noise reduction rating calculation method

8/18/2019 Noise Reduction Rating Calculation Method

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Method for Calculating and Using the Noise Reduction Rating -NRR

TheNRRisasingle-numberratingwhichisrequiredbylawtobeshownonthelabelofeachhearingprotectorsold

intheUnitedStates.TheNRRisspecifiedby40CFR(CodeofFederalRegulations)Part211,ProductNoise

Labeling,SubpartB-HearingProtectiveDevices.Itisindependentofthenoisespectruminwhichitisapplied.

ThefollowingdescriptionofthemethodofmeasurementandcalculationoftheNRRistakenfromMethod2ofthe

NIOSHComp endia(Kroesetal ., 1975)andas imilar tableca nbefo und in 44FR(Fede ral Register )page5614 2

(1979).

Calculating the NRR :

ThevaluesofsoundattenuationusedforcalculationoftheNRRaredeterminedinaccordancewithANSIS3.19

1974,"AmericanNationalStandardfortheMeasurementofReal-EarHearingProtectorAttenuationandPhysical

Attenu ationofEarmuffs."Theexperimenter-f itmethod mustbeused;thatis,theexperimen ter(notthetestsub ject)

mustfitthehearingprotectorontotheheadorintotheearofeachtestsubjectforeachoccludedtest.Mean

attenuat ionsandstandarddeviationsarecalculatedinacco rdancewiththestandard.TheNRR isthencomputedfrom

themeanattenuationsandstandarddeviationsaccordingtothefollowingequation:

8000

-APV f 98

) NRR=107.9dBC-10logΣ100.1(LA f -3dB.f=125

whereLA f istheA-weightedoctavebandlevelatfrequency f ofapinknoisespectrumwithanoveralllevelof107.9

dBC,andAPV f 98isthemeanattenuationvalueminus2standarddeviationsatfrequency f (2standarddeviations

accountsfor98%ofthevarianceinanormaldistribution).

Theequationcanbebrokendownintothestepsshowninthetableonthefollowingpage.TheNRRassumesapink

noisewithoctave-bandlevelsof100dB(line1).ThecorrectionsfortheC-weightingscale(line2)arethen

subtractedtocomputeunprotectedC-weightedoctave-bandlevelsattheear(line3).Theseoctave-bandlevelsare

logarithmicallysummedtoobtaintheoverallsoundlevelindBCattheunprotectedear;thisvalueisthefirstterm

oftheequation(i.e.,107.9dBC).ThecorrectionsfortheA-weightingscale(line4)arethensubtractedfromthe

pink-noise octave-band leve ls(line1)tocomp utetheA-weighte doc tave-b and levels attheear(lin e5). Theaverage

attenuations(line6)minustwicethestandarddeviations(line7)aresubtractedfromtheA-weightedoctave-band

levelstocompu tetheprotectedA-weightedoctave-bandlevelattheear.Notethattheattenua tiondatafor3000and

4000Hzandfor6000and8000Hzareaveragedtoderivetheattenuationsat4000Hzand8000Hz.Notealsothatthestandarddeviationsfor3000and4000Hzand6000and8000Hzaresummed(inlieuofmultiplyingby2)to

calculatethetotalstandarddeviationfor4000Hzand8000Hz.Thepro tectedA-weightedoctave-bandlevelsat

theeararethenlogarithmicallysummedtocalculatetheoverallprotectedAlevel.Thisisthesecondtermofthe

equation.TheNRRiscomputedbysubtracting3dBfromthedifferencebetweentheunprotectedC-weightedand

theprotectedA-weightedlevelsattheear.

Using the NRR:

TheHearingConservationAmendmenttotheOccupationalNoiseStandard(OSHA,1983)describessixmethods

forusingtheNRRtodetermineaworker'sprotectedA-weightednoiseexposure.Thesemethodsvaryaccordingto

the instrumentation and parameters used to determine the unprotected noise levels. However, they can be

summarizedintotwobasicformulas,dependingonwhetherunprotectedexposurelevelsweremeasuredonaC-

weightedoranA-weightedscale.

