human hearing system

8/15/2019 Human Hearing System

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ConservationConservation

TrainingTraining

Environmental Health andEnvironmental Health andSafetySafety

University of TennesseeUniversity of Tennessee


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Did You Know?

AboutAbout 30 million30 million workers are exposed toworkers are exposed tohazardous noise on the job. One in 4 ofhazardous noise on the job. One in 4 ofthese workers (or 7.5 i!!ion A eri"ans#these workers (or 7.5 i!!ion A eri"ans#wi!! de$e!op per anent hearin% !oss.wi!! de$e!op per anent hearin% !oss.

&oise'indu"ed hearin% !oss is the ost&oise'indu"ed hearin% !oss is the ost"o on o""upationa! hazard for"o on o""upationa! hazard forA eri"an workersA eri"an workers

earin% !oss fro noise is s!ow andearin% !oss fro noise is s!ow andpain!ess) *ou "an ha$e a disabi!it*pain!ess) *ou "an ha$e a disabi!it*before *ou noti"e itbefore *ou noti"e it+f *ou ust raise *our $oi"e to speak+f *ou ust raise *our $oi"e to speakwith so eone on!* , feet awa*- *ou arewith so eone on!* , feet awa*- *ou arein hi%h (hazardous# noise.in hi%h (hazardous# noise.+t is+t is 100%100% pre$entab!epre$entab!e


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earin% onser$ation

&oise onser$ation is a!so "a!!ed&oise onser$ation is a!so "a!!edthe &oise /tandard and is re%u!atedthe &oise /tandard and is re%u!atedb* O/ A (O""upationa! /afet* andb* O/ A (O""upationa! /afet* and

ea!th Ad inistration#ea!th Ad inistration#O/ A 0e%u!ation (12 30 2 .25#O/ A 0e%u!ation (12 30 2 .25#6oa! ' pre$ent hearin% !oss6oa! ' pre$ent hearin% !oss

asso"iated with hi%h noise !e$e!s atasso"iated with hi%h noise !e$e!s atworkwork


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Physical CharacteristicsPhysical Characteristics

Frequency range: 20 Hz … 20 HzFrequency range: 20 Hz … 20 HzFactor of !000Factor of !000

"ntensity range: !0"ntensity range: !0 #!2#!2 to ! $%m to ! $%m 22

Factor of !0Factor of !0 !2!2

Filtering of informationFiltering of information


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Physical characteristicsPhysical characteristics

Graphics: Rossing, Moore & Wheeler,Graphics: Rossing, Moore & Wheeler, The Science of Sound, 3The Science of Sound, 3rd rd

Ed.Ed.


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Psychophysics:Psychophysics:

o&'ective and quantitative study of theo&'ective and quantitative study of therelation of (hysical stimuli and sensoryrelation of (hysical stimuli and sensory

(erce(tions(erce(tions

PressurePressure

fluctuationsfluctuations

Middle Ear Middle Ear Transformation intoTransformation intomechanical andmechanical andthen electricalthen electricalsignalssignals Processing:Processing:

sorting, selection,sorting, selection,recognitionrecognition


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Structur Structur ee

of theof the humanhuman

ear ear


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Important parts of theImportant parts of theCochleaCochlea

)val +indo+)val +indo+Scala vesti&uli ,-lledScala vesti&uli ,-lled+ith .uid/+ith .uid/Scala tim(aniScala tim(ani

ound +indo+ound +indo+1asiliar mem&rane1asiliar mem&rane

)rgan of orti)rgan of orti *uditory nerve *uditory nerve


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Mechanism of signalMechanism of signaltransporttransport

%all,%all, Musical Acoustics, 3Musical Acoustics, 3 rd rd Ed.Ed.


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Lever action of ossiclesLever action of ossicles

Small dis(lacement over large areaSmall dis(lacement over large area&ecomes large dis(lacement over&ecomes large dis(lacement over

small areasmall area


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Hearing lossHearing loss

onduction deafness: reducedonduction deafness: reducedmo&ility of ossicles due to vi&rousmo&ility of ossicles due to vi&roustissue ,i3 e3 from re(eated otitis/tissue ,i3 e3 from re(eated otitis/4erve deafness: deterioration of hair4erve deafness: deterioration of haircells or nerve ,also age#relatedcells or nerve ,also age#related(res&ycusis/(res&ycusis/

an &e distinguished through &onean &e distinguished through &oneconductionconduction


