Iec 60118 4 2014

This stan ard sp cifies req irements for the field stren th in au io- req en y in u tion lo ps f or he rin aid purp ses, whic wi give adeq ate sig al-to-noise ratio without overlo din th

requirement s

Élect roacoust ique – Apparei s de cor ect ion audit ive –

Part ie 4: Syst èmes de boucles d'induct ion ut il sées à des fins de cor ect ion

audit ive – Ex igences de per ormances syst ème

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Copyr ight © 2 14 IEC, Ge e a, Switzer la d

Al rig ts r es r ve Unle s oth rwis s e ifi d, n p rt of this p blc tio ma b r epr od c d or uti z d in a y form

or b a y me n , ele tr onic or me h nic l in lu in p oto o yin a d microfim, with ut p rmis io in wr itin from

eith r IEC or IEC's memb r Natio al Commite in th c u tr y of th re u ste If y u h v a y q e tio s a o t IEC

c p r i ht or h v a e q ir y a o t o tainin a ditio al ri hts to this p blc tio , ple s c nta t th a dr es b low or

y ur lo al IEC memb r Natio al Commite for ur th r informatio

Dr oits d r eprod ctio ré er vé Sa f in ic tio c ntraire, a c n p rtie d c te p blc tio n p ut êtr e repro uite

ni uti s e s u q elq e forme q e c s it et p r a c n pro é é, éle troniq e o mé a iq e, y c mp s la p oto o ie

et le micr ofilms, s n la c rd é rit d l EC o d Comité n tio al d l EC d p y d d ma d ur Si v u a e d s

q e tio s s r le c p r i ht d l EC o si v u d sire o te ir d s droits s p léme tair es s r c te p blc tio , uti s z

le c ord n é s ci-a r ès o c nta te le Comité n tio al d l EC d v tr ep y d résid n e

Th Intern tio al Ele trote h ic l Commis io (IEC) is th le din glo al org niz tio th t pr ep res a d p bls e

Internatio al Sta d r ds for al ele tri al ele tr onic a d relate te h olo ie

Abo t IEC p blc tio s

Th te h ic l c nte t of IEC p blc tio s is k pt u d r c n ta t r eview b th IEC Ple s ma e s re th t y u h v th

late t e itio ,a c rrig n a or a ame dme t mig t h v b e p bls e

IEC Cat alo ue - webst ore.e ch/ cat alo ue

Th st an -alo e a plc t io for c n ultin th e tire

biblo ra hic l informat io o IEC Intern tio al Sta d rd ,

Te h ic l Sp cific tio s, Te h ic l R ep rts a d oth r

d c me ts A v aia le for P , Ma OS, A ndroid Ta let s a d

iPa

IEC p blc t i ns s arch - www ie ch/ se rch u

Th a v an e s arc e a le to fin IEC p blc t io s b a

variety of crit eria (efere c n mb r, t ext, t ec nic l

c mmitt e,…) It als gives informatio o projec t s, re la e

a d wit hdrawn p blc tio s

IEC Just P blshed - webst ore.e ch/ just pu lshed

St ay u to d te o al n w IEC p blc t io s Ju t Pu ls e

d t ais al n w p blc t io s rele s d A v aia le o ln a d

als o c a mo t h b emai

Ele t ro edia - www ele t ro edia.org

Th world's le din o ln dic t io ary of elec t ro ic a d

ele t ric l t erms c nt ainin more th n 3 0 0 terms a d

d finitio s in En ls a d Fre c , wit h e uiv ale t t erms in 14

a ditio al la g a e Als k nown a t he Int ern t io al

Elec t rot ec nic l Vo a ulary (IEV ) o ln

IEC Glos ar y - st d.e ch/ glos ary

More t ha 5 0 0 ele t rote h ic l termin lo y e trie in

En ls a d Fre c e tra t ed fom t he Terms a d Definitio s

cla s of IEC p blc t io s is u d sin e 2 0 Some e t rie

h v e b e c le te fom e rler p blc tio s of IEC TC 3 ,

7 ,8 a d CISPR

IEC Cust omer Servic Cent re - webst ore.ec ch/ csc

If y ou wis to giv e u your fe d a k o t his p blc tio or

n e furt her a sist an e, ple s c nt act h Cu t omer Serv ic

Ce t re:c c@ie c

A pro os de l'IEC

L Commis io Ele trote h iq e Inter natio ale (IEC) e t la premièr e orga is tio mo diale q i éla or e et p ble d s

Norme internatio ale p ur to t c q i a trait à léle tri ité,à léle tr oniq e et a x te h olo ie a p re té s

A pro os de p blc t i ns IEC

L c nte u te h iq e d s p blc tio s IEC e t c n tamme t rev Ve i e v u a s r er q e v u p s é e lé itio la

plu r éc nte, u c r rig n um o ame d me t p ut a oir été p blé

Cat alo ue IEC - webst ore.e ch/ cat alo ue

Applc t io a t on me p ur c n ult er t ou le re s ig eme t s

biblo ra hiq e s r le Norme intern t io ale ,

Sp cific t io s t ec niq e , R ap ort s te h iq e et a tre

d c me ts d l EC Dis o ible p ur P , Ma OS, t ablett es

A ndroid et iPa

Recherche de p blc t i ns IEC - w ww ie ch/ se rch u

L re h rc e avan é p rmet d t ro ver d s p blc tio s IEC

e uti s nt difére t s crit ère (n méro d référe c , t exte,

c mit é d’ét ud s,…) Ele d n e a s i d s informatio s s r le

projets et le p blc t io s remplac ée o retiré s

IEC Just P blshed - webst ore.e ch/ just pu lshed

R este informé s r le n uvele p blc t io s IEC Ju t

Pu ls e d tai e le n uv ele p blc t io s p ru s

Dis o ible e lg e et a s i u e fois p r mois p r emai

Ele t ro edia - www.ele t ro edia.org

L premier dic t io n ire e lg e d t erme éle tro iq e et

éle triq e I c ntie t plu d 3 0 0 terme et d finit io s e

a glais et e fa ç is, ain i q e le terme é uiv ale t s d n

14 la g e a ditio n le Eg leme t a p lé Vo a ulaire

Elec trote h iq e Int ern tio al (IEV) e lg e

Glos aire IEC - st d.e c h/ glos ar y

Plu d 5 0 0 e t ré s t ermin lo iq e électrot ec niq e , e

a glais et e fa ç is, e traite d s art icle Terme et

Définit io s d s p blc tio s IEC p ru s d p is 2 0 Plu

c rt ain s e tré s a t érie re e traite d s p blc t io s d s

CE 3 , 7 , 8 et CISPR d l EC

Serv ic Clent s - w ebst ore.e c h/ cs

Si vou d sire n u d n er d s c mme taire s r c tt e

p blc t io o si v ou av ez d s q e t io s c nt ac t ez-n u :

c c@iec c

requirement s

Élect roacoust ique – Apparei s de cor ect ion audit ive –

Part ie 4: Syst èmes de boucles d'induct ion ut il sé s à des fins de cor ect ion

audit ive – Ex igences de per ormances syst ème

FOREWORD 6

INTRODUCTION 8

1 Sco e 9

2 Normative ref eren es 9

3 Terms an def i ition 9

4 General 10 4.1 Proced re f or set ing up an commis ionin an au io-f req en y in u tion lo p s stem 10 4.2 Suita i ty of the site for the in tal ation of an au io-req en y in u tion-lo p s stem 10 4 3 Relation of the mag etic field stren th level at the telecoi to the sou d pres ure level at the micro hone 1