ForC-weightedmeasurements:

pr otecteddB A=unpro tecteddBC-NRR

wheretheprotecteddBAandtheunprotecteddBCare8-hourtime-weightedaveragesdeterminedaccordingtothe

OccupationalNoiseStandard.ThismethodishowtheNRRwasdesignedtobeused.Forexample,ifaprotector

hasanNRRof17dBanditisusedinanenvironmentalnoiselevelof95dBC ,thenoiselevelenteringtheearcould

be expected to be 78 dBA [95 - 17 = 78] or lower in 98 % of the case s if the pro tector is worn acco rding to

manufacturersspecification.

ForA-weightedmeasurements:

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Method for Calculating and Using the Noise Reduction Rating -NRR

pr otecteddB A=u npr otecteddB A-[ NRR-7]

where,again,theprotectedandunprotecteddBAare8-hourtime-weightedaveragesdeterminedaccordingtothe

OccupationalNoiseStandard.ThismethodisanadaptationforthosewhoseinstrumentationdoesnothaveC-weighting

capabilities.The7dBcorrectionfactorisusedtoaccountforthede-emphasisoflow-frequencyenergyinherenttothe

A-weightingscale.So,forexample,ifaprotectorhasanNRRof17dBanditisusedinanenvironmentalnoiselevelof95dBA,thenoiselevelenteringtheearcouldbeexpectedtobe85dBA[95-(17-7)=85]orlessin98%ofthe

cases.

NIOSHrecommendsderatin gtheN RRbyamult ipli cative factoro f75 %for ea rmuffs, 50 %for slow-recoveryf oam

earplugsandcustomearplugs,and30% forallotherearplugs.Thisvariablederatingschemeconsidersthereal-

worldperformanceofmostdifferenttypesofhearingprotector(NIOSH,1 998).Also,theNIOSHderatingscheme

doesnotaffectthe7-decibeldBCtodBA correctionasitisappliedtotheNRRonly,deratedornot.This

compendiumusestheNIOSHderatingoftheNRRwhensearchingforhearingprotectorsbasedonthecompendium

user’sinputofnoiseexposurelevelsindBAoroctavebandlevels.

Computation of the Noise Reduction Rating

OctaveBandCenterFrequency(Hz)FrequencyinHz 125 250 500 1000 2000 3000 4000 6000 8000 Log Sum

1.Assumedpinknoise(dB)

2.Cweightingcorrections(dB)

3.UnprotectedearC-weightedlevel

4.Aweightingcorrections(dB)

5.UnprotectedearA-weightedlevel

100

-.2

99.8

-16.1

83.9

100

0

100

-8.6

91.4

100

0

100

-3.2

96.8

100

100

0

100

100

-.2

99.8

1.2

101.2

100

-.8

99.2

1.0

101

100

-3

97

-1.1

98.9

107.9

6.AverageattenuationindB

ateachfrequency

21

21

22

22

23

23

29

29

41

41

43 47

45.0*

41 36

38.5*

7.StandarddeviationindB

ateachfrequency

3.7

x2

7.4

3.3

x2

6.6

3.8

x2

7.6

4.7

x2

9.4

3.3

x2

6.6

3.3 3.4

6.7**

6.1 6.5

12.6**

8.ComputeAPV98 indBateachfrequency(line6-line7) 13.6 15.4 15.4 19.6 34.4 38.3 25.9

9.ProtectedearA-weightedlevel

(averageattenuationminus2standard

deviationsdevelopstheearA-weighted

Levels(line5-line8) 70.3 76.0 81.4 80.4 66.8 62.7 73.0 85.1

___

10.NRRisunprotectedear"C"level(line3)minusprotectedear"A"level(line9)minus3dB

19.8

*averageattenuationat3000and4000Hzandat6000and8000Hz.