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Just noticable di erences Just noticable di erences

*t +hich frequency di5erence do *t +hich frequency di5erence dot+o tones sound di5erent to us6t+o tones sound di5erent to us6

'' ' %*'' ' %*'' ' %*'' ' %*'' '+ %*'' '+ %*

''' '' %*''' '' %*''' ''+ %*''' ''+ %*''' '' %*''' '' %*''' ''- %*''' ''- %*''' ''. %*''' ''. %*


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Ho! are soundsHo! are soundsresolved"resolved"

ritical &and: region on &asiliarritical &and: region on &asiliarmem&rane for rece(tion of a (uremem&rane for rece(tion of a (uretonetone"f critical &ands overla(7 only a"f critical &ands overla(7 only asingle tone is heardsingle tone is heardThe critical &and is larger at lo+The critical &and is larger at lo+frequencies3frequencies3


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#oise $%posure &t 'or( #oise $%posure &t 'or(

. +dentif* potentia! sour"es of. +dentif* potentia! sour"es ofhearin% !osshearin% !oss

1. earn how to pre$ent1. earn how to pre$enthearin% !oss at workhearin% !oss at work,.,. 8eet re%u!ator*8eet re%u!ator*

re9uire entsre9uire ents

4.4. :he purpose of audio etri":he purpose of audio etri"testin% and how it workstestin% and how it works

;e wi!! "o$er the fo!!owin% topi"s<

&oise =xposure at ;ork


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ow is &oise 8easured?

&oise !e$e!s are easured&oise !e$e!s are easuredin de"ibe!s (d>#in de"ibe!s (d>#De"ibe!s are not !inearDe"ibe!s are not !inear

easure entseasure ents:he differen"e in ener%*:he differen"e in ener%*between de"ibe!s andbetween de"ibe!s and

is not ' a"tua!!* is not ' a"tua!!*is ti es the soundis ti es the sound!e$e! pressure!e$e! pressure


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=ffe"ts of exposure to !oud noise

=xposure to !oud noise wi!!=xposure to !oud noise wi!!ine$itab!* "ause hearin% !oss o$erine$itab!* "ause hearin% !oss o$erti e.ti e.

On"e the ner$es of the inner earOn"e the ner$es of the inner earare destro*ed or da a%ed froare destro*ed or da a%ed froexposure to ex"essi$e noise- theexposure to ex"essi$e noise- theda a%e is per anentda a%e is per anent

oud noise da a%es or destro*soud noise da a%es or destro*sthe ner$es in the inner ear.the ner$es in the inner ear.

Another effe"t "an be @tinnitus orAnother effe"t "an be @tinnitus orper anent rin%in% in the ear.per anent rin%in% in the ear.


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3a"tors Affe"tin% earin% oss

:he fo!!owin% fa"tors "an affe"t:he fo!!owin% fa"tors "an affe"thearin% !oss


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;hen is &oise :oo oud?

/ &oise is easured inunits "a!!ed @ de"ibe!s or @d>

/ +f two peop!e , feet apartust shout to be heard-

the ba"k%round noise istoo !oud (abo$e C5

de"ibe!s#./ &oise abo$e 4 de"ibe!s

"auses pain andi ediate hearin% !oss.


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/i%ns of earin% oss

Diffi"u!t* hearin% peop!e speak.Diffi"u!t* hearin% peop!e speak.+nabi!it* to hear "ertain hi%h'pit"hed or soft+nabi!it* to hear "ertain hi%h'pit"hed or softsounds.sounds.&oise or rin%in% in ears.&oise or rin%in% in ears.6ettin% "o p!aints that the radio or t$ is too !oud6ettin% "o p!aints that the radio or t$ is too !oud 33


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on% :er =xposure to &oise/ Our ears "an re"o$er

fro short exposure to!oud noise- but o$er ti ener$e da a%e wi!! o""ur.

/ :he !on%er and !ouderthe noise- the %reater"han"e per anent

da a%e wi!! o""ur./ :here is no su"h thin% as

@tou%h ears or @%ettin%

used to it .


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=ffe"ts of noise to inner ear

&or a! hair "e!!s &oise'da a%ed hair "e!!s

air "e!!s in inner ear trans it noise si%na!s to the brain


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:innitus 3ro &oise =xposure

/ =xposure to hi%h noise !e$e!s"an a!so "ause per anentrin%in% in the ear or @tinnitus .

