. In Lawrence R, Ed. Proceedings of Inter-Noise 83, Edinburgh, U K
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Institute of Acoustics. 1983:879-882. Gasaway DC.Hearing protection guide directs users to manufacturers/devices by category. &cup Health Saf 1988;May: 33,35.36,38,40,42.44,46,47,50.5 I . Hughson W and Westlake H. Manual for program outline for rehabilitation of aural casualties both military and civilian. Trans Am Acad Ophthalmol Otolaryngol 1944;48(SuppI):1-15. Lempert BL and Edwards RG. Field investigationsof noise reduction afforded by insert-type hearing protectom. Am Ind Hyg Assoc J 1983;44(12):894-902. Mendez AM, Salazar EB, and Bontti HG. Attenuation measurement of hearing protectors in workplace. 12th International Congress on Acoustics, Toronto, 1986: Vol. 1, paper B10-2. NHCA (National Hearing Conservation Association). NHCA develops position on mining regulations. Spectrum 1990;7(3):13. Pekkarinen J. Industrial impulse noise, crest factor and the effects of earmuffs. Am Ind Hyg Assoc J 1987;48(10):861-866. Royster JD and Royster LH. Reducing your compensation liability for noise-induced hearing loss. In Hearing Conservation Programs: Practical Guidelines for Success. Chelsea, MI: Lewis Publishers, Inc.. 1990:llO.
Smoorenburg GF. ten Raa BH, and Mimpen AM. Real-world attenuation of hearing protectors. 12th International Congress on Acoustics, Toronto. 1986: Vol. I , paper B9-6.
Acknowledgments: Appreciation is given to Edward F. Krieg, Ph.D., for assistance with the statistical analysis, Major Phoebe Fisher (US. Air Force) for assistance with data collection and processing, Ms. Peggy Smith for editing, and Ms. Linda Can for typing of the manuscript. Mention of the name of any company or product does not constitute endorsement by the National Institute for Occupatiial Safety and Health. Address reprint requests to Card J. Merry, National Institute for Occupational Safety and Health, Physical Agents Effects Branch, Bioacoustics and Occupational Vibration Section, MS-27,4676 Columbia Parkway, Cincinnati, OH 45226. Received August 2,1991; accepted November 1.1991.
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Ear and Hearing, Vd. 13, No. 1,1992
The Effect of Fitting Procedure on Hearing Protector Attenuation Carol J. Merry, BS; Curt W. Sizemore; John R. Franks, PhD Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, Division of Biomedical and Behavioral Science, Physical Agents Effects Branch, Cincinnati, Ohio
ABSTRACT Realistically evaluating the effectiveness of hearing protectors continues to be a major problem in hearing conservation. The purpose of this study was to examine a laboratory-based fitting procedure (User Fit) that was designed to yield hearing protector attenuation values similar to that derived from field studies. Ten subjects who were naive to hearing protectors were used in a repeatedmeasures design that measured real ear attenuation at threshold for two types of plugs. Each subject was tested in two fitting conditions that varied based on the type and degree of assistance given to the subjects by the experimenter. The results showed significant differences in attenuation based on the fitting procedure used, with the User Fit best approximating field data. In addition, a generalized learning effect was noted. The results suggest that any experience with earplugs leads to subsequent improvement in attenuation despite the type of earplug used. Further testing is planned with greater numbers of subjects and additional types of hearing protectors (Ear Hear 13 1:ll-
18).