**summedstandarddeviationfor3000and4000Hzandfor6000and8000Hz.

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Method for Calculating and Using the Subject-Fit Noise Reduction Rating - NRR(SF)

TheSubject-FitNoiseReductionRating,NRR(SF),isasinglenumberratingmethodwhichdescribesreal-ear

attenuationatthreshold(REAT)measurementsobtainedaccordingtoANSIS12.6-1997, Method s for Measureing

the Real-Ear Attenuation of Hearing Pro tectors,methodB. TheNRR(SF)wasdevelopedbytheNationalHearing

ConservationAssociation’sTask Force on Hearing Protector Effectivenesstoaddresslabelingrelatedissues

(Royster,1995) .TheNRR(S F)methodisderivedfromtheNoiseReductionRating(NRR)presentedin Calculating

NR RandtheSingle-NumberRating(SNR)methodpresentedinCalculating SNRforaprotectionperformanceof

84%(EP A,1979;ISO,1992).Inprinciple,theratingmethodsexaminetheamountofnoisereductionachievedbya

hearingprotector,butminordifferencesseparatethemethods.LiketheNRR,theNRR(SF)usesapinknoiseata

levelof100dBSPLintheoctavebands.LiketheSNR,theNRR(SF)includestheoctave-bandfrequenciesbetween

63and8000HzinitscalculationwhereastheNRRincludesonlyfrequenciesbetween125and8000Hzand

includesthehalf-octavefrequencies3150and6300Hz.

Calculating the NRR(SF):

TheNRR(SF)iscalculatedwiththefollowingequation:

8000 -APV

f 84) NRR=108.5dBC-10logΣ100.1(LA f -5dB.

f =63

whereLA f istheA-weightedoctavebandlevelatfrequency f ofapinknoisespectrumwithanoveralllevelof108.5

dBC,and APV f 84isthemeanattenuationvalueminus1standarddeviationatfrequency f (1standarddeviation

accountsfor84%ofthevarianceinanormaldistribution).

TheC-weightednoiselevelsaredeterminedbysubtractingtheC-weightedcorrectionsfrom100dBforeachofthe

octavebandfrequencies(line1).ThesumoftheC-weightednoiselevelsis108.5dBC.Similarly,theLA f (line3)

aredeterminedbycorrectingthe100dBoctavebandnoiselevelswiththeA-weightcorrectionsateachfrequency

(line4).TheAPV f 84aredeterminedbysubtractingonestandarddeviation(line7)fromthemeanREATattenuation

measuredforthenoisebandcenteredatfrequency, f (line6).Thehalf-octavedataat3150and6300H zarenotused

inthecalculationoftheNRR(SF).The5dBwhichissubtractedattheendofthecalculationisrepresentativeofthe

approx imatedifferencebetweenC-weightedandA-weightedindustrialnoise(Miller,1995).Bycompensatingfor

thedifferenceinthecalculation,theNRR(SF)maybesubtracteddirectlyfromA-weightednoisetoestimatethe

noiselevelundertheprotector.Whendatafor63Hzarelacking,oneshouldusethe125Hzdatainitsplace;i.e.,usethe125-HzdatatwicewiththeappropriateAandCweightings.

Using the NRR(SF):

TheNRR(SF)isintendedtobeusedwithA-weightednoiseexposures.Hence,theNRR(SF)issubtracteddirectly

fromtheA-weightednoiseexposuretoprovidetheprotectedexposure:

protecteddBA =unprotecteddB A- NRR(SF).

IftheC-weightednoiseexposureisused,a5-dBadjustmentismadetoaccountforpredicteddifferencesintheA

andClevels(whicheffectivelyundoesthe5-dBcorrectioninline12ofthefollowingtable):

protecteddBA =unprotecteddB C-5dB- NRR(SF).

Iftheoctave-bandnoiseexposuresareused,thebestapproachistoapplytheA-weightingcorrectionsandlogsum

thelevelstoprovidetheA-weightednoiseexposurelevelforusewiththeNRR(SF).

Availability of the NRR(SF ):

TheadoptionofANSIS12.6-1997asthestandardthatreplacedANSIS3.19-1974didnotchangetheregulatory

requirementsthatallprotectorssoldintheUnitedStatesbelabeledwiththeNRRasdescribedinAppendixA1and

obtainedbytestingaccordingtotheexperimenter-fitmethodofANSIS3.19-1974.AfterANSIS12.6-1997was

adopted,manyhearingprotectormanufacturersbegantestingtheirproductsinaccordancewithMethodB,the

subject-fitmethod.AsmanufacturersbegintosubmittheirMethodBdata,thecompend iumwillshowbothsetsof

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Method for Calculating and Using the Subject-Fit Noise Reduction Rating - NRR(SF)

attenuationdataandboththesubjectfitNRRandoriginalNRR.However,theMethodBdatawillbeusedwhena

searchforhearingprotectorsisperformed.