/ :innitus sufferers usua!!*"o p!ain of "onstant whist!in%-s9uea!in%- roarin% or buzzin%in one or both ears.

/ /e$ere tinnitus a* disrupts!eep- redu"e "on"entrationand "ause irritabi!it* anddepression.


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;hat is :oo 8u"h &oise =xposure?

/ Da a%e fro noise exposuredepends on the !oudness and!en%th of exposure.

/ /"ientifi" studies ha$e shown

that hearin% !oss "an o""urwhen C'hour a$era%e noiseexposure ex"eeds C5 de"ibe!s.

/ :he risk of hearin% !ossin"reases dra ati"a!!* as noise!e$e!s in"rease.

/ =xposure to noise !e$e!s abo$e5 de"ibe!s for e$en fi$e

inutes is $er* risk*.


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O/ A /tandard

+f C'hour a$era%e ex"eeds C5 d>A then the+f C'hour a$era%e ex"eeds C5 d>A then thee p!o*er ust


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Dai!* A!!owab!e =xposure :i es to &oise

:he tab!e be!ow shows noise !e$e!s and how !on% aperson "an be exposed without hearin% prote"tionbefore there is da a%e to the ear.

&oise e$e! A!!owab!e =xposure :i e

C5 de"ibe!s C hours

2 de"ibe!s 4 hours

de"ibe!s hour

5 de"ibe!s , inutes

de"ibe!s 5 inutes

5 de"ibe!s inutes


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=xa p!es of o on &oise=xposures

Source dBA

Whisper 20

Refrigerator 40Conversation 60

Average TV 74

Blender 80

Source dBA

Ipod 90

Wood or!ing 9"#$20

%&n 'hot $"0#$40

Riding (otor)*)le 90

'no (o+ile $20

Ro)! Con)ert $40


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=xa p!es of &ois* =9uip ent

=9uip ent=9uip ent &oise e$e!&oise e$e!

>a"k oe>a"k oe C5'25 de"ibe!sC5'25 de"ibe!shain /awhain /aw de"ibe!s de"ibe!s

3ront'end oader3ront'end oader 2 '25 de"ibe!s2 '25 de"ibe!s6unshot6unshot 4 de"ibe!s4 de"ibe!s6as :urbine =n%ine6as :urbine =n%ine 1 de"ibe!s1 de"ibe!s

awn 8owerawn 8ower 2 de"ibe!s2 de"ibe!s:ra"tor:ra"tor 25' 5 de"ibe!s25' 5 de"ibe!s

ir"u!ar /awir"u!ar /aw 2 ' de"ibe!s2 ' de"ibe!s

>e!ow are so e exa p!es of noise !e$e!s of $arious>e!ow are so e exa p!es of noise !e$e!s of $ariouse9uip ent used on "a pus without the use of hearin%e9uip ent used on "a pus without the use of hearin%prote"tion.prote"tion.


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&oise e$e!s and Ber issib!e =xposure e$e!s;ithout earin% Brote"tion

hi!!er >ui!din%shi!!er >ui!din%s 4 hours4 hours0outer 27 d>A0outer 27 d>A , hour, hour0adia! ar saw 5 d>A0adia! ar saw 5 d>A hour hour

:ab!e saw 2 d>A:ab!e saw 2 d>A ,.5 hours,.5 hoursBortab!e be!t sander d>A Bortab!e be!t sander d>A 1 hours1 hours

>e!ow is the axi u a ount of ti e a person>e!ow is the axi u a ount of ti e a person"an spend exposed to this e9uip ent without"an spend exposed to this e9uip ent withouthearin% prote"tion.hearin% prote"tion.


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=xa p!es of &ois* Areas at +/:a pus

8a"hine 0oo s and hi!!er 0oo s8a"hine 0oo s and hi!!er 0oo s6enerator6enerator8a"hine ;orkin% shops8a"hine ;orkin% shopsO$er ead 8u!ti'8ediaO$er ead 8u!ti'8edia=!e"tri" E +nterior i%hts e9uip ent.=!e"tri" E +nterior i%hts e9uip ent.

onstru"tion e9uip ent- su"h as< tab!eonstru"tion e9uip ent- su"h as< tab!esaws- radia! ar saw routers- be!t sanders-saws- radia! ar saw routers- be!t sanders-dri!!sdri!!s

ands"apin% e9uip ent- su"h asands"apin% e9uip ent- su"h as!awn owers and !eaf b!owers.!awn owers and !eaf b!owers.