IT IS WELL DOCUMENTED that workers on the job frequently do not obtain the attenuation from their hearing protectors that they would expect based on the manufacturer’s noise reduction rating (NRR) that is printed on the protector packaging. One possible reason for the reduced noise attenuation may be that the overwhelming majority of workers are unable or unwilling to wear their protectors properly. A second possibility is that the NRR is an unrealistic indication of the protective ability of hearing protectors. Berger (1983) summarized 10 studies, including a National Institute for Occupational Safety and Health (NIOSH) work site study conducted by Lempert and Edwards ( 1983), that examined noise attenuation of various hearing protectors as they are worn at the job site in Ear and Hearing, Vol. 13, No. 1,1992
over 50 different industries. These studies showed considerable differences between laboratory-derived hearing protector attenuation (as reflected in the manufacturer’s NRR) and workplace attenuation for a variety of hearing protectors. The laboratory data were found to consistently overestimate the attenuation actually obtained by workers in industry. More recently, Berger (1988) reported that studies by Erlandsson, Hakanson, Ivarsson, and Nilsson (1983), Behar (1985), Mendez, Salazar, and Bontti (1986), and Smoorenburg, ten Ra, and Mimpen (1986), Edwards and Green (1987), and Pekkarinen ( 1987) also indicated substantial discrepancies between the attenuation values measured in the workplace and the standardized laboratory-based estimates of hearing protector attenuation that are currently used to assign an NRR to every hearing protector sold in the United States. The laboratory procedure currently used to assign an NRR to a protector is a carefully controlled, experimenter fit procedure. It results in a “best possible” attenuation rating for hearing protectors that may not be applicable to the work site. It has been suggested that a more realistic laboratory procedure is needed to derive an NRR that is meaningful to workers. Berger (1988) implied that the discrepancy between work site and laboratory attenuation values of hearing protectors may be minimized by altering certain test practices in the laboratory procedures. In particular, he suggested that the type of subjects used (trained versus novice), as well as the characteristics of the laboratory fitting procedure used (i.e., the degree of experimenter intervention, the level of subject training, and the motivation of the subjects) could have a major impact on measured noise attenuation. This article describes a hearing protector attenuation study in which the effects of different fitting procedures were examined. These effects were measured for two types of ear plugs across two fitting procedures: a user fit (UF), in which only the manufacturers’ instructions were available to the subjects, and an informed user fit (IUF), in which substantial fitting assistance was given to the subjects by the experimenter. The UF and IUF procedures used in this study were developed by American National Standards Institute (ANSI) working group S 12/WG 1 1, “Field Effectiveness and Physical Characteristics of Hearing Protectors.” This is the working group that has been established to directly address
0196/0202/92/1301-0011$03.00/0 EARAND HEARING Copyright 0 1992 by Williams & Wilkins
Printed in the U.S.A
Effect of Fitting Procedure
11
the issue that laboratory-based attenuation data consistently overestimate attenuation data collected in field studies. Their charge is to develop a reproducible, laboratory-based testing protocol that will yield results which more realistically approximate the attenuation obtained by typical motivated workers on the job. The study described in this paper is the NIOSH evaluation of the initial ANSI working group test protocol. The data from this study were compared to the NIOSH work site study data (Lempert & Edwards, 1983) as well as to the attenuation data provided by the manufacturer or printed on the hearing protector packages. It was hypothesized that the attenuation values obtained from the two fitting procedures would differ significantly based upon the degree of instruction and fitting assistance given to subjects, with subjects receiving better attenuation in the IUF condition than in the UF condition. Because workers on the job may receive very little instruction/reinstruction or assistance with hearing protector fitting on a day to day basis at the typical work site, the attenuation values measured in the lab by the UF procedure might best approximate the work site data. METHOD
Subjects Ten subjects were paid to participate in this study. Six females and four males (ages 21-36) were recruited predominantly from two universities in Cincinnati, OH. An effort was made to draw from a broad subject pool by having few selection criteria. The subjects were primarily required to be normal-hearing, novice users of hearing protectors. Subjects responded to posted advertisements for subjects and met the following criteria: 1. Pinnae and ear canals: free of pathology as noted by