5 Usin comp nents of a sou d s stem in an ind ction-lo p s stem 1

5.1 General 1

5.2 Micro hones 1

5.3 Mixer 1

5.4 Power ampl fier 1

6 Meters an test sig als 1

6.1 Meters 1

6.1.1 Meters in general 1

6.1.2 Req irements common to b th typ s 1

6.1.3 True- m.s meter 12 6.1.4 Pe k programme meter (PPM) 12 6.2 Test sig als in general 12 6 3 Sp ec sig als 1

3

6.3.1 Live sp ec sig als 13

6.3.2 Recorded sp ec material 13

6.3.3 Simulated sp ec material 13

6.4 Pin noise sig al 13

6.5 Sin soidal sig al 13

6.6 Combi sig al 14

7 Mag etic bac grou d noise level of the in tal ation site 14

7.1 Method of me s rement 14

7.2 Recommen ed maximum mag etic noise levels 15

8 Characteristic to be sp cif ied, method of me s rement an req irements 15

8.1 General 15

8.2 Mag etic field stren th 16

8.2.1 Characteristic to b sp cif ied 16

8.2.2 Method of me s rement with a simulated sp ec sig al 16

8.2.3 Method of me s rement with pin noise 17

8.2.4 Method of me s rement with a sin soidal sig al 17

8.2.5 Method of me s rement with a combi sig al 17

8.2.6 Method of me s rement – Other 17

8.2.7 Req irements 17

8.3 Freq en y resp n e of the mag etic f ield 18

8.3.2 Method of me s rement with a simulated spe c sig al 18

8.3.3 Method of me s rement with pin noise 18

8.3.4 Method of me s rement with a sin soidal sig al 18

8.3.5 Method of me s rement with combi sig al 19

8.3.6 Method of me s rement – Other 19

8.3.7 Req irements 19

8.4 Useful mag etic f ield volume 19

8.4.1 Characteristic to b sp cif ied 19

8.4.2 Method of me s rement 19

8.4.3 Req irements 19

9 Smal-volume s stems 19

9.1 Ina pl ca i ty of the 'u eful mag etic field volume' con e t 19

9.2 Disa led ref uge an simi ar cal -p ints 2

9.3 Req irements f or disa led ref uge an simi ar cal -p ints 2

9.4 Cou ter s stems 2

9.5 Req irements f or cou ter s stems 2

10 Setin up (commis ionin ) the s stem 2

10.1 Proced re 2

10.2 Mag etic noise level d e to the s stem 2

10.2.1 Explanation of term 2

10.2.2 Method of me s rement with a sp ec sig al 2

10.2.3 Method of me s rement with pin noise 2

10.2.4 Method of me s rement with a sin soidal sig al 2

10.2.5 Method of me s rement with a combi sig al 2

10.2.6 Method of me s rement – Other (no input sig al) 2

10.2.7 Req irements 2

10.3 Ampl f ier overlo d at 1,6 kHz 2

10.3.1 Explanation of term 2

10.3.2 Method of test 2

10.4 Req irements 2

An ex A (informative) Sy tems f or smal u eful mag etic f ield volumes 2

A.1 Overview 2

A.2 Bod -worn au io s stems 2

A.3 Smal volume, defined se tin , mainly in hou ehold 2

A.4 Sp cif i location s c as help an information p ints, tic et an b n cou ters, etc 2

An ex B (informative) Me s rin eq ipment 3

B.1 Overview 3

B.2 Sig al sources 3

B.2.1 Re l sp ec 3

B.2.2 Simulated sp ec 3

B.2.3 Pin noise 3

B.2.4 Sine wave 3

B.3 Mag etic field stren th level meter 31

B.3.1 General recommen ation 31

B.3.2 Pe k-programme meter (PPM) typ 31

B.3.3 True r.m.s meter typ 31

An ex C (informative) Provision of information 3

C.1 General 3

C.2 Inf ormation to b provided to the he rin aid u er 3

C.3 Inf ormation to b provided to s stem in tal ers an by them to u ers 3

C.4 Inf ormation to b provided by the man facturer of the ampl fyin eq ipment 3

An ex D (inf ormative) Me s rin sp ec sig als 3

An ex E (informative) Basic the ry an practice of au io-req en y in u tion-lo p s stems 3

E.1 Pro erties of the lo p an its mag etic field 3

E.2 Directional resp n e of the telecoi of a he rin aid 3

E.3 Sup lyin the lo p c r ent 4

E.4 Sig al sources an ca les 4

E.4.1 Micro hones 4

E.4.2 Other sig al sources 4

E.4.3 Ca les 4

E.5 Care of the s stem 4

E.6 Mag etic u its 4

An ex F (informative) Efects of metal in the bui din stru ture on the mag etic field 4

An ex G (inf ormative) Calbration of ield-stren th meters 4

An ex H (inf ormative) Ef fect of the asp ct ratio of the lo p on the mag etic field stren th 4

H.1 Overview 4

H.2 Ef fect of asp ct ratio on field p t ern 4

An ex I (informative) Overspi of mag etic field f rom an in u tion-lo p s stem 51

I 1 General 51

I 2 Examples of overspi is ues 51

I 3 Ad res in overspi is ues 51

Biblogra h 5

Fig re 1 – Flow c art for the o eration in this stan ard 10 Fig re 2 – Me s rement p ints f or disa led refu e an simi ar cal -p ints 21

Fig re 3 – Me s rement p ints f or a cou ter s stem 2

Fig re A.1 – Field p tern of a vertical lo p 2

Fig re A.2 – Contour plot of field stren th of vertical lo p 2

Fig re C.1 – Gra hical s mb l: in u tive coupl n 3

Fig re E.1 – Persp ctive view of a lo p, s owin the mag etic field vector p th 3

Fig re E.2 – Stren th of the comp nents of the mag etic f ield d e to c r ent in a horizontal rectan ular lo p at p ints in a plane a ove or b low the lo p plane 3

Fig re E.3 – Field p tern of the vertical comp nent of the mag etic field of a horizontal lo p 3

Fig re E.4 – Field p tern of the vertical comp nent of the mag etic field of a vertical lo p 0,7 m s uare 4

Fig re E.5 – Persp ctive view of the variation of the vertical field stren th level at an o timum heig t a ove a horizontal rectan ular lo p 41

Fig re E.6 – Directional resp n e of the mag etic pic -up coi ( elecoi) of a he rin aid 4

Fig re F.1 – Mag etic f ield p t ern of a 10 m by 14 m lo p, 1,2 m a ove its plane 4

Fig re F.2 – Mag etic field p t ern of a 10 m by 14 m lo p, 1,2 m a ove its plane,

s owin the eff ect of metal (iron) in the flo r 4

Fig re G.1 – Triple Helmholtz coi f or cal bration of meters 4

Fig re H.1 – Variation of the c r ent req ired to prod ce a sp cified mag etic f ield

stren th at a sp cific p int with the dimen ion an asp ct ratio of the lo p 4

Fig re H.2 – Sq are an rectan ular lo ps 5

Ta le 1 – Ap l cation of sig als 1

2

Ta le 2 – Sp cification of the combi sig al 14

Ta le 3 – Mag etic field stren th typical y prod ced by dif ferent test sig als, with an

ampl f ier havin p ak-detectin AGC 17

INTERNATIONAL ELECTROTECHNICAL COMMISSION

1 Th Intern tio al Ele trote h ic l Commis io (IEC) is a worldwid org niz tio f or sta d rdiz tio c mprisin

al n tio al ele trote h ic l c mmite s (IEC Natio al Commite s) Th o je t of IEC is to promote

intern tio al c -o eratio o al q e tio s c n ernin sta d rdiz tio in th ele tric l a d ele tro ic f i ld To

this e d a d in a ditio to oth r a tivitie , IEC p bls e Intern tio al Sta d rd , Te h ic l Sp cific tio s,

Te h ic l Re orts, Pu lcly Av ia le Sp cif i atio s (P S) a d Guid s (h re f ter refer e to a “IEC

Pu lc tio (s)”) Th ir pre aratio is e tru te to te h ic l c mmite s; a y IEC Natio al Commite intere te

in th s bje t d alt with ma p rticip te in this pre aratory work Intern tio al g v rnme tal a d n

n-g v rnme tal org niz tio s laisin with th IEC als p rticip te in this pre aratio IEC c la orate clo ely

with th Intern tio al Org niz tio f or Sta d rdiz tio (ISO) in a c rd n e with c n itio s d termin d b

a re me t b twe n th two org niz tio s

2) Th f ormal d cisio s or a re me ts of IEC o te h ic l maters e pre s, a n arly a p s ible, a intern tio al

c n e s s of o inio o th rele a t s bje ts sin e e c te h ic l c mmite h s re re e tatio fom al

intere te IEC Natio al Commite s

3) IEC Pu lc tio s h v th f orm of re omme d tio s f or intern tio al u e a d are a c pte b IEC Natio al

Commite s in th t s n e Whie al re s n ble ef forts are ma e to e s re th t th te h ic l c nte t of IEC

Pu lc tio s is a c rate, IEC c n ot b h ld re p n ible f or th wa in whic th y are u e or for a y

misinterpretatio b a y e d u er

4) In ord r to promote intern tio al u iformity, IEC Natio al Commite s u d rta e to a ply IEC Pu lc tio s

tra s are tly to th ma imum e te t p s ible in th ir n tio al a d re io al p blc tio s An div rg n e

b twe n a y IEC Pu lc tio a d th c re p n in n tio al or re io al p blc tio s al b cle rly in ic te in

th later

5) IEC its lf d e n t pro id a y ate tatio of c nf ormity In e e d nt c rtific tio b die pro id c nf ormity

a s s me t s rvic s a d, in s me are s, a c s to IEC mark of c nformity IEC is n t re p n ible f or a y

s rvic s c rie o t b in e e d nt c rtif i atio b die

6) Al u ers s o lde s re th t th y h v th late t e itio of this p blc tio

7) No la i ty s al ata h to IEC or its dire tors, emplo e s, s rv nts or a e ts in lu in in ivid al e p rts a d

memb rs of its te h ic l c mmite s a d IEC Natio al Commite s f or a y p rs n l injury, pro erty d ma e or

oth r d ma e of a y n ture wh ts e er, wh th r dire t or in ire t, or for c sts (in lu in le al f ee ) a d

e p n e arisin o t of th p blc tio , u e of, or rela c u o , this IEC Pu lc tio or a y oth r IEC

Pu lc tio s

8) Ate tio is drawn to th Normativ refere c s cite in this p blc tio Us of th refere c d p blc tio s is

in is e s ble f or th c r e t a plc tio of this p blc tio

9) Ate tio is drawn to th p s ibi ty th t s me of th eleme ts of this IEC Pu lc tio ma b th s bje t of

p te t rig ts IEC s al n t b h ld re p n ible for id ntif yin a y or al s c p te t rig ts