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&oise ontro! 8easures:he e p!o*er ust take so e steps to "ontro! noise:he e p!o*er ust take so e steps to "ontro! noiseexposure in the workp!a"e- su"h as


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=n%ineerin% ontro!s

=n%ineerin% "ontro!s are=n%ineerin% "ontro!s arethe ost preferred optionthe ost preferred optionin redu"in% or e!i inatin%in redu"in% or e!i inatin%the noise hazard b*


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Ad inistrati$e ontro!s+f en%ineerin% "ontro!s do not work to+f en%ineerin% "ontro!s do not work toe!i inate the noise hazard- thene!i inate the noise hazard- thenad inistrati$e "ontro!s shou!d bead inistrati$e "ontro!s shou!d be"onsidered


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Bersona! Brote"ti$e =9uip ent

+f en%ineerin% or+f en%ineerin% orad inistrati$e "ontro!s doad inistrati$e "ontro!s donot work to e!i inate thenot work to e!i inate the

noise hazard- thennoise hazard- thenpersona! prote"ti$epersona! prote"ti$ee9uip ent shou!d bee9uip ent shou!d be"onsidered as a !ast"onsidered as a !ast

resort.resort.:his in"!udes usin%:his in"!udes usin%hearin% prote"tion- su"hhearin% prote"tion- su"has ear p!u%s or ear uffs.as ear p!u%s or ear uffs.


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:here are three t*pes of hearin%:here are three t*pes of hearin%prote"tion H ear uffs- earp!u%sprote"tion H ear uffs- earp!u%sand ear "aps.and ear "aps.

=ar uffs and earp!u%s pro$ide=ar uffs and earp!u%s pro$ideabout e9ua! prote"tion- ear "apsabout e9ua! prote"tion- ear "apsso ewhat !ess.so ewhat !ess.

:*pes of earin% Brote"tion

ear uffs

earp!u%s ear "aps


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A!! hearin% prote"tors are desi%ned toredu"e the intensit* (!oudness# of noiseto the inner ear.:he* work u"h better than wads of"otton or bits of "!oth stuffed in the ear.A!! three t*pes ha$e ad$anta%es anddisad$anta%es and peop!e $ar* onwhi"h the* prefer to use.

:*pes of earin% Brote"tors

otton doesnItworkJJ


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=arp!u%s are ade of foa -rubber or p!asti" and are eitherone'size'fits'a!! or in sizes s a!!-

ediu and !ar%e./o e are disposab!e- so e arereusab!e.:he* are !i%htwei%ht- andre9uire no aintenan"e.:he* are inserted into the ear"ana!.

earin% Brote"tion H =ar B!u%s


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/o e peop!e a* find ear p!u%sun"o fortab!e to wear for !on%periods at first.=ar p!u%s rare!* "ause infe"tionor pro!on%ed irritation of the ear"ana!.8ost peop!e "an find a"o fortab!e fit b* tr*in% se$era!different sizes- t*pes or brands.

usto ' o!ded earp!u%s "an beobtained for axi u "o fort.

=ar B!u% o fort

"usto o!dedearp!u%s


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3oa t*pe earp!u%s are one'size'fits'a!! and3oa t*pe earp!u%s are one'size'fits'a!! andust be inserted proper!* into the ear.ust be inserted proper!* into the ear.

+nsertin% 3oa =arp!u%s

0o!! earp!u% into s a!! "*!inder first- then insert in ear.


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+nsertin% 3oa =arp!u%s+nsertin% 3oa =arp!u%s

=arp!u% in"orre"t!* inserted =arp!u% "orre"t!* inserted


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=ar uffs "o$er the who!eear and are preferred b*so e peop!e.

:he* ha$e rep!a"eab!epads and so e hi%h'te"hst*!es fi!ter out spe"ifi"noise pit"hes.

:he* !ast !on%er than ostp!u%s.

=ar 8uffs


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/o e uffs are atta"hed to/o e uffs are atta"hed tohard hats or %o%%!es.hard hats or %o%%!es.

/o e hi%h'te"h uffs "an fi!ter/o e hi%h'te"h uffs "an fi!ter

out "ertain fre9uen"ies or ha$eout "ertain fre9uen"ies or ha$eradios inside for "o uni"ationradios inside for "o uni"ationin hi%h noise areas.in hi%h noise areas.