otoscopic exam. 2. Little prior experience with hearing protectors. This meant subjects stated they had not worn earplugs for any purpose, including swimming or sleeping more than five times, and they had never received instruction in the use of hearing protectors. 3. Ability to pass a screening audiogram with hearing threshold levels not in excess of 25 dB at 500 to 8000 Hz. 4. Ability to read small print typically found on hearing protector packages. Subjects were accepted into the study as they applied and passed the screening criteria noted above. Three male applicants were rejected because they had too much experience wearing hearing protectors. Otherwise, all applicants were accepted until a test population was obtained that was comprised of approximately equal numbers of males and females (&lo%). Hearing Protectors The E-A-R formable foam plug and the V5 I R premolded, plastic plug have been included in various field and laboratory investigations in the past. They were selected for this study so that comparisons to existing attenuation literature would be possible. The E-A-R is a unisize, expandable foam plug that 12
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must be compressed and rolled into a thin cylinder by the user before insertion into the ear canal. The V5 1R is a roundtipped, single flange, soft plastic plug that is available in various sizes from extra small to extra large. All five sizes were available to the subjects at each fitting. The V5 I R plugs used in this study were not color-coded by size, thus, subjects selected the appropriate size based solely on fit. To use this plug, a user must select the proper size for each ear canal and then insert the device by twisting the plug into the ear canal until a seal is obtained. Apparatus Testing was carried out in a double-walled, double-floored reverberant chamber (Tracoustics RE245) that met the requirements of both ANSI S3.19 (1 974) and ANSI S 12.6 ( 1984) for a diffuse sound field with acceptable background noise levels. Test stimuli for the real ear attenuation at threshold (REAT) measurements were */3 octave bands of noise presented as three pulses within a 200 msec period, with a 20 msec rise and fall interspersed with a 200 msec off time. Pink noise generated by a Grason-Stadler Noise Generator was fed into a Hewlett-Packard Filter Set to yield seven Y3 octave bands of noise centered at 125 Hz, 250 Hz, 500 Hz, 1 kHz, 2 kHz, 4 kHz, and 8 kHz. The I/3 octave bands were fed into a custom control box developed at NIOSH. (The schematics for the prototype control box are available upon request from the authors.) For each threshold measurement, the selected test band was gated by a Wilsonics Cosine Switch and intensity was controlled by a Wilsonics Programmable Attenuator. Output of the programmable attenuator was passed to a Hewlett-Packard 20 dB Gain Preamplifier and then to an Altec Power Amplifier. The power amplifier output was sent through a passive 20 dB T pad attenuator to the loudspeakers in the chamber. This equipment array is diagrammed in Figure 1. When appropriate, a fitting noise was generated by a second Grason-Stadler Noise Generator, sent to a General Radio Multifilter, shaped, and, as needed, switched into the HewlettPackard 20 dB Preamplifier, which fed through the Altec Power Amplifier and then passed through the passive 20 dB T pad attenuator to the loudspeakers in the chamber (see Fig. 1). The low-pass-filtered pink fitting noise was presented at an A-weighted SPL of 60 dBA. The subject responded to the stimulus by triggering a silent optical hand switch, which was developed at NIOSH for this project. The subject’s response was registered by the computer, which determined threshold automatically to the nearest decibel after a modified Hughson-Westlake audiometric procedure (Hughson & Westlake, 1944). Hearing protector attenuation was calculated using REAT procedures and the equipment described above. In REAT testing, a subject’s binaural threshold of hearing is measured with and without hearing protection. The difference between the two thresholds is the measure of the hearing protector’s attenuation. Each day before testing began, the equipment and sound field were calibrated using a Briiel and Kjaer (B & K) 4145 low noise microphone and a 2627 power amplifier. The pickup was displayed on a B & K 2131 digital frequency analyzer. The system was calibrated with a B & K 4220 piston phone. Procedures On the first day of testing, subjects participated in an initial audiometric threshold screening and a minimum of five pracEar and Hearing, Vol. 13, No. 1,1992
Compaq Deskpro Computer
Computer Interface
Hewlett-Packard 8056A Filter Set
General Radio 1382 Noise Generator
Wilsoni~BSIT Cosine Switch
Wilsonics PATT Attenuator
I
- -I
Fitting Noise Switch
I
General Radio
1925 Multifilter
General Radio I381 Noise Generator
Figure 1. Schematic of equipment and sound field.