IfandwhentheU.S.EnvironmentalProtectionAgencyissuccessfulinpromulgatingarevisedHearingProtector

LabelingregulationthatrequirestheuseoftheNRR(SF)orsomeothersubject-fitbasedratinginlieuofthepresent

NR R,then manufacturer swillstartp rovidingda tathat theya lreadyhave.

IntheabsenceofMethodBdata,NIOSHrecommendsderatingtheNRRusingNIOSHderatingschemesas

describedintheNIOSHrevisednoisecriteriadocument(CriteriaForaRecommendedStandard:OccupationalNoise

Exposure Revised Criteria 1998 . DHHS(NIOSH)Pub.No.98-126).Thederatingschemeswhicharespecificto

hearingprotectortypeandbasedonreal-worlddataareexplainedinCalculating NRR.

NIOSHsuggests thatcon sumerscontac tmanu facturerstorequesttheNRR(SF)foranyprotectorsthey are

consideringpurchasingorrecommending.

The NRR(SF) Calculation

FrequencyinHz 63 125 250 500 1000 2000 4000 8000 Log Sum

1.AssumedPinkNoise(dB) 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0

2.C-weightingcorrection(dB) -0.8 -0.2 0.0 0.0 0.0 -0.2 -0.8 -3.0

3.UnprotectedearC-weighted 99.2 99.8 100.0 100.0 100.0 99.8 99.2 97.0 108.5

level(dB) line1-line2

4.A-weightingcorrection(dB). . . -26.2 -16.1 -8.6 -3.2 0 1.2 1.0 -1.1

5.UnprotectedearA-weighted . . . . 73.8 83.9 91.4 96.8 100.0 101.2 101.0 98.9

level(dB)line1-line4

6.AverageREATattenuation 19.7* 19.7 19.7 20.7 21.2 28.3 36.6 37.8

ateachfrequency(dB)

7.REATstandarddeviation 8.7 8.7 9.7 9.4 8.4 7.4 9.1 10.4

ateachfrequency(dB)

8.AssumedProtectionValue 11.0 11.0 10.0 11.3 12.8 20.9 27.5 27.4

for84%performance(dB)

line6-line7

9.ProtectedearA-weighted 62.8 72.9 81.4 85.5 87.2 80.3 73.5 71.5 90.7

level(dB)line5-line8

10. Logsumof line3 (dB) 108.5

11.Logsumofline9(dB) 90.7

12.NRR(SF)(dB) . . 12.8

line10-line11-5dB

* thedatafor125areusedat63Hzwithoutalterationbecause63Hzisoptional.

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Method for Calculating and Using the Single Number Rating -SNR

TheSNRisasingle-numberratingwhichiscalculated inaccordancewithISO48692.2(19 92),"Estimation ofEffective

A-weightedSoundPressureLevelsWhenHearingProtectorsAreWorn."ItisverymuchliketheNRRintermsofits

calcula tionmetho dwithacoupleofnotableexceptions.First,theSNRcanbecalculatedforvariouslevelsofprotection

perfor manc e - that is, percen tage of the populati onwhichcanbe considered to receive no less than the calculated

attenua tion-whiletheNRRisfixedat98% .Second,thereisnotanadditional3dB subtractionasintheNRR . Various

protect ionperformances levels (%)are showni nTab le1a longwith theappropria temultipl ier(�)whichisappliedto

thestandarddeviation.

Table 1.

Value of � for various protection performances, x

Whenaprotectionperformancelevelisselected,theSNRisso

designatedbyasubscript.Thusforaprotectionperformanceof ProtectionPerformance80%,thedesignationwouldbeSNR 80. FortheSNRtobesimilar xin%Valueof�totheNRR,itsdesignationwouldbeSNR 98-3dB. Whenthe 75 0.67

protec tion perfo rmance is yet to be selected, the design ation is 80 0.84SN R x. 84 1.00

85 1.04TheISO48692.2testprocedurecallsfortheuseof16subjects 90 1.28whofittheprotectorsunderthesupervisionoftheexperimenter. 95 1.64

Each subject provides one unoccluded and one occludedthreshold.