International Stan ard IEC 6 1 8-4 has b en pre ared by IEC tec nical commit e 2 :

Electro cou tic

This third edition can els an re laces the secon edition publ s ed in 2 0 This edition

con titutes a tec nical revision

This edition in lu es the fol owin sig if i ant tec nical c an es with resp ct to the previou

edition: Ad ition of An exes G, H an I where more inf ormation is provided a out practical

con ideration an method of me s rement

The text of this standard is b sed on the fol owin doc ments:

Ful inf ormation on the votin for the a proval of this stan ard can b foun in the re ort on

votin in icated in the a ove ta le

This publ cation has b en draf ted in ac ordan e with the ISO/IEC Directives, Part 2

A l st of al p rts of the IEC 6 1 8 series, publ s ed u der the general title Electro c ustics –

H earin aids, can b f ou d on the IEC we site

The commit e has decided that the contents of this publ cation wi remain u c an ed u ti

the sta i ty date indicated on the IEC we site u der "htp:/we store.iec.c " in the data

related to the sp cific publ cation At this date, the publ cation wi b

• recon rmed,

• with rawn,

• re laced by a revised edition, or

IMPORTANT – Th 'colour inside' logo on the cover pa e of this publ c tion indic te

that it contains colours whic are consid re to be us f ul f or the cor e t

understa din of its conte ts Us rs s ould theref ore print this doc me t usin a

colour printer

Au io- req en y in u tion-lo p s stems are widely u ed to provide a me n for he rin aid

u ers, whose he rin aid are fit ed with in u tion pic -up cois, general y k own as 'telecoi s',

to minimise the pro lems of lstenin when at a distan e f om a source of sou d, s ielded

f rom the p rson sp akin by a protective win ow, an /or in a b c grou d noise Bac grou d

noise an distan e are two of the main cau es of he rin aid u ers b in u able to he r

satisfactori y in other than face-to-f ace q iet con ition In u tion-lo p s stems have b en

widely in tal ed in c urc es, the tres an cinemas, for the b nefit of hearin -imp ired p o le

The u e of in u tion-loo s stems has b en exten ed to man tran ient commu ication

situation s c as tic et of fices, b n cou ters, drive-in/drive-throu h service location ,

l f ts/elevators etc The widespre d provision of tele hone han sets that provide in u tive

coupl n to he rin aid is another sig ificant a pl cation, where ITU-T Recommen ation

P3 0 [1]

1

a pl es

Tran mis ion of an au io- req en y sig al via an in u tion-lo p s stem can of ten esta l s

an ac e ta le sig al-to-noise ratio in con ition where a purely acou tical tran mis ion would

b sig ificantly degraded by reverb ration an b c grou d noise

One f orm of au io f req en y in u tion-lo p s stem comprises a ca le in tal ed in the form of

a lo p u ual y arou d the p rimeter of a ro m or are in whic a group of he rin imp ired

p rson wis to l sten The ca le is con ected via an ampl fier to a micro hone s stem or

other source of au io sig al, s c as a radio receiver, CD player etc The ampl fier prod ces

an au io-f req en y electric c r ent in the in u tion lo p ca le, cau in a mag etic field to b

prod ced in ide the lo p The desig an implementation of the in u tion lo p is determined

by the con tru tion of the bui din in whic it is in tal ed, p rtic larly by the presen e of large

amou ts of iron, ste l or aluminium in the stru ture In ad ition the layout an p sition of

electrical ca les an equipment may generate hig levels of b c grou d au io feq en y

mag etic field that may inter ere with the rece tion of the lo p sig al

Another form of in u tion-lo p s stem employ a smal lo p, inten ed for commu ication with

a he rin -aid u er in its immediate vicinity Examples are: nec lo ps, tic et-cou ter s stems,

self contained 'p rta le' s stems an c airs in orp ratin in u tion lo ps (Se An ex A)

The pic -up device for an au io- req en y in u tion-lo p s stem is u ual y a p rsonal he rin

aid, of a typ fit ed with a pic -up coi ( elecoi ); however, sp cial in u tion lo p receivers may

b u ed in certain a plcation

_ _ _ _ _ _ _

This p rt of IEC 6 1 8 is a plca le to au io-req en y in u tion-lo p s stems prod cin an

alternatin mag etic field at au io f eq en ies an inten ed to provide an input sig al for

he rin aid op ratin with an in u tion pic -up coi ( elecoi) Throu hout this stan ard, it is

as umed that the he rin aid u ed with it conf orm to al relevant p rts of IEC 6 1 8

This stan ard sp cifies req irements for the field stren th in au io- req en y in u tion lo ps

f or he rin aid purp ses, whic wi give adeq ate sig al-to-noise ratio without overlo din

the he rin aid The stan ard also sp cifies the minimum f req en y resp n e req irements

f or ac e ta le intel gibi ty

Method for me s rin the mag etic f ield stren th are sp cif ied, an inf ormation is given on

a pro riate me s rin eq ipment (se An ex B), inf ormation that s ould b provided to the

o erator an u ers of the s stem (se An ex C), an other imp rtant con ideration

This stan ard do s not sp cify req irements f or lo p driver ampl f iers or as ociated

micro hone or au io sig al sources, whic are de lt with in IEC 6 4 9-1, or for the field

stren th prod ced by eq ipment, s c as tele hone han sets, within the s o e of ITU-T

P.3 0

The f ol owin doc ments, in whole or in p rt, are normatively referen ed in this doc ment an

are in isp n a le f or its a pl cation For dated ref eren es, only the edition cited a ples For

u dated referen es, the latest edition of the referen ed doc ment (in lu in an

amen ments) a pl es

IEC 6 2 8-3:2 13, So nd system e uipme t – Part 3: Amp fiers

IEC 6 2 8-10:1991, Sou d system e uipme t – Part 10: Pe k pro ramme lev l meters

IEC 616 2-1:2 13, Ele tro c ustics – So nd level meters – P art 1: Sp cific tions

IEC 6 4 9-1:2 10, El ectro c ustics – Audio-fe u n y ind uction-lo p systems for as isted

h aring – Part 1: Met hods of me suring a d sp cifying t he p rorma c of system

3.2

us f ul ma netic field volume

volume (of 3-dimen ional sp ce) within whic the s stem provides he rin -aid u ers with a

sig al of ac e ta le q al ty (se 8.4)

Note 1 to e try: In th f irst e itio of this sta d rd, th c n e t of 's e ifie ma n tic field are ' wa d fin d,

b c u e th t e itio did n t c n id r th v ry imp rta t 'h ig t' dime sio (th p rp n ic lar dista c b twe n th

The f low c art in Fig re 1 s ows the seq en e of o eration detai ed in this stan ard

Figure 1 – Flow c art for the operations in this sta dard

4.2 Suitabi ity of th site f or the instal ation of a a dio- re ue c in uction-loop

It may not b p s ible to o tain ac e ta le con ition f or an in u tion-lo p s stem in al

places where it is desira le It is therefore es ential, in the planning stage, to examine a

prop sed location with resp ct to the fol owin con ition :

• the mag etic noise level fom electric in tal ation , e.g he tin s stems in the f lo r or ro f ,

the electrical control of l g tin s stems (esp cial y in the tres), (se Clau e 7);

• the influen e of mag etiza le an electrical y-con u tin materials in the stru ture in

whic the lo p is inten ed to b in tal ed;

• the presen e of other in u tion-lo p s stems in the neig b urho d, the sig als of whic

may interf ere with that of the plan ed lo p s stem

NOT Te h iq e e ist to re u e th ma n tic f i ld stre gth o tsid a in u tio lo p, b t pre io sly-in tale

4.3 Relation of the ma netic field stre gth le el at the tele oi to the sound pre s re

le el at the microphone

An acou tic input sou d pres ure level of 70 dB an a lon -term average mag etic field

stren th level (L

e ,6 s) of −12 dB re 4 0 mA/m, i.e 10 mA/m, at the telecoi in a he rin

aid are as umed to give the same acou tic output level

5 Using components of a sound system in an induction-loop system

It may se m economical y at ractive to derive sig als f or an in u tion-lo p s stem f om a

sou d s stem servin the same sp ce, but it may not b tec nical y straig tf orward

5.2 Microphone

Micro hones f or a sou d s stem may not b p sitioned at the o timum places to o tain a

sig al as fe as p s ible f rom ambient acou tic noise an reverb ration It is es ential to

l sten to the sig al, prefera ly with hig -q alty he dphones, to as es its s ita i ty This

s ould b done for al micro hone sig als that the sou d s stem can prod ce in dif ferent

modes or con g ration

5.3 Mix r

The sig al for the ind ction-lo p s stem s al b taken f rom the mixer at a p int where the

level of that sig al is control ed in e en ently f om the sig al level in the c ain le din to the

lou sp akers of the sou d s stem

5.4 Power ampl f ier

It is p s ible that a s ita le sig al can b o tained f om the output of a p wer ampl fier, but

s c a sig al can b satisf actory only if it is a pl ed to an in u tion-lo p amplfier provided

with an input of a pro riate sen itivity an imp dan e, an with automatic gain control of a

range s f ficient to ac ommodate c an es in the sig al level in the sou d s stem

In general, it is not ad isa le to atempt to derive fom a sou d s stem a sig al s ita le for

con ection directly to an in u tion lo p Su h an intercon ection mu t b in ivid al y

desig ed to s it the electrical c aracteristic of the sou d s stem an the lo p s stem