I
Hewlett-Packard 465A Preamplifier
Altec W A Power Amplifier
Tracoustics RE-245
DoublcWalllDouble Floor Room
tice sound field threshold tests. Subjects were required to demonstrate repeatability of their audiometric thresholds to within 6 dB at each frequency on their last three practice runs. Subjects then came to the laboratory on 4 additional days. Each visit was divided into two 1 h sessions so that repeated measures on both earplugs were obtained each day. The UF protocol was administered during the first and ihird test days and the IUF protocol was administered during the second and fourth test days. Presentation order of the occluded (plugs in) and unoccluded (no plugs) conditions and the presentation order of the two earplugs were counterbalanced across the 10 subjects. In the UF protocol, before the attenuation testing, subjects were simply handed the hearing protectors along with the manufacturer’s instructions as printed on the package and as noted on the dispensing box. They were instructed to practice (up to a maximum of 5 min) with the protectors until they believed them to be properly inserted into their ears. The subjects received no verbal or physical assistance from the experimenter. At the conclusion of the practice period, subjects removed their plugs and were escorted into the test chamber, where they reinserted them for testing. Subjects remained in the chamber for two complete sets of open and occluded thresholds for that plug. Before each insertion of the protectors for threshold testing, subjects could refer to the written instructions; but again, no assistance or feedback was given by the experimenter. Session A ended when the testing of the first plug was complete. Subjects exited the chamber Ear and Hearing, Val. 13, No. 1,1992
for a brief rest period; then session B began with the second plug and the same unassisted fitting process. In the IUF protocol, subjects were given the hearing protector along with the same manufacturer’s written instructions as in the UF condition. In addition, during the practice fitting of the protector, the experimenter facilitated the fitting by offering verbal and physical assistance in adjusting the plugs. In the test chamber before each occluded threshold, subjects refitted hearing protectors by themselves using the written instructions. At this point, the experimenter was not permitted to offer advice or feedback to the subjects. A fitting noise was introduced into the chamber and subjects were instructed to use this noise in order to obtain maximum attenuation from the plugs. (No fitting noise was available in the UF condition because it was designed to yield values approximating work site data. It was felt that a fitting noise in a laboratory setting would permit subjects to attain attenuation values that would be too high to meet this goal.) As in the UF condition, two complete sets of open and occluded thresholds were obtained for each plug. RESULTS
The mean attenuations and standard deviations for the E-A-R plug and the V5 1R plug, as measured in this study, are shown in Table 1. This was a repeated measures design where each subject was tested four Effect of Fitting Procedure
13
times with each plug in each fitting procedure. The events cannot be considered independent; therefore, for statistical accuracy, the standard deviations are computed using the subject means (N = 10) at each frequency. Overall, there was a statistically significant difference between the IUF and UF procedures, with the IUF yielding larger attenuation values than the UF (F[ 1,9] = 7.92, p < 0.02). Additionally, there was a statistically significant difference between the attenuation of the two plugs in general (F[ 1,9] = 41.68, p < 0.0001) and in relation to the two fitting procedures (F [ 1,9] = 11.50, p < 0.008). Figure 2 displays the data for both plugs plotted according to fitting procedure. It can be noted that the E-A-R plug provides more attenuation than the V51R plug in both the IUF and the UF procedures, but there is less of a difference between IUF and UF with the E-A-R than with the V5 1R. A significant “day” effect was noted, in that subjects received more hearing protector attenuation in both fitting conditions the second day they worked with the protectors (F [1,9] = 7.61, p < 0.02). The effect was not differentiated by plug, trial, or fitting technique, and may be attributed to a generic learning effect in which the novice subjects quickly became familiar with earplugs in general. The manufacturers of the hearing protectors used in this study indicate on their packaging that these plugs offer more protection at the higher frequencies tested
than the lower ones. We found this was true for both plugs regardless of fitting procedure. However, as noted in Figure 3, subjects were able to obtain approximately 50% better attenuation at each frequency with E-A-R earplugs than with the V5 1R. Many second and third order comparisons were not statistically significant and are noted in the analysis of variance summary table, Table 2. In particular, the fit x day interaction was not significant, somewhat alleviating concern about the alternating UF-IUF procedure. Originally, there was interest in knowing if a moderately experienced panel of subjects could be used in the UF procedure instead of totally novice subjects. This was a logistical consideration, as many laboratories maintain a stable panel of listeners for hearing protector testing. Thus, the study was designed to “create” a panel of experienced listeners in a controlled and consistent way. When the subjects returned on the third day for their second UF test session, they were considered “experienced.” The approach was problematic. As expected, there was a significant overall learning effect, but some inconsistencies were seen; perhaps this was based on the different degrees of difficulty inherent in fitting each type of plug. In any revised procedure, only novice subjects will be used and all UF conditions will be tested before subjects are given any instructions for the IUF conditions. DISCUSSION
Table 1. Real ear attenuation at threshold by fitting technique and plug. Mean attenuation in decibels. Standard deviations” based on N = 10.
125 E-A-R UF E-A-R IUF V51R UF V51R IUF
250
500
1000
2000 4000 8000
20(7) 21 (6) 23(7) 24 (5) 28(5) 39 (5)40(7) 22 (6) 24 (7) 26 (7) 25 (6) 30 (5) 40 (4)40 (6) 9 (9) 9 (9) lO(9) 13 (10)19 (10)24 (8)16 (12) 15(11) 15(10) 14(10) 18(9) 23(9) 25 (7)24(10)
Figures in parentheses show standard deviations.