Calculating the SNR :

ThecalculationoftheSNR xisbasedonanassumedbackgroundofpinknoisewithanoveralllevelof100dBCand

AssumedProtectionValues,APV x,ofthehearingprotector(seeCalculating APV ).TheSNR xisindependentofthe

actualnoisespectrumtowhichitisappliedandiscalculatedusingtheequation:

8000

SNR x=100dBC-10logΣ100.1(LA f -APV f x)

f =63

where: LA f istheA-weightedoctave-bandlevelatfrequency f asshowninTableA.3.2,andAPV f xisthe

AssumedProtectionValueforeachfrequency.Ifdataarenotavailablefor63Hz,thesummationbegins

at125Hz.

8000

Note1: Theterm 10 logΣ100.1(LA f -APV f x)equalsLA x

f =63

Note 2:100 dBr epresentsthet otalC -weightedsound pressure levelo f thereference pinknoisein Ta ble2 .

ThevaluesofLA f arederivedinTable2.Theyaresimplytheoctavebandlevelsofa100dBpinknoisecorrectedto

reflecttheA-weightingscale.

5


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Method for Calculating and Using the Single Number Rating -SNR

Using the SNR:

TheSNR xisusedtoestimatethenoiselevelunderthehearingprotectorforaspecificprotectionperformancelevel

(L 'A x)givenaspecificC-weightednoiselevel(L C).TheeffectiveA-weightedsoundpressurelevel,L'A x,underahearing

prot ector is calculated by subtract ing theSNR x fromtheC-weightedsoundpressurelevelof aspecificnoise. For

example,ifL C=103dB,andtheSNR 84=21.6dB,LA84iscalculatedusingtheequation:

L'A x=LC-SNR x,

sothat

L'A84=103dB-21.6dB=81.4dB.

ItcanbestatedthattheeffectiveA-weightedsoundpressurelevel,L'A84,willbelessthanorequalto81.4dBin84%of

thesituationswhenthehearingprotectorisproperlywornbyvarioususersinthisnoiseenvironment.

Table 2.

A-weighted octave-band sound pressures levels, LA f , of a pink noise which has a C-weighted sound pressure level of 100 dB

Octave-bandcenterfreq. f inHz 63 125 250 500 1000 2000 4000 8000 Log Sum

Assumedpinknoisebandlevel 91.5 91.5 91.5 91.5 91.5 91.5 91.5 91.5 100.0 dBC

Totalpinknoise 100

A-weightingcorrectionfactors -26.2 -16.1 -8.6 -3.2 0 +1.2 +1.0 -1.1

LA f 65.3 75.4 82.9 88.3 91.5 92.7 92.5 90.4

LA(totalA-weightedsoundlevel) 98.5 dBA

AnexampleSNRcalculationisshowninTable3.TheAPVvaluesusedaretakenasderivedinCalculating APV for

1standarddeviation-aprotectionperformancelevelof84%.

L

L

Table 3.

Calculation difference between APV f 84 and LA f . All values are in decibels.

Octave-bandcenterfreq. f inHz 63 125 250 500 1000 2000 4000 8000 Log Sum

A f ,fromTableA.3.2 65.3 75.4 82.9 88.3 91.5 92.7 92.5 90.4

ExampleAPV f 84 4.1 6.4 10.8 15.0 18.8 22.4 33.4 28.9

A f -APV84 61.2 69.0 72.1 73.3 72.7 70.3 61.1 61.5 78.4 dBA

SNR x=100dBC-78.4=21.6dB

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Method for Calculating and Using the High, Middle, Low (HML) Rating

TheHMLisaratingwhichiscalculatedinaccordancewithISO48692.3(1992),"EstimationofEffectiveA-weighted

SoundPressureLevelsWhenHearingProtectorsAreWorn."UsingtheH,M,andLratingsrequiresbothC-weighted

(LC)andA-weighted(LA)soundpressurelevelsofthenoise.TheHandMvaluesareusedinthecalculationofthe

protectedexposurelev elforno ises whic hhave primaryenergyinthemiddleandhigh frequenciesandforwhichthe LCandLAlevelsdifferby2dBorless.TheMandLvaluesareusedinthecalculationoftheprotectedexposurelevelfor

noiseswhichhaveappreciablelow-frequencycomponentsandforwhichtheL CandLAlevelsdifferbymorethan2dB.

