Experimenter trained / Inexperienced Subject Fit. ⢠ANSI S12.42-2009 MIRE and ATF Testing. Methods. ⢠Impulsive Noise methods / Continuous Noise. Methods.
Hearing Protection for Continuous Noise William J. Murphy, PhD Hearing Loss Prevention Team National Institute for Occupational Safety & Health Disclaimer: The findings and conclusions in this report are those of the authors and do not necessarily represent the views of the National Institute for Occupational Safety and Health. Products mentioned in this presentation do not constitute an endorsement by the CDC or NIOSH
Types of Hearing Protection Devices • Passive earmuffs • Passive earplugs • Passive semi aural insert (Canal Caps) • Active Noise Cancellation HPDs • Level Dependent Electronic HPDs • Level Dependent Passive HPDs
Standards and Regulations • United States • EPA Regulation 40 CFR 211B Labeling for Hearing Protective Devices • OSHA 1910.95 Hearing Conservation Amendment • Europe – ISO 4869 standards, EN 352 directives • Canada – CSA Z94.2-02 (2002) HPD Performance, Care, Selection and Use • Brazil – National Association of the Industry of Safety Materials and Work Protection 3/25/03 • Australia – AUS/NZS 1270:2002 Acoustics – Hearing Protectors 3
ASA Z24.22 Real Ear Attenuation at Threshold
• Tested with Pure Tones • Progressive Free-field • Anechoic Environment. • Minimum 10 subjects, 3 trials
4
Second Standard for REAT • ANSI S3.19-1974 • Real Ear Attenuation at Threshold • 1/3rd Octave Narrow Band Noise • Reverberant Environment • 10 Subjects, 3 Trials • Subject Fit (with instruction) • Experimenter Fit (Optimum fit)
5
Who’s responsible for the NRR? • 1979 US Environmental Protection Agency Noise Reduction Rating
• Established the ANSI S3.19-1974 as THE method. • Selected Experimenter Fit • Chose the NIOSH NRR computation Method 2 from Kroes et al. 1976, a C-weighted statistic • Uses a Pink Noise for the determination of the protector performance
6
International Standards Organization • 4869-1 Subjective method for measurement of sound attenuation • 4869-2 Estimation of effective A-weighted sound pressure levels when hearing protectors are worn. • 4869-3 Simplified method for the measurement of insertion loss of ear-muff type protectors for quality inspection • 4869-4 Measurement of effective sound pressure levels for level-dependent sound restoration ear-muffs. • 4869-5 Method for estimation of noise reduction using fitting by inexperience test subjects • 4869-6 Determination of attenuation of active noise reduction earmuffs 7
EU Hearing Protector Directives • EN 352-1 - Ear muffs • EN 352-2 - Ear plugs • EN 352-3 - Ear muffs attached to a safety helmet • EN 352-4 - Ear muffs attached to a safety helmet • EN 352-5 - Active noise reduction ear muffs • EN 352-6 - Ear muffs with electrical audio input • EN 352-7 - Level dependent ear plugs • EN 352-8 - Entertainment audio earmuffs
8
American National Standards • ANSI S12.6-1984 REAT Testing Methods • Experimenter Supervised
• ANSI S12.6-1997 REAT Testing Methods • Exp-Supervised / Naïve Subject Fit
• ANSI S12.68-2007 Rating Methods • Noise Reduction Rating Statistic and Graphical Method
• ANSI S12.6-2008 REAT Testing Methods • Experimenter trained / Inexperienced Subject Fit
• ANSI S12.42-2009 MIRE and ATF Testing Methods • Impulsive Noise methods / Continuous Noise Methods 9
Hearing Conservation Program • OHSA 1910.95 • Employers shall provide a hearing conservation program to workers at no cost if they are exposed at levels of 85 dBA TWA or more than 50 percent of the dose at 90 dBA. • Workers exposed above 85 dBA TWA will be provided hearing protection • The employer shall establish and maintain an audiometric testing program as provided in this paragraph by making audiometric testing available to all employees whose exposures equal or exceed an 8-hour time-weighted average of 85 decibels.