Atta"hed =ar uffs


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8uffs "an be un"o fortab!e

in hot weather.

8uffs donIt sea! we!! for so eonewith %!asses or hea$* sideburns.

=ar 8uff o fort E 6!asses


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=ar "aps are !ikeearp!u%s- ex"ept the* donot %o into the ear"ana!- the* on!* b!o"k it.

:he* are %ood foro""asiona! use or forpeop!e who findearp!u%s un"o fortab!e.

:he* are not asprote"ti$e as earp!u%sor uffs.

=ar aps


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:he @noise redu"tion ratin%or @&00 of hearin%prote"tion is easured inde"ibe!s.

:he &00 is found on theear uff or earp!u% pa"ka%e.:he hi%her the nu ber- the%reater the prote"tion.

&oise 0edu"tion of earin% Brote"tion


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Fsin% ear uffs or p!u%s innois* areas "an a"tua!!*

ake it easier to hear"oworkers or a"hiner*.

:he* redu"e o$erwhe! in%!oud ba"k%round noise.

:he* are si i!ar to dark%!asses that redu"e the sunIs%!are akin% it easier to see.

ow "an *ou hear an*thin%with ear uffs on?


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=ar uffs and p!u%spro$ide %ood prote"tionon!* when usedproper!*.

/o eti es peop!e wi!!re o$e hearin%prote"tion for @just a

inute in a nois* area.

+n areas of $er* hi%hnoise exposure- this"ou!d resu!t in noiseo$erexposure.

Broper Fse of earin% Brote"tion

+t wonIt prote"t *our ears ifit is around *our ne"kJJJ


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+nstru"tions on /e!e"tion- 3ittin%- Fse-and are of earin% Brote"tors

=ar p!u%s=ar p!u%sKeep "!ean and free of ateria!sKeep "!ean and free of ateria!s

;ash in i!d !i9uid deter%ent and war water;ash in i!d !i9uid deter%ent and war water

/9ueeze ex"ess water and air dr*/9ueeze ex"ess water and air dr*Dis"ard p!u%s when hardened or do notDis"ard p!u%s when hardened or do notre'expandre'expand

=ar ana!s=ar ana!s!ean !ike nor a! ear p!u%s!ean !ike nor a! ear p!u%s

Do not ta per with the headband andDo not ta per with the headband andthe a"ousti" sea!the a"ousti" sea!


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+nstru"tions on /e!e"tion- 3ittin%- Fse-and are of earin% Brote"tors

=ar 8uffs=ar 8uffsKeep "!ean and free of debrisKeep "!ean and free of debris

!ean "ushions with war soap* water!ean "ushions with war soap* waterDo not ta per with the a"ousti" sea!Do not ta per with the a"ousti" sea!between the "ushions and thebetween the "ushions and theheadbandheadband

Do not odif* the ear uffs in an*Do not odif* the ear uffs in an*wa*wa*Do not stret"h or abuse the headbandDo not stret"h or abuse the headband


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+t takes just a few inutes ofunprote"ted exposure at noiseabo$e 5 de"ibe!s to riskhearin% da a%e.

=arp!u%s not we!! inserted intothe ear "ana! wi!! not pro$ide"o p!ete prote"tion.

ikewise- ear uffs not snu%a%ainst the head wi!! @!eaknoise into the ear.

Broper Fse of earin% Brote"tion


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earin% aids do not b!o"k outenou%h sound for ost

workp!a"e noise./o e hearin% aids "an a"tua!!*in"rease the noise !e$e! at theear.

ust turnin% off the hearin%aids wi!! not pre$ent furtherhearin% !oss fro noiseexposure.

earin% Aids Are &ot earin% Brote"tion


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Bortab!e 0adiosG D B!a*ersGiBodsBortab!e 0adiosG D B!a*ersGiBods

/ :hese de$i"es do not pro$ideprote"tion fro noise.

/:he earphones are notear uffs and the usi" on!*adds to ba"k%round othernoise.

/ :he usi" !e$e! in theearphones the se!$es "anex"eed C5 de"ibe!s and "ause

hearin% !oss.


