SachsFelix

Information about SachsFelix

Published on February 26, 2008

Author: Maitane

Source: authorstream.com

Content

Slide1:  Hearing protector fit and usage considerations in impulse noise damage risk criteria Felix Sachs Hearing Conservation Program DSN 584-3797 (410) 436-3797 [email protected] 25 April 2001 U.S. ARMY CENTER FOR HEALTH PROMOTION & PREVENTIVE MEDICINE Hearing protector fit and usage considerations:  Hearing protector fit and usage considerations In steady noise Attenuation measurement Use and misuse in exposure estimates In current impulse noise criteria CHABA criteria MIL-STD-1474 Artillery in acoustic free field A-weighted energy In the AHAAH model Hearing protector modeling Exposure estimates Slide3:  Basic but fundamental Firearms produce hazardous noise at the operator position Soldiers’ impulse noise exposure is occupational. Soldiers presumed to be wearing hearing protective devices (HPD) in training Hearing conservation objective- at least 95% of soldiers will sustain no more than a mild hearing loss after a 20-year career Slide4:  Army HPD HPD attenuation:  HPD attenuation Measurement Real ear attenuation at threshold (REAT) Acoustic text fixture (ATF) Microphone in real ear (MIRE) Protector fit Experimenter-supervised fit (ANSI S12.6) Subject fit (ANSI S12.6 method B) Trained subject fit (as soldiers are supposed to be) As-trained user fit (as soldiers are) Slide6:  HPD attenuation variation with training Yellow foam earplug, 4 groups of 10 subjects, 3 measurements each (Goldstein et al, USAARL 1980) Standard deviations Mean attenuations 50 30 20 0 10 40 Who uses what and why:  Who uses what and why Mandatory EPA label: NRR (a) C weight 100 dB octave band (pink) noise (b) A weight 100 dB pink noise NRR = (a) – {(b) - (mean expert fit attenuation - 2 std dev)} OSHA: one half the dB of NRR Army health hazard assessment Assumes trained user Shares responsibility between user and developer Population-based HPD effectiveness Actual noise exposure spectrum Mean trained user fit attenuation If not known, expert fit minus 1 standard deviation Slide8:  Attenuation histogram User fit, yellow foam earplugs, 28 subjects, 140 measurements, from Edwards, et al (NIOSH), 1982 Flat spectrum Armored vehicle spectrum (BFV) Armored vehicle spectrum, mean = 12.7 dB, std dev = 4.0 dB NRR = 29 dB Attenuation in steady noise Flat spectrum, mean = 21 dB, std dev = 6.6 dB NRR Population-based HPD effectiveness:  Population-based HPD effectiveness Determine unprotected damage-risk relation Threshold of damage, exposure time (years) Use appropriate protector fit Define acceptable level of risk Effectiveness = shift in damage risk relation due to HPD at defined risk level Army example: 20 year career Damage threshold: TTS2 10 [email protected], [email protected] 2kHz, [email protected]+ 85 dBA acceptable risk Effectiveness = mean - 0.3 x std dev CHABA*-1968 interim impulse noise DRC:  CHABA*-1968 interim impulse noise DRC Unprotected ears (no fit consideration) Parameters: dBP, B-duration, # shots/per day Protects 95% of population Threshold: TTS2 10 [email protected], [email protected] 2kHz, [email protected]+ Normal incidence, + 5 dBP for grazing incidence + 5 dBP per decade decrease in # shots *CHABA- NAS-NRC Committee on Hearing, Bioacoustics, and Biomechanics (reaffirmed with restrictions in 1992) Slide12:  100 shots per day CHABA DRC, unprotected Slide13:  MIL-STD-1474 Currently used by US Army Based on CHABA 1968 unprotected DRC Developed by ARL HRED (Garinther and Hodge 1972) Considers all