6 Meters and test signals

6.1 Meters

6.1.1 Meters in ge eral

For historical re son , two typ s of mag etic field stren th meter are in u e, an it is not

practica le to disal ow the u e of either of them The res lts of meas rements with the two

typ s of meters are exactly eq al only for sin soidal sig als but in most cases the diff eren es

are not so large as to cau e seriou pro lems In ication are given in this stan ard of

diff eren es that may b exp cted in some cases In case of doubt, the res lt of me s rement

with the meter sp cif ied in 6.1.3 s al b definitive

6.1.2 Re uireme ts common to both type

The meter s al have a f req en y resp n e flat within ± 1 dB f rom 5 Hz to 10 kHz, fal n at

an ultimate rate of at le st 6 dB/octave outside this ran e A-weig tin s al also b provided

6.1.3 True-r.m.s meter

This meter was derived f rom the IEC sou d level meter sp cified in IEC 616 2-1 by re lacin

the micro hone by a mag etic pic -up coi an an ampl fier with feq en y resp n e

cor ection This meter has a true- m.s detector an a 12 ms averagin time con tant in 'F'

mode

A u ef ul ad itional fe ture is a p ak-hold in ication

6.1.4 Pe k programme meter (PPM)

This meter was derived fom the PPM Typ I sp cified in IEC 6 2 8-10 by ad in a mag etic

pic -up coi , u ual y together with a modern display (pref era ly a 'b r' typ ) in place of the

original movin -coi p inter in trument

It s al have d namic resp n es conf ormin to the relevant req irements of IEC 6 2 8-10, i.e

an at ac time-con tant of a proximately 5 ms an a rele se time-con tant of a proximately

1,0 s

6.2 Te t sig als in g neral

It is p s ible to u e several diff erent typ s of test sig al f or the setin -up an me s rement

of the f req en y mid-b n value (in case of doubt, the average value over the octave b n

centred on 1 kHz) an the f eq en y resp n e of the mag etic field stren th However, some

sig als are not s ita le for some purp ses, an the s ita i ty de en s on the ampltu e

c aracteristic of the ampl f ier in the s stem (se IEC 6 4 9-1) Ta le 1 s ows the ran e of

a pl cation of the sp cif ied test sig als The test sig al sp cified by the ampl f ier

man f acturer s al b u ed, u les the u e of a dif ferent sig al can b ju tif ied

Table 1 – Appl c tion of sig als

The u e of a wide an sig al an wide an meter to determine the ac ievement of the

ref eren e mag etic field stren th req ires a sp cial proced re to prevent seriou er ors First

the mag etic b c grou d noise level s al b me s red, to en ure s ff i ient sig al to noise

ratio, fol owed by the f req en y resp n e of the wanted mag etic f ield, af ter makin an

adju tments to the amplfier controls so as to ac ieve the flat est p s ible resp n e The

ac ievement of the referen e mag etic f ield stren th can then b determined

The f req en y- esp n e controls are set to ac ieve the f latest p s ible resp n e, otherwise

it is p s ible that the referen e mag etic field stren th is not ac ieved at 1 kHz Partic larly in

ro ms with metal reinf orcement, this may cau e con idera le er ors Also, if the sig al

-to-noise ratio is not s f ficient, p rtic larly if there are stron comp nents in the noise, this

method may not b ac urate

6.3 Spe c signals

Live sp ec is s ita le only for u e as a test sig al for the final verification (commis ionin )

of the o eration of an in u tion-lo p s stem However, l ve sp ec is an es ential element in

the s bjective as es ment of lo p s stems

Sp ec that has b en recorded u der control ed con ition an evaluated b th s bjectively

an o jectively may b u ed f or test purp ses Se also B.2.1

6.3.3 Simulate spe c material

Simulated or s nthetic sp ec material contain the f eatures of spe c in terms of its

ampl tu e, f req en y comp nents an temp ral c aracteristic , but has no recog iza le

The ISTS (International Spe c Test Sig al) [3] is recommen ed f or makin o jective

me s rements It was develo ed by EHIMA (Euro e n He rin In trument Man facturers'

As ociation) is derived f om 21 f emale sp akers in six dif ferent mother ton ues (American

En l s , Ara ic, Chinese, Fren h, German an Sp nis ) an is b sed on natural recordin s

but is largely non-intel gible b cau e of segmentation an remixin This was then analy ed

an comp red to the original recordin s in resp ct of diff erent criteria (in olvin man time,

f req en y an ampltu e distribution ) an f ou d to b entirely re resentative

6.4 Pink nois signal

The sig al s al b b n width lmited, with a p ak-to-p ak voltage (as me s red with an

os i os o e) to true r.m.s voltage ratio of at le st 18 dB (crest factor = 4), with a third

-octave-b n sp ctrum f lat within ±1 dB fom 10 Hz to 5 kHz

Ban width l mitation s al b car ied out by me n of at le st one-third-order Buterworth hig

p s an low p s fi ters givin − dB resp n es at 7 Hz an 6,5 kHz Se also B.2.3

NOT 1 This s e if i atio is giv n to e s re th t th te t sig al stimulate th s stem in a ma n r simiar to

n rmal s e c

NOT 2 Th tolera c of ±1 dB is n c s ary b c u e th th oretic l re p n e of th s e ifie 3rd ord r

Buterworth fiters are − ,8 dB at 10 Hz a d − ,7 dB at 5 kHz, a d c mp n nt tolera c s af fe t th e a t v lu s

This ef f ect is ta e into a c u t in th meth d of me s rin fe u n y re p n e u in th pin n is sig al

6.5 Sinusoidal signal

The sig al source s ould provide at le st the thre f eq en ies 10 Hz, 1 kHz an 5 kHz (one

at a time or simultane u ly, or b th), with les than 2 % total harmonic distortion in a 2 kHz

an 0 V to 1 V (so as to b s ita le for micro hone an l ne-level tests) The output source

imp dan e s ould b 6 0 Ω or les

A sin soidal sig al can b u ed f or the tests sp cified in 8.2 an 8.3 if the ampl tu e

c aracteristic of the ampl fier (se IEC 6 4 9-1) in lu es a ran e of input sig al voltages for

whic the output c r ent is pro ortional to the input voltage Ampl f iers with automatic gain

control normal y ex ibit s c a ran e Amplfiers with exp n ion or more complex sig al

-proces in can ot u ual y b me s red satisfactori y, other than for the ampl tu e

c aracteristic an the f ield stren th prod ced at 1 kHz, with sin soidal sig als Se also 6.2

6.6 Combi signal

This is a sig al con istin of s a ed tone-bursts of 1 kHz sine wave interle ved with pin

noise, an as s c is s ita le for an me s rements sp cified in this stan ard that the 1 kHz

sine wave or pin noise sig al is s ita le for

The level of the 1 kHz sine wave can b me s red, an with an ampl f ier employin a p ak

detectin AGC (automatic gain control), the lower r.m.s levels of the pin noise segments

me n that overal it prod ces con idera ly les he tin in the amplfier, whic is an

ad antage when test times are lon

The len th of the 1 kHz sine wave is l mited to a minimum of 1 s to alow either meter

sp cified in 5.1 to re c the cor ect me s rement level an b come sta le enou h for

ac urate re din b fore the tran ition

The d ration of the pin noise sig al s al b lon enou h in relation to that of the sine wave

sig al (ratio ≥ 4:1) in order to lower the overal ampl fier he tin , whie providin re sona ly

f req ent bursts of the sine wave sig al for me s rement purp ses

The sp cification of the combi sig al is s mmarized in Ta le 2

Table 2 – Spe ific tion of the combi signal

Me s rements of mag etic noise levels s al b p rormed by u in an A-weig tin network

in the me s rin in trument The me s red values of the mag etic field stren th, me s red

with a meter as sp cif ied in 6.1, with a pic -up coi whose magnetic axis is vertical (u les

otherwise sp cified, se 8.1), s al b expres ed as levels refer ed to the ref eren e mag etic

f ield stren th level (se 3.1) in dB

The in u tion-lo p s stem, if alre dy in tal ed, s al b switc ed off , but al other eq ipment

normal y o erated at the site s al b switc ed on an dimma le l g tin s al b set to half

dimmed The mag etic b c grou d noise level s al b me s red at a s f ficient n mb r of

p ints in the req ired u eful mag etic field volume The selection of p ints may b ran omly

distributed, but s ould b influen ed by the heig t ran e of the u ers (normal y 1,2 m f or

se ted l steners an 1,7 m for stan in l steners s ould b u ed), sp cific se tin

req irements, ph sical layout of the location an p tential influen e f om metal an interf erin

sig als

NOT It is u ef ul to lste to th n is , to f orm a s bje tiv impre sio of its s e trum, a d th s th lk ly

disturbin eff ect o lste ers

7.2 Re omme de ma imum ma netic nois le els

Ide l y, the dif feren e b twe n the ref eren e mag etic field stren th level an the A-weig ted

mag etic b c grou d noise level, whic for clarity is refer ed to as 'referen e sig al-to-noise

ratio' in this stan ard, s ould b gre ter than 4 dB This value is a pro riate in situation

where the a sthetic value of the sp ec is imp rtant an the b c grou d acou tic noise level

is very low, i.e in the tres an simi ar location Su h low levels of mag etic (an acou tic –

se Note 1) noise may not alway b present

NOT 1 He rin -aid u ers are e p s d to a o stic n is at th site, a wel a ma n tic n is Th re is n rmaly

n p int in re uirin a ma n tic n is le el far lower th n th a o stic n is le el a p rc iv d b h arin -aid

u ers, ta in into a c u t th ef f ect of h arin lo s o th a dibi ty of th a o stic n is Howe er, this d e n t

a ply if th u ers we r e r d fe d rs

In cases where commu ication takes priority over a sthetic con ideration , a hig er mag etic

noise level may b tolera le It s ould also b con idered that hig levels of mag etic

b c grou d noise can b tirin an th s s ould only b tolerated where the commu ication is

of a s ort an es ential nature For this re son, a sig al-to-noise ratio of 3 dB is

recommen ed as a minimum If the actual ratio is les than 3 dB, this s al b re orted an

agre d with the s stem o erator, so that con ideration can b given to remedial me s res