The attenuation values noted from the two conditions used in this study (UF and IUF) were compared to the attenuation values received by workers in the unsupervised self fit of the NIOSH work site study, and the attenuation values derived from the highly supervised experimenter fit of the ANSI S3.19-1974 procedure. The subjects in this study were primarily college graduate students. It is possible that they, and the subject panels used to perform the ANSI S3.19- 1974 procedure, differ from industrial workers on their mo-
Figure 2. Comparison of the attenuation for the E-A-R and V51 R earplugs in both fitting conditions.
125
14
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4000
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Ear and Hearing, Vol. 13, No. 1,1992
E-Afl PLUG
v5l-R PLUG
m
0 45
@ 40 U
*0
35
$30
E 4
9 9
r-l
25 20
15
Ll I5
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Figure 3. Comparison of the overall frequency-specific attenuation for both earplugs collapsed across subjects, fitting conditions, and trials.
II 4000
0 FREQUENCY W HERTZ Table 2. Analysis of variance summary table. Source
df
Sumsof Squares
Mean Squares
F
Pr > F
Repeated measures subject effects 1 40916.19 40916.19 41.68 0.0001 Plug 9 8835.36 981.71 Enor (plug) 1 3150.68 3150.68 7.92 0.0202 Fit technique 9 3578.94 397.66 Error (fit) 1 1863.02 1863.02 7.61 0.0221 Day 9 2202.28 244.70 Error (day) Plug :fit 1 856.63 856.63 11.50 0.0080 74.49 Error (plug * fit) 9 670.42 Nonsignificant effects 0.98 0.3474 Trial (session A/B) Plug :Day 0.10 0.7642 Fit :Day 0.02 0.8960 0.05 0.8232 Plug * Trial Fit * Trial 0.81 0.3925 Day :Trial 0.11 0.7439 0.23 0.6409 Plug :Fit :Day Plug :Fit * Trial 2.63 0.1396 1.11 0.3191 Plug * Day :Trial Fit :Day :Trial 0.90 0.3673 Plug * Fit * Day :Trial 0.08 0.7856
tivation to carefully follow the manufacturer’s instructions. However, because the instrumentation parameters in these studies were acoustically similar, it is reasonable to suggest that to some degree, the differences in attenuation are the result of differences in fitting procedure. The mean attenuations and standard deviations from the manufacturers, the NIOSH work site study, and this study are included in Table 3. Because the manufacturers and NIOSH calculated their standard deviations based on the Environmental Protection AgencyEar and Hearing, Vol. 13, No. 1, 1992
8000
mandated technique that uses the grand mean (EPA, 1979), the standard deviations for the data in this study were recalculated based on N = 40. Although statistically incorrect, this is the traditionally used method in attenuation testing and it is appropriate in this case for purposes of comparison. Both the IUF and UF groups received substantially less attenuation than the manufacturers’ published data. Additionally, both groups received better attenuation than that reported for these two plugs in the NIOSH work site data (Lempert & Edwards, 1983). Hearing protector manufacturers use a fitting technique (ANSI S3.19-1974) that was developed, in part, to standardize procedures so that psychoacoustic testing of hearing protectors on human subjects could be uniformly performed and accurately replicated with little interlaboratory variability. Thus, the procedure could be used to create a labeling scheme for hearing protectors. Although the ANSI S3.19-1974 standard technically allows for an experimenter fit or an experimentersupervised subject fit, the experimenter fit is the procedure generally used because it is called for by the EPA in the determination of the NRR (EPA, 1979). In experimenter fit, the investigator personally fits the hearing protectors for the subject before each occluded threshold. Three separate trials must be made pairing open and occluded threshold measurements, with each trial requiring a refit of the hearing protector on the subject by the experimenter. The test results are summarized for each hearing protector tested in terms of a grand mean and standard deviation of attenuation at each of the nine test frequencies. This is derived by computing the mean value in decibels of the occluded threshold with the hearing protector in place minus the open threshold for all listeners on all trials under identical test conditions ( N = 30). When reporting the results, the standard permits the experimenter to exEffect of Fitting Procedure
15
Table 3. Real ear attenuation data in decibels: E-A-R plug and V51 R plug (UF/IUF standard deviations' recalculated with N = 40 for Comparison).