HPD as equal: 29 dB attenuation in peak level for plugs or muffs 6.5 dB additional for plugs and muffs Based on expert-fit data Slide14:  MIL-STD-1474 criteria, with single HPD 5 shots per day 100 shots per day 2,000 shots per day HPD required Slide15:  Artillery in free field Based on USAARL research (Patterson and Johnson): Human exposure with supervised fit Normal incidence Leaky noise muff to simulate degraded HPD Features: As parameters: dBP, pulse shape No more than 2 peaks with highest one first A-duration > 2 ms B-duration < 60 ms Requires “good quality” HPD Slide16:  Muff and pulse shapes Patterson and Johnson, 1994 Slide17:  Volunteer subjects Slide18:  Damage risk criteria, single HPD MIL-STD-1474, 5 shots per day MIL-STD-1474, 2,000 shots per day Free field, 6 shots per day Free field, 100 shots per day, double # for – 3 dB Slide19:  Weapons impulse noise MIL-STD-1474, 5 shots per day MIL-STD-1474, 2,000 shots per day Free field, 6 shots per day Free field, 100 shots per day Rifle Howitzer, low charge Howitzer, high charge MAAWS Slide20:  A-weighted energy Favored by France Daily at-ear sound energy < 8.7 J/sq m Equivalent to US 8-hr TWA <85 dBA using 3 dB exchange rate At-ear criterion automatically includes HPD HPD fit considerations All steady noise HPD fit considerations apply Slide21:  AHAAH* Model Leading candidate to replace MIL-STD-1474 Computer model Input: pressure time history at eardrum Output: auditory damage units (ADU) Includes HPD module Input: HPD band attenuation, angle of incidence Output: pressure time history at eardrum HPD fit considerations apply *AHAAH: Auditory Hazard Assessment Algorithm-Human, developed by US Army Research Laboratory (Price and Kalb) Slide22:  AHAAH model as damage risk criterion Differentiates warned, unwarned exposure Threshold of hazard for 95th %ile susceptibility 500 ADU ADUs are cumulative, 24 hour horizon Hazard effect for 50th %ile susceptibility ADU for 10 dB down pulse ADU ratio varies with attenuation Slide23:  User fit, yellow and white foam earplugs, 56 subjects, 280 measurements, from Edwards, et al (NIOSH), 1982 0 40 10 20 30 Mean Standard deviation AHAAH exposure estimate, user fit, batch ver T Slide24:  MAAWS Time, ms 186.8 dBP Slide25:  Twin .50 cal Gatling guns Slide26:  Foam plug, MAAWS, 95th %ile Mean user fit plug, 98 ADUu Manufacturer’s mean – 2 std dev, 35 ADUu Slide27:  Foam plug, MAAWS, 95th %ile Median ADUu = 130 Slide28:  Foam plug, MAAWS, 50th %ile Mean user fit plug, 41 ADUu Manufacturer’s mean – 2 std dev, 14 ADUu Slide29:  Foam plug, MAAWS, 50th %ile Median ADUu = 55 Slide30:  Foam plug, .50 cal, 95th %ile Mean user fit plug, 220 ADUu Manufacturer’s mean – 2 std dev, 108 ADUu Slide31:  Damage risk criteria, steady noise Percent of population with damage Exposure metric Steady noise: dBA vs % population with X PTS 85 dBA 5 to 10 % dBA Slide32:  Sample of calculations, MAAWS pulse Slide33:  Damage risk criteria, AHAAH Percent of population with damage Exposure metric 500 ADU 5 % AHAAH: ADU at 95% population with X PTS AHAAH: ADU at 50% population with X PTS 50% ?? ADU ADU Slide34:  Conclusions HPD fit influences attenuation measurements Population-based effectiveness: Can be calculated for steady noise Mean minus fraction of standard deviation of user fit data gives good estimate Fit effects not in Army impulse noise DRC AHAAH model allows fit considerations ADU seems not an absolute hazard measure AHHAH DRC needs further refinement to make population-based estimates

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