Un er some circ mstan es, a sig al-to-noise ratio of 3 dB mig t b u ac e ta le

However, if the mag etic noise has no sig ificant u desira le tonal q al ty or is mostly at low

f req en ies, then a hig er level of interf erin sig al may b ac e ta le For example, a

ref eren e sig al-to-noise ratio as low as 2 dB may b tolera le f or s ort p riod only The

actual au ible imp ct of the interf erin sig al s ould then b con idered in as es in whether

the overal b nefit of the s stem to he rin -aid u ers is pref era le to the a sen e of a lo p

s stem or the u e of an alternative tec niq e req irin a sp cial receiver (i.e inf ra- ed or

radio)

NOT 2 Su h a s stem c n b u e o ly with h a p o e s ita le f or h arin aid we rers, or a c s orie , s c

a wirele s mo ule , givin a dire t ele tric l in ut to th h arin aid, b c u e, if a n c lo p were u e , th

ma n tic b c gro n n is wo ld b pic e u b a h arin aid switc e to ‘T’

8 Characteristics to be specif ied, methods of me surement and requirements

The man facturer of the s stem (an in some cases, the ampl fier alone) s ould sp cify

values for those c aracteristic that are in e en ent of the p rtic lar in tal ation The in tal er

s ould me s re the values of those c aracteristic that are sp cif i to the in tal ation, an

provide the res lts to the s stem o erator for f uture ref eren e

Me s rements s al be made with a receiver havin the pic -up coi axis vertical, u les

sp cifical y req ested d e to a sp cial situation These may b en ou tered in places of

wors ip, hospitals an recovery are s, where p o le may b k e l n , prone or s pine

Where these situation exist, al me s rements s al b made with the me s rin receiver

p sitioned at the a pro riate orientation The recommen ation for b c grou d interf erin

sig al levels an the requirements for f ield stren th an f req en y resp n e remain

u c an ed f om the req irements f or a normal s stem

The levels of sp ec sig als, an ambient magnetic noise levels, may vary over p riod of

several secon s or even min tes It is imp rtant to o serve the sig al levels for a s ff i ient

time so that s c variation can b o served an taken into ac ou t

Durin the me s rements, the in u tion-lo p s stem s al b o erated u der the con ition

de med to b normal d rin u e Other p wer in tal ation s c as l g tin s al b adju ted

to the con ition de med to b normal d rin the u e of the in u tion-lo p s stem

A warm up time of at le st 10 min s al b al owed b f ore an me s rements take place

When c an in the input levels to the s stem, con ideration s al b given to the rele se time

of the AGC circ it, if an , whic may b several ten of secon s

The c aracteristic of AGC circ its vary, an this may af fect the res lts of me s rements on

the in tal ed lo p s stem If the AGC c aracteristic of the amplfier are not k own, the

relevant me s rements sp cified in IEC 6 4 9-1 s ould b car ied out first

AGC do s not cre te s ort-term non-l ne r distortion, an it is neces ary to distin uis

b twe n gain c an es whic are ne ded to ke p the average sig al level fairly con tant whi e

maintainin lne r s ort-term ampl fication an actual tran f er c aracteristic non-l ne rities

introd ced by some sp cial zed sig al proces in , whic c an e the p ak-to-average ratio A

cor ectly-p r ormin AGC s stem is not non-lne r over s ort time intervals (mi isecon s) in

common u derstan in

The method of me s rement u in diff erent sig als are alternatives: it is req ired to u e one

method only, but res lts for dif ferent method may b presented if desired If s c res lts

dif fer, the res lts of me s rements u in the method sp cified in this stan ard, or u ed or

recommen ed by the man facturer, are def i itive

8.2 Ma netic field stre gth

8.2.1 Chara teristic to be spe ifie

The maximum value of the mag etic field stren th, me s red with a meter as sp cif ied in 5.1

an a pic -up coi whose mag etic axis is vertical (u les otherwise sp cified – se 6.1),

prod ced by the s stem at one p int, at le st, within the u eful mag etic field volume

(se 8.4)

The method of me s rement given in 8.2.2 to 8.2.5 are b sed on the u e of an ampl f ier

whic has a 'o p drive' gain control fol owin an AGC stage, an are inten ed

ONLY to s ow

that the ampl fier is ca a le of prod cin the req ired mag etic f ield stren th If s c a

control is not provided, the man facturer's in tru tion s al nevertheles b fol owed In

order to determine that the whole s stem is ca a le of prod cin the req ired mag etic field

stren th fom the micro hone(s) an an other sig al source(s), the proced re des rib d in

Clau e 10 is neces ary

8.2.2 Method of me s reme t with a simulate spe c sig al

Ap ly the simulated sp ec sig al sp cified in 6.3.3 to the ampl fier an adju t its controls in

ac ordan e with the man f acturer's in tru tion so that the req irements sp cified in 8.4 are

satis ed

8.2.3 Method of me s reme t with pink nois

Ap ly the pin noise sig al sp cified in 6.4 to the ampl fier an adju t its controls in

ac ordan e with the man facturer's in tru tion u ti the req irements sp cified in 8.4 are

satis ed

8.2.4 Method of me s reme t with a sinusoidal signal

Ap ly a 1 kHz sin soidal sig al to the amplfier an adju t its controls in ac ordan e with the

man facturer's in tru tion u ti the req irements sp cified in 8.4.3 are satisfied

NOT Th ma uf acturer is fe to s e if y a ma imum d ratio of th te t, whic is lo g e o g to p rf orm th

me s reme t b t d e n t re ult in e c s iv temp rature ris in th amplf i r

8.2.5 Method of me s reme t with a combi signal

Ap ly the combi sig al sp cified in 6.6 to the amplfier an adju t its controls in ac ordan e

with the man f acturer's in tru tion u ti the req irements sp cified in 8.4 are satisfied For

an in trument with the 0,12 s averagin time, a p ak hold fe ture is p rtic larly u eful for

this me s rement

8.2.6 Method of me s reme t – Other

Not a pl ca le

8.2.7 Re uireme ts

The maximum value of the mag etic field s al b 4 0 mA/m, me s red with a meter as

sp cified in 6.1, an a sin soidal test sig al (or eq ivalent combi sig al, se 6.6) Becau e of

the p ak-detectin AGC u ed in most ampl f iers, other sig als are lkely to give diff erent

me s red values, de en in on whic typ of meter is u ed

If the man facturer recommen s the u e of a p ak programme meter an a non-sin soidal

test sig al, the me s red value of mag etic f ield stren th obtained with the specified test

sig al, ful y activatin the AGC circ it, s al b sp cified, when the ampl fier is set up to

prod ce, fom a sin soidal test sig al also f ul y activatin the AGC circ it, a f ield stren th of

4 0 mA/m, me s red with a meter as specif ied in 6.1

Typical values for the f ield stren th to b prod ced by the dif ferent test sig als, when an

ampl fier with p ak-detectin AGC is set up to prod ce 4 0 mA/m with a sin soidal sig al, are

given in Ta le 3 For an ampl fier with AGC whic is not p ak-detectin , the diff eren es are

e c e th 0,12 s r.m.s v lu a d c n c u e clp in of th sig al c re t It h s b e s own th t, u le s v ry

s v re, s c clp in h s n sig if i a t eff ect o s e c intel gibi ty Se [4]

8.3 Fre ue c re pon e of the ma netic field

8.3.1 Chara teristic to be spe if ie

The f eq en y resp n e of the mag etic f ield stren th, me s red with a pic -up coi whose

mag etic axis is vertical (u les otherwise sp cified, se 8.1)