Audiometric Monitoring • "Annual audiogram." At least annually after obtaining the baseline audiogram, the employer shall obtain a new audiogram for each employee exposed at or above an 8-hour time-weighted average of 85 decibels. • If the annual audiogram shows that an employee has suffered a standard threshold shift, the employer may obtain a retest within 30 days and consider the results of the retest as the annual audiogram. • If a comparison of the annual audiogram to the baseline audiogram indicates a standard threshold shift as defined in paragraph (g)(10) of this section has occurred, the employee shall be informed of this fact in writing, within 21 days of the determination.
Audiometric Evaluation & HPDs 1910.95(g) • Employees not using hearing protectors shall be fitted with hearing protectors, trained in their use and care, and required to use them. • Employees already using hearing protectors shall be refitted and retrained in the use of hearing protectors and provided with hearing protectors offering greater attenuation if necessary.
Standard Threshold Shift • As used in this section, a standard threshold shift is a change in hearing threshold relative to the baseline audiogram of an average of 10 dB or more at 2000, 3000, and 4000 Hz in either ear. • In determining whether a standard threshold shift has occurred, allowance may be made for the contribution of aging (presbycusis) to the change in hearing level by correcting the annual audiogram according to the procedure described in Appendix F: "Calculation and Application of Age Correction to Audiograms."
How is attenuation measured? • Real Ear Attenuation at Threshold • • • •
Frequency bands 125 to 8000 Hz 1/3rd Octave Band Noise Reverberant Room Diffuse sound field in the location of the subject’s head
• Measure the occluded & open ear thresholds • Compute the differences • Measure REAT for a pool of subjects • ANSI S12.6 - 10 to 20 subjects twice • ANSI S3.19 - 10 subjects three times • ISO 4869-1 – 16 subjects once
Passive Hearing Protectors (REAT) • Passive attenuation is measured with REAT • Electronic HPDs are tested while turned off. • Electronics may provide amplify sounds. • Electronics may increase background noise.
• Passive HPDs with filters need to be tested in all conditions. • HPDs that can be worn in different positions must be tested in all positions.
Total and Active Attenuation • First REAT measured for the sample pool. • Two methods can be used to estimate the active contribution Microphone In Real Ear (MIRE)
Acoustic Test Fixture (ATF)
Impulse Peak Attenuation • Measured using a set of high level impulses with an acoustic test fixture. • Impulse levels at 130, 150, 170 dB peak SPL. Acoustic Shock Tube
Acoustic Test Fixture
Different Hearing Protection Ratings • Noise Reduction Rating (NRR) • C-weighted Noise reduction
• Single Number Rating (SNR) • C-weighted Noise Reduction
• High/Middle/Low (HML) • A-weighted Noise Reduction (requires LC-LA)
• Noise Reduction Statistic for A-weighting (NRSA) • A-weighted Noise Reduction
• Noise Reduction Statistic Graphical (NRSG) • A-weighted Noise (requires LC-LA)
• Octave Band Calculation • Requires Octave Band Spectra 125 – 8000 Hz
Attenuation Data for Premolded Earplug
Noise Reduction Rating Freq Pink
125
250
500
1000
2000
4000
8000
Total
100.0
100.0
100.0
100.0
100.0
100.0
100.0
C weighting
-0.2
-0.0
-0.0
0.0
-0.2
-0.8
-3.0
Pink + Cwt
99.8
100.0
100.0
100.0
99.8
99.2
97.0
A weighting
-16.1
-8.2
-3.2
0.0
1.2
1.0
-1.1
Attenuation