V51 R Manufacturer IUF UF NIOSH E-A-R Manufacturer IUF UF NIOSH
125
250
500
1000
2000
4000
8000
NRR
20 (2) 15 (11) 9 (10) 8 (11)
22 (2) 15 (10) 9 (10) 8 (9)
24 (2) 14 (11) 10 (10) 6 (10)
28 (2) 18 (10) 13(11) 10 (10)
34 (2) 23 (10) 19 (11) 21 (12)
37 (3) 25 (8) 24 (10) 19 (9)
37 (3) 24 (11) 16 (13) 15 (10)
23 -3 -9 -9
37 (6) 22 (7) 20 (8) 15 (9)
41 (5) 24 (7) 21 (7) 15 (8)
45 (3) 26 (8) 23 (9) 16 (7)
44 (4) 25 (7) 24 (7) 18 (8)
36 (5) 30 (5) 28 (6) 28 (10)
43 (3) 40 (5) 39 (7) 33 (9)
47 (3) 40 (7) 40 (9) 27 (9)
29 13 10 3
Figures in parentheses show standard deviations.
-
UF ,--+--+
IUF
MANUFACTURER'S DATA
...... +......
NOSH FIELD DATA
*.-.*.-.+
E-A-R Figure 4. E-A-R earplug. Compari-
2 HI
son of attenuation from the four fitting procedures.
Eu
30
a
U w 40
125
250
500
1000
2000
4000
8000
FREQUENCY IN HERTZ
clude data from a subject for whom the experimenter cannot obtain an adequate fit of the hearing protector. The NIOSH study (Lempert & Edwards, 1983) reported on workplace investigations that determined the actual noise reduction achieved by workers using several types of earplugs in various industrial settings. Testing was carried out in a specially instrumented mobile van on the work sites by using circumaural headphone sets that could be worn over earplugs without touching the earplugs or disturbing their fit. Data on the foam plug were provided by 56 subjects, whereas 84 subjects provided data on the V51R plug. The occluded threshold was determined first, followed by the open threshold. Means and standard deviations of attenuation were calculated by pooling the data from all subjects for each plug in order to provide results comparable to ANSI S3.19 summary statistics. When the manufacturers' data (experimenter fit) are compared to the NIOSH work site data (Table 3), it is clearly evident that the attenuation obtained with these two plugs in the workplace is substantially less than 16
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their laboratory-derived, experimenter fit values. Because this experimenter fit procedure does not give a reasonable estimate of hearing protector performance for the average user, hearing conservation managers are advised to routinely derate the published NRRs by 50% in an effort to more realistically compare products for their employees (NHCA, 1990; Royster & Royster, 1990). Unfortunately, this is not a satisfactory solution because an across the board 50% derating ignores the differences between types of protectors. A 50% derating would be too much for an earmuff and perhaps not enough for the V5 1R earplug. When the data from this study, the NIOSH work site data, and the manufacturers' data are plotted for each plug (Figs. 4 and 5), it appears that the UF procedure used in this study with the E-A-R and V51R plugs shows promise as a more realistic indicator of the amount of attenuation that can be expected by average users. The manufacturers' data are derived by testing a minimum of 10 subjects; the carefully controlled exEar and Hearing, Vol. 13, No. 1, 1992
IUF
UF ,--+
t---c.
:
NIOSH FIELD DATA
MANUFACTURERS DATA
---,
0
..... .....0
* . + a +
20
T
e
YD 0
.................+................. ..................*.................. .................................... .*
*.
Figure 5. V51 R earplug. Compari-
son of attenuation from the four fitting procedures.
T
125
250
.-*............