8.3.2 Method of me s reme t with a simulate spe c signal

Proce d as fol ows

a) Ap ly the simulated sp ec sig al sp cified in 6.3.3.3 to the ampl fier an adju t its

controls in ac ordan e with the man facturer's in tru tion on the con ition u der whic

the f req en y resp n e s al b me s red (se IEC 6 4 9-1)

b) Me s re the one-third-octave b n sp ctrum of the sig al source

c) Me s re the one-third-octave b n sp ctrum of the mag etic field at a s ff i ient n mb r

of p ints within the u a le mag etic field volume

d) Subtract the res lts of b) f rom the res lts of c), so as to make the final res lts in e en ent

of the sp ctrum of the source

NOT Th u e of simulate s e c or oth r sig als of simiar c mple ity f or me s rin fe u n y re p n e is

rath r dif fic lt to c r y o t a c rately, a d is more s ite to re e rc a d d v lo me t a plc tio s th n for

c mmis io in

8.3.3 Method of me s reme t with pink nois

Proce d as fol ows

a) Ap ly the pin noise sig al sp cif ied in 6.4 to the ampl f ier an adju t its controls in

ac ordan e with the man f acturer's in tru tion on the con ition u der whic the

f req en y resp n e s al b me s red (se Clau e C.4)

b) Me s re the one-third-octave sp ctrum of the sig al source

c) Me s re the one-third-octave sp ctrum of the mag etic f ield at a s ff i ient n mb r of

p ints within the u a le mag etic field volume

d) Subtract the res lts of b) f rom the res lts of c), so as to make the final res lts

in e en ent of the sp ctrum of the source

Me s rements s al b made at le st in the one-third-octave b nd centred on 10 Hz, 1 kHz

an 5 kHz, at a s ff i ient n mb r of p ints within the u eful mag etic field volume (se 8.4)

Pref era ly, an analy is s ould b made of the variation of feq en y resp n e within the

volume, an the one-third-octave b n centred on 5 kHz is recommen ed f or the initial test

This is to en ure that an spuriou los es d e to con u tive metal stru tures are identif ied

8.3.4 Method of me s reme t with a sinusoidal signal

Proce d as fol ows

a) Ap ly the sin soidal sig al sp cified in 6.5 to the ampl fier an adju t its controls in

ac ordan e with the man facturer's in tru tion on the con ition u der whic the

f req en y resp n e s al b me s red (se Clau e C.4), so that the ampl fier is o eratin

b low the thres old of AGC, as sp cif ied by the man f acturer or determined ac ordin to

b) Me s re the mag etic field stren th prod ced.

c) Me s rements s al b made at le st at 10 Hz, 1 kHz an 5 kHz, at a s ff i ient n mb r

of p ints within the u eful mag etic field volume (se 8.4) Prefera ly, an analy is s ould

b made of the variation of f eq en y resp n e within the volume, an a feq en y of

5 kHz is recommen ed as a primary test f eq en y This is to en ure that an spuriou

los es d e to con u tive metal stru tures are identified

8.3.5 Method of me s reme t with combi signal

Proce d as fol ows

a) Ap ly the combi sig al as sp cified in 6.6 to the ampl f ier an adju t its controls in

ac ordan e with the man f acturer’s in tru tion on the con ition u der whic the

f eq en y resp n e s al b me s red (se Clau e C.4)

b) Using the meter sp cif ied in B.3.3, with no ad itional lon -term averagin , me s re the

one-third-octave sp ctrum of the sig al source an mag etic field as sp cif ied in 8.3.3 b)

to d), but ig ore the res lts o tained d rin the sine wave burst p rt of the combi sig al

8.4 Us ful ma n tic field volume

8.4.1 Chara teristic to be spe if ie

The volume within whic the req irements recommen ed or sp cified in Clau e 7, 8.2.7, 8.3.7

an 10.2.7 are met

8.4.2 Methods of me s reme t

Se Clau e 7, 8.2, 8.3 an 10.2 The me s rements s al b made at a s f ficient n mb r of

p ints in the predicted or req ired u ef ul mag etic f ield volume The selection of p ints

s ould b influen ed by where the u ers are l kely to b , their ran e of heig ts, sp cif i

se tin req irements, the ph sical layout of the location an p tential in uen e f om metal

an intererin sig als Normal y, me s rement heig ts of 1,2 m for se ted lsteners an

1,7 m for stan in l steners s ould b u ed

8.4.3 Re uireme ts

The me s red levels of mag etic field stren th at the selected p ints s al b within ± 3dB of

the level sp cified ac ordin to 8.2.7 This do s not a ply to smal volume s stems, f or whic

a wider ran e is ac e ta le Se Clau e 9 an An ex A F r the other c aracteristic , the

recommen ation in Clau e 7, an the requirements in 8.3.7 an 10.2.7 a ply

9.1 In ppl c bi ity of the 'us ful ma netic f ield volume' conc pt

For these s stems it is p s ible an neces ary to sp cify in the stan ard the p sition of the

me s rement p ints an not to u e the 'u eful mag etic f ield volume' con e t, where s f or

other s stems it is not p s ible, so for the lat er the 'u eful mag etic field volume' a pro c

9.2 Dis ble refug a d simi ar c l -p ints

Unles sp cifical y re laced by contractual req irements, me s rements s al b made at the

six p ints sp cified in Fig re 2 The referen e p int (or l ne) is the face or s r ace of the cal

-p int, intercom, or help p int closest to the u er, an is not neces ari y the location of the

mag etic field source

The semi-circ lar layout is more s ita le f or smal mag etic f ield sources an the rectan ular

layout is more s ita le for vertical or f lo r lo p sources Only one of the diagrams s al b

u ed for a given s stem

NOT An off set b twe n th ref ere c p int (or ln ) a d th p sitio of th ma n tic field s urc promote

e e n s of field p tern o er th are wh re p o le are e p cte to sta d

Dime sion in mil l imet re

The six me s rement p ints are req ired at b th 1,2 m an 1,7 m Se Fig re 3 b), but there

is no req irement to me s re at a heig t of 1,4 m

9.3 Re uireme ts for dis ble refu e a d simi ar c l -p ints

Unles sp cifical y re laced by contractual req irements, the s stem s al comply with the

requirements sp cif ied in 9.3 an 8.3.7 at al me s rement p ints sp cif ied in Fig re 2,

acros b th vertical an horizontal ran es The mag etic field stren th level at these p ints

s al b ± dB ref 4 0 mA/m, me s red ac ordin to 8.2 At one p int at le st, it s al b

≥ dB re 4 0 mA/m

The mag etic field stren th level s al not b ab ve + dB ref 4 0 mA/m in the are where

p o le are exp cted to stan

NOT 1 This hig field stre gth is in vita le f or a simple v rtic l lo p of pra tic ble dime sio s If th sig al is

to lo dor distorte , th u er c n ju t mo e as ort dista c f urth r awa f rom th s urc of th ma n tic field

NOT 2 Cla s s 4 a d 7 d al with th s bje t of ma n tic b c gro n n is le el It is n t pra tic ble to s e ify

a re uireme t, a this mig t rule o t th pro isio of a s stem th t wo ld b at le st h lpf ul to u ers

9.4 Counter s stems

Unles sp cifical y re laced by contractual req irements, me s rements s al b made at the

p ints sp cif ied in Fig re 3 The referen e p int is the face or s r ace of the cou ter closest

to the u er, an is not neces ari y the p sition of the mag etic f ield source

For cou ter s stems there is of ten a req irement to control overspi to an adjacent cou ter

p sition Control n this overspi is l kely to b a sig ificant factor in desig , an as s c may

res lt in a compromise of even es of field over the are where p o le are exp cted to stan

NOT 1 It is n t n c s ary to re u e th ma n tic s i b twe n c u ter p sitio s b low a le el c mp ra le with

th a o stic s i A dif fere c gre ter th n 2 dB b twe n e uiv le t p sitio s at th two c u ters c uld b

e o g

NOT 2 Th b u d rie of th are wh re p o le are e p cte to sta d c n ot b sta d rdiz d a th y d p n

o th b idin la o t a d oth r f actors

NOT 3 For v rtic l lo p , a off set b twe n th refere c p int a d th p sitio of th lo p promote e e n s

of ield p tern o er th are wh re p o le are e p cte to sta d, b t re u e th ef fe tiv n s of s i c ntrol to

th n xt c u ter p sitio

Dime sion in mil l imet re

Me s rements at the thre p ints s own in the plan view are req ired at 1,2 m, 1,4 m an

1,7 m

9.5 Re uireme ts for counter s stems

Unles sp cifical y re laced by contractual req irements, the s stem s al comply with the

requirements sp cif ied in 9.3 an 8.3.7 at al me s rement p ints sp cif ied in Fig re 3,

acros b th vertical an horizontal ran es The mag etic field stren th level at these p ints

s al b ± dB ref 4 0 mA/m, me s red ac ordin to 8.2 At one p int at le st, it s al

b ≥ dB re 4 0 mA/m

The f ield stren th s al not b a ove + dB re 4 0 mA/m in the are where p o le are

exp cted to stan

NOT 1 This hig field stre gth is in vita le f or a simple v rtic l lo p of pra tic ble dime sio s If th sig al is

to lo dor distorte , th u er c n ju t mo e as ort dista c f urth r awa f rom th s urc of th ma n tic field

NOT 2 Cla s s 4 a d 7 d al with th s bje t of ma n tic b c gro n n is le el It is n t pra tic ble to s e ify

a re uireme t, a this mig t rule o t th pro isio of a s stem th t wo ld b at le st h lpful to u ers

