Alternative Field Methods of Measuring Hearing Protector Performance John R. Franks, Dave A. Harris, Jennifer L. Johnson, and William J. Murphy
Introduction: The recently-approved ANSI S12.6-1997 standard for measuring the real-ear attenuation at threshold (REAT) of hearing protectors incorporates a true subject-fit method. The subject is given the device and manufacturer’s instructions and instructed to fit the device. The experimenter is not allowed to assist in fitting the device. The REAT is the difference between pairs of occluded and unoccluded thresholds using a third-octave narrow-band noise stimulus. The subject-fit method produces lower REAT estimates than the experimenter fit method. However, the measurements are consistent across laboratories and are more representative of real-world studies (Royster et al. 1996; Berger et al. 1998). The subject-fit method requires 20 subjects for earplugs and 10 subjects for earmuffs to determine a Noise Reduction Rating. Two protector testing methods have been proposed for use in occupational hearing loss prevention programs. FitCheck is a laptop unit which generates narrow-band noise stimuli presented via circumaural headphones. The Bone Conduction Loudness Balance method uses a 2000 Hz bone-conducted stimulus which the subject loudness matches to an air-conducted narrow-band noise. This poster reports the results of testing the E•A•R Pod Plug with five methods: FitCheck Headphone, Modified FitCheck Sound Field, Modified Bone Conduction Loudness Balance Subject Fit re ANSI S12.6-1997 Method B, and Experimenter Fit method re ANSI S3.19-1974.
History of Protector Testing Standards
1957
ANSI Z24.22: Anechoic room, pure-tone stimulus, free-field condition, single speaker. 10 subjects 3 paired trials per subject Experimenter-Supervised fit.
ANSI S3.19-1974: Third-octave band noise 1974
stimuli; diffuse soundfield. Min & Max reverberation times. Experimenter and Experimenter-Assisted protocols. 40 CFR 211 EPA: Experimenter Fit protocol
1979
for REAT measurements adopted for labeling standard. Noise Reduction Rating is standardized.
ANSI S12.6-1984: Third-octave band noise stimuli. Maximum reverberation times. 1984
Experimenter-Supervised Fit.
ANSI S12.6-1997: Experimenter-Supervised and Subject Fit Methods. 2 trials per 1997
subject/device. 10 subjects for muffs, 20 subjects for plugs.
Methods: Stimulus:SF, EF, FCH, MFCSF methods 1/3rd Octave narrow-band noise centered at 0.125, 0.25, 0.5, 1, 2, 3.15, 4, 6.3 and 8 kHz. MBCLB: 250-6300 Hz. Rooms: RE-245 Double Wall/Floor for EF, SF and MBCLB. 3 orthogonal panels of speakers Radio-Ear B-72 bone conduction transducer IAC-401 Single Wall/Floor for FCH and MFCSF. FitCheck headphones or 3 BOSE B-25 speakers, driven in series Stewart M-1 preamplifier and PA1400 Power amp Testing: SF, EF: Modified Hughson-Westlake procedure MBCLB - Bekesy Tracking with XY Plotter FCH and MFCSF - Proprietary Bekesy paradigm Subjects: 10 male and 10 female subjects aged 18-45 years old recruited through advertisements 3 Days of testing: Day 1: Qualification and Training Day 2: SF and 2 of (FCH, MFCSF or MBCLB) Day 3: SF, remaining method and EF Continuity of earplug placement was maintained throughout trials for subject-fit protocols. Two paired occluded/unoccluded thresholds were collected for the subject-fit methods.Three pairs were collected for the experimenter fit method Data Analysis: Model: a = m + f + s + r + m*f + s*f + s*r a=attenuation, m=method, f=frequency, s=subject gender, r=repeated measure, m*f =method by frequency, s*f=subject gender by frequency, s*r= subject gender by repetition.
Experiment configuration for testing with the FitCheck system. Audio stimuli are generated by a computer which controls the FitCheck attenuators. FitCheck could either drive headphones as it was designed, or could pass the signal to power amplifier which drove the Bose B-25 speakers. Although FitCheck was designed to test earplugs, the speaker arrangement permitted testing earmuff hearing protectors.
Results: 0
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40 FitCheck Headphone
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12 5 25 0 50 0 10 00 20 00 31 50 40 00 63 00 80 00 12 5 25 0 50 0 10 00 20 00 31 50 40 00 63 00 80 00
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Figure 1. Attenuations measured for five different methods. The data points represent the mean attenuation for each subject at each test frequency. The error bars denote one standard deviation about the grand mean indicated by the solid lines. The MFCSF data exhibited the least attenuation below 1000 Hz. Experimenter Fit data exhibited the smallest standard deviations while the FCH and MBCLB methods exhibited the largest.