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8onitorin%

=n$iron enta! ea!th and /afet* or=n$iron enta! ea!th and /afet* orthe Depart ent of earin% andthe Depart ent of earin% and/pee"h "an "ondu"t noise onitorin%/pee"h "an "ondu"t noise onitorin%to deter ine the !e$e! of noiseto deter ine the !e$e! of noise

exposure.exposure./ound !e$e! eter ' %i$es an/ound !e$e! eter ' %i$es aninstant readin%instant readin%/ound !e$e! dosi eter ' pro$ides/ound !e$e! dosi eter ' pro$ides

an inte%rated !on%'durationan inte%rated !on%'duration(exa p!e hour# readin%(exa p!e hour# readin%= / aintains re"ord of noise= / aintains re"ord of noisesa p!in%sa p!in%

0ound 1e el Meter 0ound 1e el Meter

0ound 1e el 2osimeter 0ound 1e el 2osimeter


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Audio etri" :estin%A!! Fni$ersit* e p!o*ees in"!udedA!! Fni$ersit* e p!o*ees in"!udedin the earin% onser$ationin the earin% onser$ationBro%ra ust under%o initia! andBro%ra ust under%o initia! andannua! audio etri" testin%.annua! audio etri" testin%.Audio etri" testin% is re9uired b*Audio etri" testin% is re9uired b*:O/ A for an* e p!o*ees:O/ A for an* e p!o*eesexposed to ex"essi$e noiseexposed to ex"essi$e noise:hese tests- whi"h are done b*:hese tests- whi"h are done b*trained te"hni"ians- are used totrained te"hni"ians- are used to%au%e hearin% !oss in affe"ted%au%e hearin% !oss in affe"tede p!o*ees and wi!! be pro$ided ate p!o*ees and wi!! be pro$ided atno "ost to the e p!o*ees.no "ost to the e p!o*ees.:he initia! test is used as a:he initia! test is used as abase!ine and the annua! test isbase!ine and the annua! test isused to deter ine if hearin% !ossused to deter ine if hearin% !osshas o""urredhas o""urred 33


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Burpose of Audio etri" :estin%

:he purpose of Audio etri" :estin% is:he purpose of Audio etri" :estin% isto


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Audio etri" :estin%Audio etri" :estin%

Audio etri" testin% resu!ts "anbe used to "he"k the fo!!owin%<

+f the hearin% prote"tion in use+f the hearin% prote"tion in useis ade9uate-is ade9uate-

+f there is a "han%e in noise+f there is a "han%e in noiseexposure-exposure-

+f there is a edi"a! "ondition+f there is a edi"a! "onditionof the ear unre!ated to noiseof the ear unre!ated to noiseexposure.exposure.


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;hen *ou are first hired- abase!ine test is taken.

:he testin% is repeated

e$er* *ear after that and"o pared to the base!inetest resu!t.

+f a hearin% !oss is

dete"ted- *ou wi!! bereferred to a do"tor oraudio!o%ist.

ow Does Audio etri" :estin% ;ork?

A dio etri" :estin%Audio etri" :estin%


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Audio etri" :estin%Audio etri :estin%Audio etri" testin% produ"es

printed audio%ra s whi"hshow hearin% abi!it* at se$era!pit"hes or fre9uen"ies.

:hese fre9uen"ies in"!udethose of the hu an $oi"e.

:he se"ond and fo!!owin% *eartests are "o pared to the first*ear tests or base!ine.

+f there is hearin% !oss of de"ibe!s orore in the hu an $oi"e fre9uen"ies- *ou

wi!! be sent to the do"tor or audio!o%ist.


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An audio%ra is a printed "hart of the resu!ts of theAn audio%ra is a printed "hart of the resu!ts of thehearin% test. :he* !ook si i!ar to the resu!ts be!ow.hearin% test. :he* !ook si i!ar to the resu!ts be!ow.

;hat is an Audio%ra ?

&or a! hearin%/e$ere hearin% !oss


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= p!o*er 0esponsibi!it*

= p!o*ers are responsib!e for the= p!o*ers are responsib!e for thefo!!owin%


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= p!o*ee 0esponsibi!it*

= p!o*ees are responsib!e to


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opies of the O/ A /tandardopies of the O/ A /tandardand 0e!ated +nfor ationand 0e!ated +nfor ation

F: earin% onser$ation Bo!i"* foundF: earin% onser$ation Bo!i"* foundin = / /afet* 8anua! H 6/ 1in = / /afet* 8anua! H 6/ 1

O/ A standard a$ai!ab!e in the = /O/ A standard a$ai!ab!e in the = /offi"e or on!ine at


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8uestions

human hearing system

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