Calculating the HM L:

ThecalculationoftheH,M,andLvaluesisbasedoneightreferencenoisespectrawithdifferent(LC-LA)valuesas

showninTable1andtheAssumedProtectionValues(APV f x seeCalculating APV )ofthehearingprotector.Theeight-referencenoisespectrawerederivedfromtheNIOSH 100noises(JohnsonandNixon,1974).Theindex,i,isusedto

refer to a particular noise spectrum. The constant, d i is an empirically-determined value. The HML values are

independentoftheactualnoisesituationtowhichtheyareapplied.

TheHMLvaluesarecalculatedusingthefollowingequations:

4 4

H x=0.25ΣPNR xi-0.48Σ(di*PNR xi)

i=1 i=1

8 8

M x=0.25ΣPNR xi-0.16Σ(di*PNR xi)

i=5 i=5

8 8

L x=0.25ΣPNR xi+0.23Σ(di*PNR xi)

i=5 i=5

where: 8000

PNR xi=100dBA-10logΣ 100.1(LA f i-APV f x)

f =63

No te1.PNR xireferstothepredictednoisereductionfor apro tectionperformancelevel xandareferencenoisespectrumi. LA f ianddiaregiveninTable 1. Ifdataarenotavailable for63Hz,thensummationbeginsat125 Hz.Thevalue 100

dBrepresentsthetotalA-weightedsoundpressurelevelofeachofthenoisesinTable1.TheresultingH x,M x,andL x

valuesaretoberoundedtothenearestinteger.

No te2 .Tocalcul atetheH, M, and Lvalues for apart icularprotector, itis necessarytoob ta intheAssumedPro tection

Valueforthedesiredprotectorperformanceforeachfrequency(see Calculating APV ). Forexample,ifthesample

pr otectorfro mCalculating APV wereused,theAPV f 84wouldbeasinTable2.

No te3.After theAPV f xareobtained,theA-weightedoctave-bandnoiselevelsundertheprotectorarecalculatedfor

eachofthereferencenoises(LA f i-APV f 84). Fortheprotectorinthisexample,theselevelswouldbeasshownin

Table3.

TheeightPNR i,84valuesarethencalculatedbysubstitutingthedifferencesfromTable3intoitsequationasshowninTable4.

TheH84,M 84,andL84valuesarecalculatedfortheexampleusingtheappropriateequations,thePNR i,84valuesfrom

Table4,andtheconstantsdifromTable1. ThecalculationsareshowninTable5forCalculating APV andthe

valuesareroundedtothenearestinteger.

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Method for Calculating and Using the High, Middle, Low (HML) Rating

Table 1.

A-weighted octave-band so und pressure levels, LA f i, of eight reference n oises normalized to an A-weighted

sound pressure level of 100 dB, LC - LA values and constants d i. All values are in decibels.

Index Octave-band CenterFrequency( f ) Difference Constant

i 63 125 250 500 1000 2000 4000 8000 (LC-LA) di

1 51.4 62.6 70.8 81.0 90.4 96.2 94.7 92.3 -1.2 -1.20

2 59.5 68.9 78.3 84.3 92.8 96.3 94.0 90.0 -0.5 -0.49

3 59.8 71.1 80.8 88.0 95.0 94.4 94.1 89.0 0.1 0.14

4 65.4 77.2 84.5 89.8 95.5 94.3 92.5 88.8 1.6 1.56

5 65.3 77.4 86.5 92.5 96.4 93.0 90.4 83.7 2.3 -2.98

6 70.7 82.0 89.3 93.3 95.6 93.0 90.1 83.0 4.3 -1.01

7 75.6 84.2 90.1 93.6 96.2 91.3 87.9 81.9 6.1 0.85

8 77.6 88.0 93.4 93.8 94.2 91.4 87.9 79.9 8.4 3.14

Table 2.

Example APV f 84 for the hearing protector in Calculating APV .