Mean REAT Values and Standard Deviations 0 5
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10 15 20 25 Experimenter Fit FitCheck Headphone FitCheck Sound Field Bone-Conducted Loudness Balance Subject Fit, Day 1 Subject Fit, Day 2
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Figure 2. The mean attenuations and standard deviations are compared for each of the methods. The MBCLB attenuations yield the greatest attenuation at low frequencies and the least attenuation at high frequencies. The Experimenter Fit data tends to produce greater attenuation than the other methods (exclusive of the MBCLB) and small standard deviations. The other methods yield comparable attenuations and deviations.
Test of Fixed Effects Source NDF methods 4 frequency 6 subject 1 m*f 24 f*s 6 Contrast against S12.6 Method B data Source NDF MFCSF 1 FCH 1 BCLB 1 MFCSF/FCH 1
DDF 1340 1340 18 1340 1340
Type III F 2.91 92.93 0.19 9.50 2.33
Pr >F 0.0206 0.0001 0.6684 0.0001 0.0306
DDF 1340 1340 1340 1340
Type III F 2.37 7.30 0.05 9.36
Pr > F 0.1240 0.0070 * 0.8161 0.0023 *
* * * *
Statistical analysis of the subject-fit data without the 125 and 8000 Hz data indicates that significant effects were observed for the test method, stimulus frequency, method by frequency interaction and frequency by subject interaction. As well, the analysis also showed that the method which differed significantly from the SF method re: ANSI S12.6-1997 was the FCH. The FCH and MFCSF were significantly different from each other. The EF data was not included in the statistical analysis. Test of Fixed Effects Source NDF methods 3 frequency 8 subject 1 repetitions 1 m*f 24 f*s 8 s*r 1 Contrast against S12.6 Method B data Source NDF MFCSF 1 FCH 1 MFCSF/FCH 1
DDF 1375 1375 18 1375 1375 1375 1375
Type III F 2.84 119.30 0.16 0.03 5.12 1.85 2.45
Pr >F 0.0369 * 0.0001 * 0.6938 0.8671 0.0001 * 0.0641 0.1178
DDF 1375 1375 1375
Type III F 1.74 4.57 7.29
Pr > F 0.1876 0.0327 * 0.0070 *
Additional analysis without the MBCLB data confirmed the significant effects for method, frequency, and method by frequency interaction. The FitCheck Headphone method was significantly different than the ANSI S12.6 data and the MFCSF data. The FCH REAT estimates tended to be greater than the other methods.
Discussion: Modified Bone Conduction Loudness Balance Method Contrary to the method of Rimmer and Ellenbrecker (1997), the BC stimulus frequency tracked the noise stimulus frequency and was 40 dB above the airconducted pure-tone thresholds. Poor subject performance. –Large standard deviations, increased test time. Overestimated low frequency REAT. Underestimated high frequency REAT. Lacking 125 and 8000 Hz data. Modified Fit-Check Sound Field Method High frequency REATs consistent with ANSI S12.6-1997. Low frequency REATs underestimated, but not significant. Clipping, spectral splatter and room acoustics could have affected the REAT measurement. These factors were minimized in modifying the FitCheck system to perform a sound field test. If a FitCheck system were to be used in an occupational setting, modifications would include better signal generation and amplification suitable for driving speakers. Fit-Check Headphone Method Easily implemented in standard audiometric booth Elevated low-frequency REAT estimates –Possibly due to occlusion effect and head-noise. Adjustments necessary to compare to ANSI S12.6-1997 data.
Conclusions: The only method which proved to exhibit significant differences with the ANSI S12.6-1997 data was the FitCheck headphone. The differences are minor and could be corrected for if estimating a Noise Reduction Rating. Because time away from the job must be minimized, the time to administer a hearing protector test is of concern. The MBCLB method would be difficult to incorporate in a hearing loss prevention program due to the different psychophysical task and the instrumentation requirements. Similarly, the FitCheck Sound Field method is not yet feasible due to instrumentation requirements. The FitCheck Headphone method proved to be the easiest test to administer. With continued research using this method, correction factors accounting for the occlusion effect may be developed to permit better comparison with standard REAT techniques.
Correspondence: William J. Murphy, Ph.D. John R. Franks, Ph.D. NIOSH, Mailstop C-27 NIOSH Mailstop C-27 4676 Columbia Parkway 4676 Columbia Parkway Cincinnati, OH 45226-1998 Cincinnati, OH 45226-1998
[email protected] [email protected] 513-533-8125 513-533-8151