10 Set ing up (commissioning) the system

10.1 Proc d re

The commis ionin proced re s al in lu e a test with the sou d sources ( alker, etc.) in their

normal p sition with resp ct to the s stem micro hone(s), an with an other sources, s c

as a CD player Me s rements s al b made to c ec that the controls of the amplfier, etc

are set so that the mag etic f ield stren th sp cif ied in 8.2.7 is ac ieved If the amplf ier has a

gain control precedin the AGC stage, an an in icator that the AGC is o eratin , it is

normal y s f ficient to set the control so that the in ication sp cified by the man f acturer is

ac ieved The referen e sp ec sig al as def i ed in 6.3.3.3 can also b u ed f or a more

o jective test, but as in al cases it s ould not b neces ary to adju t the ' o p drive' control

(gain control af ter the AGC stage) of the ampl fier

It is desira le that a smal n mb r of he rin -aid u ers s ould b present when a s stem is

b in set up for the first time or after exten ive c an es, to c ec that the s bjective res lts

are con istent with the me s rements It is imp rtant to c ec these he rin aid u ers for

cor ect o eration of their aid , an to en ure that they actual y u derstan what they are

s p osed to b l stenin to

It is es ential that the trained p rson (s) sp cif ied in Clau e E.5 are present, with the

receivers they wi u e for normal s stem c ec in

NOT Some h arin aid u ers s t th ir v lume c ntrols mu h to hig , a d s me old r h arin aid te d d to

o erlo d at a rath r low field stre gth Wh re sig if i a t v riatio s of o inio are e p rie c d b twe n h arin aid

u ers, o th p rorma c of a s stem, it c uld b n c s ary to c e k th s tin of th aid

10.2 Ma n tic nois le el due to the s stem

10.2.1 Expla ation of term

The mag etic field stren th, me s red with a pic -up coi whose mag etic axis is vertical

(u les otherwise sp cified, se 8.1), d e to the combination of bac grou d f ield an the

f ield d e to ampl f ier noise, with al the sig al inputs muted

NOT This v lu c n ot b me s re c re tly u ti th c mmis io in pro e ure h s b e c rie o t

10.2.2 Method of me s reme t with a spe c signal

Not a pl ca le

10.2.3 Method of me s reme t with pink nois

10.2.6 Method of me s reme t – Other (no input signal)

The mag etic f ield stren th s al b me s red as sp cified in Clau e 7, with A-weig tin , at a

s f ficient n mb r of p ints within the u eful volume, with the s stem switc ed on but with al

the sig al inputs muted

NOT If th sig al is d riv d f rom s u d s stem e uipme t, th mutin is a ple at th in uts of th t e uipme t

10.2.7 Re uireme ts

If the referen e sig al-to-noise ratio as me s red in 7.2 is gre ter than 4 dB, the mag etic

f ield stren th level at an p int with the s stem switc ed on s al not ex e d – 7 dB If the

ref eren e sig al-to-noise ratio is les than 4 dB then the mag etic f ield stren th level at any

p int with the s stem switc ed on s al not ex e d that with the s stem switc ed of by more

than 1 dB

10.3 Ampl fier ov rload at 1,6 kHz

10.3.1 Expla ation of term

If f req en y resp n e cor ection is a pl ed to the ampl fier in order to comp n ate f or metal

los , the amplfier may b ca a le of prod cin the req ired maximum mag etic field stren th

at 1 kHz but may b overlo ded at a hig er f eq en y A test u in a sig al at 1,6 kHz is

con idered s f ficient

10.3.2 Methods of te t

Ap ly a sine wave sig al at 1 kHz an adju t its level so that a mag etic f ield stren th 1 dB

les than the required value is o tained at a given p int Ap ly this sig al only for the s ortest

practica le time, in order to prevent overhe tin of the amplf ier Chan e the f req en y to

1,6 kHz without c an in its level

NOT 1 Th ma n tic field stre gth is inte tio aly in re s d b th fe u n y-e p n e c mp n atio

To determine whether the ampl f ier is overlo ded, a ply one of the fol owin tests:

• o serve the 'cl p in icator' on the ampl f ier, if one is provided;

• comp re the meas red output voltage with the man facturer's sp cified value;

• examine the output curent waveform with an os i os o e

NOT 2 Th c re t c n b c e k d b in lu in a low-v lu re istor, s c a 0,2 Ω, in s rie with th lo p, b t

n ith r e d of th re istor is lk ly to b at e rth p te tial For ma y amplfiers, a me s reme t c n b ma e

b twe n th 'c ld' lo p o tp t termin l a d th amplfier sig al e rth

10.4 Re uireme ts

The maximum value of the mag etic f ield stren th o tained f om the referen e sp ec sig al

For the ref eren e sig al an al re l sou d sources, the me s red value de en s on the

c aracteristic of the AGC circ it as wel as the sig al source itself, an as a res lt the

me s red r.m.s levels are l kely to deviate f rom the target value Provided the me s rement

time is lon enou h to o serve true maximum levels, the s stem s ould u ual y ac ieve ± dB

ref 4 0 mA/m (2 3 mA/m to 5 6 mA/m)

No c an e f rom 4 0 mA/m s al b made if the general publ c has ac es to, an u es, the

s stem The f ield stren th may b adju ted only if the s stem is solely u ed by a closed group

of he rin -aid u ers who in icate that, u les adju ted, the sig al level is u satisfactory ( he

s stem desig er has no control over where s stem u ers set the gain controls of their he rin

aid ) As explained in the Note b low, there are con ition in whic a close commu ity of

u ers may fin a field stren th of 4 0 mA/m u satisf actory

If a field stren th of 4 0 mA/m ± 3 dB is not ac ieved with re l sig als, the me s rement s al

b re e ted u in the sig al sp cif ied in 6.3.3.3 If the req irement is sti not ac ieved, the

s stem sp cification an the set-up proced re given in 4.1 s al b reviewed, to determine

whether the s stem as a whole, an the amplf ier, have b en cor ectly sp cified

NOT Be a s th f i ld stre gth v rie fom pla e to pla e, it is b u d to b e u l to th v lu d termin d

a c rdin to 8.2 at s me pla e , more at oth rs a d le s els wh re Th s bje tiv lo d e s als d p n s o th

v lume c ntrol s tin s of th h arin aid , whic are n t u d r th ma uf acturer's or in taler's c ntrol It is

th ref ore in p ro riate to ma e a s e ific v lu of f i ld stre gth ma d tory, wh n th re is a c n e s s th t a

c a g of th f i ld stre gth c n b ma e

Annex A

(inf or mative)

Systems f or smal useful magnetic field volumes

A 1 Ov rview

There is of ten a req irement to s p ly an in u tion lo p sig al to a he rin aid u er in

sp cial sed circ mstan es These can normal y b divided into thre major categories

A 2 Body-worn audio systems

Bod worn s stems general y u e a nec lo p, whic is es ential y a smal lo p worn rou d

the nec l ke a nec lace These lo ps are general y driven f rom an output on normal au io

eq ipment desig ed to drive stan ard he dphones, or to b con ected to cel phones or

mo i e phones The p sition of the pic -up coi in the he rin aid can normal y b control ed

e si y, an theref ore the location of the l stenin sp ce where the p rf orman e can b

me s red is e si y def i ed Per orman e as def i ed in this doc ment s ould normal y b

exp cted Se also IEC 6 4 9-1

A 3 Smal volume, def ined seating, mainly in households

A smal volume s stem, of ten in a hou ehold en ironment, may b either a nec lo p, a

sp cial sed c s ion whic the l stener sits on or a lo p emb d ed in the c air of the u er In

s c situation , the lo p is normal y driven by a smal dedicated ampl f ier For c s ion or

c air lo ps, al owan e s ould b made for the l stener's he d p sition, whic may b

sig if i antly af fected by the heig t of the actual p rson Per orman e as sp cified in this

stan ard s ould normal y b exp cted Sometimes, toward the l mits of the sp ce lkely to b

oc upied by a u er's he rin aid, the field stren th req irements may not b met

A 4 Specif ic locations such as help a d information points, ticket a d ba k

counters, etc

Information p ints an simi ar are of ten u ed in man fixed location The lstenin sp ce is

of ten diff i ult to def i e e si y, as it is neces ary f or the in talation to al ow for the p tential

he d heig t of the l stener, whic may b 1 m f or a c i d, 1,2 m f or a whe lc air u er, an

1,7 m f or a tal p rson Horizontal y, there are l kely to b sig ificant displacements f om the

o timal p sition Also in this situation, it may e si y oc ur that sig if i ant amou ts of metal

are present This cre tes diff i ulties with ac ievin go d p rorman e of the s stem, an the

req irements for b c grou d noise (of ten computer generated), sig al stren th an f eq en y

resp n e s ould b a pl ed with in ig t, takin into ac ou t that a s stem that is a go d as it

can b , given the restraints u der whic it has to o erate, is u ual y b t er than no s stem at

al

L o s for cou ter s stems vary in size an con g ration, an lo ps that exten into thre

dimen ion are not e s to tre t analytical y A vertical lo p is u ual y e s to in tal , but

ar an ed con entional y, its f ield p t ern is far f rom ide l, havin a large volume of low field

stren th centred on the horizontal axis, as s own, for a typical lo p 7 cm s uare, in