......+.................*
500 1000 2000 FREQUENCY IN HERTZ
perimenter fit procedure in that protocol results in less variability than that of the UF and IUF procedures of this study. Further testing of the revised UF and IUF procedures with more than 10 subjects is planned to see if a larger N will result in decreased variability. Also to be considered is the learning effect: any laboratory experience with earplugs seems to affect subsequent attenuation. Perhaps the laboratory procedure best approximating real world attenuation values will be a modified UF procedure in which many novice subjects test the plugs only once. Unfortunately, the logistic and economic difficulty with such a protocol would be great. Gasaway ( 1988) lists approximately 280 different types of hearing protectors that are currently on the market. Should a new standard such as the UF protocol be adopted, manufacturers may have difficulty finding enough novice subjects for testing. Certainly, it is important for hearing conservation professionals to continue to emphasize better supervision and education of the workers in their programs. Unfortunately, studies at NIOSH and elsewhere seem to indicate that consistent, effective use of hearing protectors by workers on a day to day basis does not always occur. The factors affecting a hearing protector's effectiveness include both the physical characteristics and fitting (demands) of the protector as well as the physical characteristics and motivation of the user. One way to potentially improve protection for users is to assure that hearing protectors on the market are rated by a method that takes the human motivation component into consideration. Thus, the significance of developing a laboratory methodology that will accurately predict hearing protector effectiveness in the workplace is profound. Workers and hearing conservation managers who select hearing protection based on current NRR labeling may Ear and Hearing, Val. 13, No. 1,1992
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8000
be disappointed to find that the devices, in the hands of the average person, do not perform nearly as well as the packaging indicates. Until a laboratory procedure has been validated that correlates well with workplace data, perhaps it would be prudent to derate the NRR and select hearing protectors based on an analysis of the type of noise encountered and on the attenuation values that have been published thus far in work site studies. It would be helpful if an updated compendium was published of those hearing protectors for which actual work site attenuation data are available. A hearing conservation manager could consult such a reference and take the workplace attenuation values into consideration as an additional factor influencing the selection of hearing protectors for employees. REFERENCES American National Standards Institute. Method of the measurement of real-ear protection of hearing protectors and physical attenuation ofearmuffs. ANSI S3.19-1974. New York: ANSI, 1975. American National Standards Institute. Method for measurement of the real-ear attenuation of hearing protectors. ANSI S12.6-1984. New York ANSI, 1984. Behar A. Field evaluation of hearing protectors. Noise Control Eng 1985;24( 1): 13-1 8. Berger EH. Using the NRR to estimate the real world performance of hearing protectors. Sound Vib 1983;17( 1):12-18. Berger EH. Can real-world hearing protector attenuation be estimated using laboratory data? Sound Vib 1988;22(12):26-31. Edwards RG and Green WW. Effect of an improved hearing conservation program on earplug performance in the workplace. Noise Control Eng 1987;28(2):55-65. Environmental Protection Agency. Noise labeling requirements for hearing protectors. Federal Register 1979;44( 190), 40CFR Part 21:56 120-56 147.
Erlandsson B, Hakanson H, lvarsson A, and Nilsson P. Noise dose measurements inside and outside earmuffs on shipyard workers. In Lawrence R, Ed. Proceedings of Inter-Noise 83, Edinburgh, U K
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Institute of Acoustics. 1983:879-882. Gasaway DC.Hearing protection guide directs users to manufacturers/devices by category. &cup Health Saf 1988;May: 33,35.36,38,40,42.44,46,47,50.5 I . Hughson W and Westlake H. Manual for program outline for rehabilitation of aural casualties both military and civilian. Trans Am Acad Ophthalmol Otolaryngol 1944;48(SuppI):1-15. Lempert BL and Edwards RG. Field investigationsof noise reduction afforded by insert-type hearing protectom. Am Ind Hyg Assoc J 1983;44(12):894-902. Mendez AM, Salazar EB, and Bontti HG. Attenuation measurement of hearing protectors in workplace. 12th International Congress on Acoustics, Toronto, 1986: Vol. 1, paper B10-2. NHCA (National Hearing Conservation Association). NHCA develops position on mining regulations. Spectrum 1990;7(3):13. Pekkarinen J. Industrial impulse noise, crest factor and the effects of earmuffs. Am Ind Hyg Assoc J 1987;48(10):861-866. Royster JD and Royster LH. Reducing your compensation liability for noise-induced hearing loss. In Hearing Conservation Programs: Practical Guidelines for Success. Chelsea, MI: Lewis Publishers, Inc.. 1990:llO.
Smoorenburg GF. ten Raa BH, and Mimpen AM. Real-world attenuation of hearing protectors. 12th International Congress on Acoustics, Toronto. 1986: Vol. I , paper B9-6.
Acknowledgments: Appreciation is given to Edward F. Krieg, Ph.D., for assistance with the statistical analysis, Major Phoebe Fisher (US. Air Force) for assistance with data collection and processing, Ms. Peggy Smith for editing, and Ms. Linda Can for typing of the manuscript. Mention of the name of any company or product does not constitute endorsement by the National Institute for Occupatiial Safety and Health. Address reprint requests to Card J. Merry, National Institute for Occupational Safety and Health, Physical Agents Effects Branch, Bioacoustics and Occupational Vibration Section, MS-27,4676 Columbia Parkway, Cincinnati, OH 45226. Received August 2,1991; accepted November 1.1991.
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