Octave-bandcenterfreq.( f )inHz 63 125 250 500 1000 2000 4000 8000

ExampleAPV f 84 4.1 6.4 10.8 15.0 18.8 22.4 21.4 28.9

Table 3.

Calculated difference spectrum for LA f i - APV f 84

Octave-bandcenterfreq. f inHz 63 125 250 500 1000 2000 4000 8000

LA f 1-APV f 84 47.3 56.2 60.0 66.0 71.6 73.8 73.3 63.4

LA f 2-APV f 84 55.4 62.5 67.5 69.3 74.0 73.9 72.6 61.1

LA f 3-APV f 84 57.7 64.7 70.0 73.0 76.2 72.0 72.7 60.1

LA f 4-APV f 84 61.3 70.8 73.7 74.8 76.7 71.9 71.1 59.9

LA f 5-APV f 84 61.2 71.0 75.7 77.5 77.6 70.6 69.0 54.8

LA f 6-APV f 84 66.6 75.6 78.5 78.3 76.8 70.6 68.7 54.1

LA f 7-APV f 84 71.5 77.8 79.3 78.6 77.4 68.9 66.5 53.0

LA f 8-APV f 84 73.5 81.6 82.6 78.8 75.4 69.0 66.5 51.0

Table 4.

PN R i,84 values for the sample hearing protector.

PN R 1,84=100dBA-10log(100.1*47.3+...+100.1*63.4) = 21.7dB

PN R 2,84=100dBA-10log(100.1*55.4+...+100.1*61.1) = 20.7dB

PN R 3,84=100dBA-10log(100.1*57.7+...+100.1*60.1) = 19.6dB

PN R 4,84=100dBA-10log(100.1*61.3+...+100.1*59.9) = 18.4dB

PN R 5,84=100dBA-10log(100.1*61.2+...+100.1*54.8) = 17.3dB

PN R 6,84=100dBA-10log(100.1*66.6+...+100.1*54.1) = 16.1dB

PN R 7,84=100dBA-10log(100.1*71.5+...+100.1*53.0) = 15.2dB

PN R 8,84=100dBA-10log(100.1*73.5+...+100.1*51.0) = 13.3dB

8


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Table 5.

Calculated H84, M 84, and L84 values

H84=0.25*(17.7+...+14.4)-0.48*(-1.20*17.7+...+1.56*14.4)= 20.1dB.

M84=0.25*(13.2+...+9.4)-0.16*(-2.98*13.2+...+3.14*9.4)=15.5dB.

L84=0.25*(13.2+...+9.4)+0.23*(-2.98*13.2+...+3.14*9.4)= 15.5dB.

Using the HML:

ThevaluesH84,M 84,andL84maybeusedtoestimateL'84(totalA-weightednoiselevelattheear)foraparticular

pro tector inaspecificnoisesituat ion.Usingthecalculate dvalu esf rom Ta ble5 ,an example is shown below.

1. Calculate LC - Thedifference(LC-LA)iscalculated.

LA.

2. Calculate PNR x. Ifthisdifferenceis


10/11

Method for Calculating and Using the Assumed Protection Values (APV fx )

AssumedProtectiveValues(APV fx)areameasureofhearingprotectorattenuationbyfrequency( f ).TheAPV fxis

usedincalculatingotherhearing-protectorrelatedvalues,suchastheSingle-NumberRating(SNR;see Calculating

SNR)andtheHighMediumLowrating(HML;seeCalculating HML). Itmayalsobeusedaloneasadirectestimate

ofnoisereductionataparticularfrequencybandwhentheoctave-bandlevelsofthenoiseareknown.

Calculating the APV fx :

TheAPV fxiscomputedbysubtractingamultipleofthestandarddeviationoftheattenuationsfromthemean

attenuationforeachfrequencyband.Thestandarddeviationmultiplier(α

)ischosenbasedonthedesiredprotection perfo rman celeve l( x). Table1givesthevaluesforvariousprotectionperformancelevels.Onceaprotection

pe rformancel eve lis selected ,theA PVis sode signa tedbythesubscr ipted x. Forexample,APV500(84%)designates

theAPVatthe500Hzoctavebandforaprotectorperformancelevelof84%.

Table2illustratesthecalculationoftheAPV fxforaprotectorperformancelevelof84%.