27.0
27.6
31.2
29.5
34.0
36.7
42.2
Std Deviation
5.7
5.4
6.6
6.9
5.7
6.8
7.8
Assumed Prot Value
15.6
16.8
18.0
15.7
22.7
23.2
26.7
Pink + Awt – (Atten-2SD)
70.3
76.0
81.4
80.4
66.8
62.7
108.0
73.0
87.3
LC – LA Atten – 3
17.7
20
Single Number Rating Freq
63
125
250
500
1000
2000
4000
8000
Total
Pink
91.5
91.5
91.5
91.5
91.5
91.5
91.5
91.5
C weighting
-0.8
-0.2
-0.0
-0.0
0.0
-0.2
-0.8
-3.0
Pink + Cwt
90.3
91.3
91.5
91.5
91.5
91.3
90.7
88.5
A weighting
26.2
-16.1
-8.6
-3.2
0.0
1.2
1.0
-1.1
Attenuation
27.0
27.0
27.6
31.2
29.5
34.0
36.7
42.2
Std Deviation
5.7
5.7
5.4
6.6
6.9
5.7
6.8
7.8
APV 84
21.3
21.3
22.2
24.6
22.6
28.3
29.9
34.5
Pink + Awt – (Atten-1SD)
44.0
54.1
61.1
63.7
68.9
64.4
62.6
55.9
LC – LA Atten-3
99.5
72.2 27.3
21
NRSA Attenuation Freq NIOSH 100 Noise #33
125
250
500
1000
2000
4000
8000
Total
91
89
92
92
92
88
80
-16.1
-8.2
-3.2
0.0
1.2
1.0
-1.1
Noise +Awt
74.9
80.4
88.8
92.0
93.2
89.0
78.9
Attenuation
11.0
13.0
12.0
17.5
27.5
33.0
22.0
NAwt –Atten
63.9
67.4
76.8
74.5
65.7
56.0
56.9
A weight
97.4 79.5 17.9
Awt Atten
NRSA84 = 25.3 dB
NRSA50 = 26.2 dB NRSA16 = 35.6 dB
22
Premolded Earplug Rating Comparison
ANSI S12.68 NRS Graphical
NRSG
Estimating Exposure when HPDs are worn • OSHA/NIOSH Noise Reduction Rating • Lexp = LC – NRR • Lexp = LA – (NRR-7)
• ISO SNR • Lexp = LC – SNR
• ISO HML • PNR = M – (H-M)/4 (LC-LA-2) for LC-LA ≤ 2 • PNR = M – (M-L)/8 (LC-LA-2) for LC-LA ≥ 2 • Lexp = LA – PNR
• MIL-STD 1474E NRSA/NRSG • Lexp = LA – NRSA • Lexp = LA – NRSG
Four Example Noise Spectra
NRR EPA/OSHA/NIOSH
Performance Rating for Active HPD’s • Passive component – ANSI S12.6 / S12.68 • Experimenter-Trained Fit, Real Ear Attenuation at Threshold (REAT) • Active component – ANSI S12.42 • Microphone in Real Ear (MIRE) modified Active Noise Reduction devices • Measure with ANR turned ON and OFF • Passive REAT + Active MIRE • Performance rating determined with ANSI S12.68
Performance Rating for Active HPD’s • Active component – MIRE Testing Muffs • Miniature microphone centered in ear canal • Measure with the muff turned OFF • Measure with the muff turned ON • The difference in the octave band measurements will be used to estimate the Active component.
Performance Rating for Active HPD’s • Active component – MIRE Testing Plugs • Need to be tested on a mannequin. • Measure with the plug turned OFF • Measure with the plug turned ON • The difference in the octave band measurements will be used to estimate the Active component.
Combining REAT & MIRE data
Corrections for Bone Conduction
Differences ANSI and ISO ratings • ANSI required that the same subjects be used with REAT and MIRE measurements • ISO proposed that different subjects could be allowed, but the latest draft (11/9/2016) seems to have required the same subjects be used.
• ANSI required 5 samples of the protectors to be tested in a counterbalanced order over 10 or 20 subjects. • ISO has proposed 4 samples of the headsets be counterbalanced across 16 subjects
Active Noise Cancellation Headsets Bose A20
Pilot DNCXL PA1779T
Sennheiser HMEC250
Telex Stratus HeliXT
Bose Aviation X Headset
Pilot DNCXL PA1779T
Sennheiser HMEC250 Headset
Telex Stratus HeliXT
Four Example Noise Spectra
NRSA, NRSG, Oct. Band Protection Levels
NRSA, NRSG & Oct. Band Exposures
Level Dependent HPDs • What is the performance of these HPDs? • Some exhibit level dependent attenuation
• Some have level limiting electronics
Murphy & Tubbs 2007
Murphy et al. 2012
Murphy et al. 2015
Peltor™ TacticalPro™ Earmuff
3M™ Combat Arms™ earplug open