Using the APV fx :

TheAPV fxmaybeusedasadirectestimateofthenoiseexposurelevelataparticularfrequencybandwhenthe

octave-bandnoiselevelsareknown.TheAPVisappliedbysubtractingitfromthecorrespondingoctave-bandnoise

levelofthenoisefromwhichprotectionisbeingsought.Forexample,iftheoctave-bandlevelforanoiseis92dB

SPLat500HzandtheAPV500(84%)is13dB,thentheprotectedlevelwouldbeequaltothedifferencevalueof79dB

SPL.Thedifferencesmayalsobelogsummedacrossfrequencytoprovideapredictionofoveralleffectivenoise

exposurelevelunderthehearingprotector.

IntheexampleinTable2,theAPV f 84valuesforahearingprotectorarecalculated;thatis,theprotectionperformance,α,of

84%(1standarddeviation)isselected. TheseAPV f 84valuesthencanbeusedalone(asdescribedabove)orutilizedin

calculationsfortheSNR(seeCalculating SNR)andHML(seeCalculating HML).

Table 1.

Values of α for various protection performances, x

Protectionperformance

xin%Valueof�

75 0.67

80 0.84

84 1.00

85 1.04

90 1.28

95 1.64

98 2.00

Table 2.

Calculations of APV f 84. All values are in decibels.

Octave-bandcenter

freq. ( f )inHz 63 125 250 500 1000 2000 4000 8000

MeanattenuationindB,m f 7.4 10.0 14.4 19.6 22.8 28.6 38.8 34.1

Standarddeviation

indB,s f 3.3 3.6 3.6 4.6 4.0 6.2 7.4 5.2

α∗s f ,α=1.00 3.3 3.6 3.6 4.6 4.0 6.2 7.4 5.2

APV f 84=m f - αs f ,indB 4.1 6.4 10.8 15.0 18.8 22.4 31.4 28.9

10


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References

AmericanNationalStandardsInstitute.(1974)AmericanNationalStandardfortheMeasurementofReal-Ear

HearingProtectorsandPhysicalAttenuationofEarmuffs.ANSIS3.19-1974,AmericanNationalStandardsInstitute,

NewY ork,N Y.

AmericanNationalStandardsInstitute.(1997)MethodsforM easuringthereal-earattenuationofhearingprotectors.

ANSIS12.6-1997AmericanNationalStandardsInstitute,NewYork,NY.

EnvironmentalProtectionAgency.(1979).40CFRPart211-Productnoiselabeling,SubpartB-Hearingprotective

devices.44FederalRegister56139-56147.

InternationalStandardsOrganization.(1992 ).Acoustics-Hearingprotectors-Part2:EstimationofeffectiveA-

weightedsoundpressurelevelswhenhearingprotectorsareworn.ISO/DIS48692. 2,InternationalOrganizationfor

Standardization,Geneva,Switzerland.

JohnsonDLandNixonCW(1974)Simplifiedmethodsforestimatinghearingprotectorperformance.Soundand

Vib8(6):20-27.

KroesP,FlemingR;LempertB.(1975).Listofpersonalhearingprotectorsandattenuationdata,NIOSH Technical

Report,HEWPublicationNo.(NIOSH)76-120.

MillerTW (1995).Ananalysisofoctavebandsoundpressureleveldatabasesandvariousmetricsbasedonthedata

bases studied .Unpublished maste rsthesis,NorthCarolinaStateUnivers ity ,Ral eigh.

NIOSH.(1998). Cr iter iafor aRecommendedS tandard-Occupation al No iseExposure,R evised Cr iteria .N ational

InstituteforOccupationalSafetyandHealth,Cincinnati,OH.DHHS( NIOSH )PublicationNo.98-12 6.

RoysterLH.(1995).Insearchofameaningfulmeasureofhearingprotectoreffectiveness;Recomendationsofthe

NHCA's tas kforceo nhearingpro tectoref fectiveness. Spectrum1 2( 2):1,6-13 .

U.S.DepartmentofLabor,OccupationalSafetyandHealthAdministration.(1983).OccupationalNoiseExposure:

HearingConservationAmendment;FinalRule.48(46)FederalRegister9738-9785.

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noise reduction rating calculation method

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