Fig re A.1

Fig re A.1 – Field pat ern of a v rtic l loop

Sin e the ran e of l stenin heig ts is 5 cm, it can b se n f rom the a ove that an o timum

range of field stren th , takin into ac ount the variation in distan e fom the lo ps, is fom

a out 12 cm a ove the centre of the lo p to a out 6 cm a ove the centre (Note that the lo p

con u tor is 3 cm a ove the centre, so the up er lo e of the field p t ern in Figure A.1,

in lu in the f ield outside the lo p, is u ed.) This me n that the centre of the lo p s ould b

10 cm a ove the flo r, so the lower ed e s ould b 7 cm a ove an the upp r ed e 14 cm

a ove the flo r It is eq al y eff ective to set the centre of the lo p at 18 cm a ove the flo r,

so that the lower lo e of the f ield p t ern is u ed

NOT Th s lo p p sitio s are f or a lo p 7 cm s u re: f or oth r dime sio s th o timum lo p p sitio s c n b

d termin d f rom a f i ld p tern plot simiar to th t in Fig re A.1

Fig re A.2 s ows that with the former ar an ement, there is no pro lem with coverage f rom

side-to-side The dimen ion are in metres, an an in erted diagram s ows that this a pl es

Fig re A.2 – Contour plot of f ield stre gth of v rtic l loop

This is for a 7 cm by 7 cm lo p, at a distan e of 0,3 m The field stren th level is relative to

that at the centre of the lo p an in its plane

The me s rement heig ts of 1,2 m an 1,7 m relate to the u ers, not the s stems, so a ply

eq al y to cou ter s stems, but an ad itional set of me s rements at a heig t of 1,4 m is

es ential, b cau e a p orly-desig ed in tal ation may have a n l there At this heig t, the

f ield stren th level ref 4 0 mA/m at some of the p ints may b up to, but not gre ter than,

+12 dB

For the p ints in the plan view, a les strin ent req irement is a pro riate for a cou ter

s stem, b cau e several p o le are lkely to close arou d a refu e an al wis to he r,

where s f or a cou ter s stem, ide l y only one p rson s ould b a le to he r

Annex B

(inf or mative)

Measur ing equipment

B.1 Ov rview

To fulfi the u derlyin aim of the stan ard – to en ure that in u tion-lo p s stems are

desig ed, in tal ed an set up cor ectly – it is neces ary to make the recommen ation for the

tec nical req irements for the me s rin eq ipment as simple as p s ible, b cau e if only

costly eq ipment can b u ed, the p rorman e of man in tal ation is u l kely to b

me s red The statu of a recommen ation is that a re son is req ired f or not o servin it

B.2 Signal sources

The recommen ed sources are CD recordin s of sp ec , without data compres ion Other

stated sources may b u ed, but it s ould b noted that there may b sig ificant dif feren es

in the res lts o tained f om me s rements u in dif ferent sp ec sig als

Where recorded sp ec is u ed, the recordin s ould b re rod ced with s ita le eq ipment,

whose output voltage s ould b ca a le of b in set to the ran es 0 mV to 10 mV an 0 V to

1 V The output source imp dan e s ould b 1 0 0 Ω or les

If local sp ec sources are u ed, several diff erent sp ec samples s ould b tested to en ure

that varia i ty b twe n sp akers do s not in al date the me s rements

B.2.2 Simulate spe c

The recommen ed sources are as f ol ows

• The recordin on CD s p led as a s p lement to the ITU P.5 stan ard [2] The male

sp ec s ould b u ed

• The ref eren e sp ec sig al (ISTS) Se 6.3.3.3

The recordin s ould b re rod ced with s ita le eq ipment, whose output voltage s ould b

ca a le of b in set to the ran es 0 mV to 10 mV an 0 V to 1 V The output source

imp dan e s ould b 1 0 0 Ω or les

B.2.3 Pink nois

This source s ould prod ce pin noise with a p ak-to-p ak voltage (as me s red with an

os i os o e) to true r.m.s voltage ratio of at le st 8, with a one-third-octave b n sp ctrum

f lat within ±1 dB f om 10 Hz to 5 kHz, b n -l mited to b – dB or b low at 7 Hz an

6,5 kHz The output voltage s ould b ca a le of b in set to the ran es 0 mV to 10 mV an

0 V to 1 V The output source imp dan e s ould b 1 0 0 Ω or les

The b nd-l mitin fi ters s ould b at le st of the third-order; s c an active f ilter req ires one

o erational ampl fier section The b n lmitation is sp cif ied so that the sig al interacts with

AGC s stems in a man er simi ar to sp ec

B.2.4 Sine wa e

Se 6.5

s ould b gre ter than the me n diameter Its p sition within the in trument, an the direction

of its maximum sen itivity, s ould b cle rly marked

B.3.1.2 Me s reme t ra ge a d in ic tion

The me s rement ran e, whic may b divided into two or more s b- an es f or in re sed

resolution, s ould ide l y cover – 2 dB to + dB relative to 4 0 mA/m However, for many

purp ses, a ran e of – 2 dB to + dB is s f ficient, an this can b o tained u in f e l

y-avai a le low-cost display driver devices F r b th of the recommen ed typ s of meter, these

recommen ation f or the ran es, an th s the markin s of the in icator, refer to r.m.s values

of a sin soidal sig al The resolution s ould b ±1 dB or bet er in the ran e of levels fom

– dB to + dB relative to 4 0 mA/m Meters s ould b calbrated to re d 0 dB in a sin soidal

mag etic field at 1 kHz, whose stren th is 4 0 mA/m r.m.s

The in ication may b by me n of a movin -coi meter, an LED dot or b r display or an LED

or LCD digital display A 'p ak hold' f eature may b provided; in whic case, the me s red

'p ak hold values' may b gre ter than the 6 s average values (se 8.2.7) by a proximately

2 dB A preset control for set in the sen itivity may b provided (se An exes E an F)

B.3.1.3 External conne tions

One or more output con ectors s ould b provided, for the con ection of he dphones an

other me s rin eq ipment, s c as a sp ctrum analy er F r he dphones, the output s ould

conform to the relevant req irements of IEC 619 8 [5] but se also Clau e E.5 For external

me s rin eq ipment, an output of a proximately 1 V r.m.s at maximum level in ication is

u ual y s ita le The source imp dan e s ould b 1 0 0 Ω or les Con ectin a lo d

conformin to the sp cification of the meter s ould not cau e a c an e in the me s red res lt

by more than 0,2 dB

B.3.2 Pe k-programme meter (PPM) type

The p ak-programme meter typ may b a sp cial y desig ed in trument It s ould

in orp rate a ful-wave p ak rectif ier, givin d namic c aracteristic simiar to those of the

Typ I meter sp cified in IEC 6 2 8-10

Simplfied sp cification f or the meter dy amic , derived f rom IEC 6 2 8-10, are:

• a 10 ms tone burst at 5 kHz s ould prod ce a re din of −2 dB ±1 dB b low that prod ced

by a contin ou 5 kHz sin soidal sig al;

• the time b twe n the removal of a 1 kHz sin soidal sig al prod cin an in ication of 0 dB

an the in ication f al n to − 0 dB s al b 2,3 s ± 0,5 s If the in icator is not a movin

-coi meter, this may b determined by me s rin the variation with time of an a pro riate

voltage internal to the eq ipment, u in a storage os i os o e

B.3.3 True r.m.s meter type

The true r.m.s meter typ may b a sp cial y-desig ed in trument, or a sou d level meter

whose micro hone is re laced by a mag etic pic up coi , with an eq al zer ad ed to prod ce

a s bstantialy flat f eq en y resp n e in the u weig ted me s rement mode It is neces ary

that the meter in orp rates a true r.m.s rectifier, an me ts the relevant req irements for a

Clas 2 sou d level meter sp cif ied in IEC 616 2-1, ex e t as sp cif ied in B.3.1.2

B.4 Field stre gth lev l meter c l brator

A cal brator s ould prod ce a mag etic field stren th level of 4 0 mA/m r.m.s at 1 kHz within

a volume large enou h to in lu e the whole of the pic up coi of the meter with whic it is

inten ed to b u ed Ad itional f eq en ies of 10 Hz an 5 kHz s ould b provided, to

ena le the resp n es at these feq en ies to b c ec ed Se also An ex F

B.5 Spectrum analyzer

A sp ctrum analy er s ould provide one-third-octave b n analy is over at le st the

f req en y range 10 Hz to 5 kHz The f ilter c aracteristic s ould conform to those sp cif ied

Annex C

(inf or mative)

Provision of inf ormation

The fol owin req irements are inten ed to en ure that the en u er of the s stem, the

p rson resp n ible for the in tal ation an /or o eration of the eq ipment, an the

man f acturers of the eq ipment b provided with adeq ate information to en ure that

in u tion-lo p s stems o erate in ke pin with the req irements of this stan ard

The in tal er s ould provide at le st the f ol owin information

C.2 Inf ormation to be provided to the hearing aid user

A sig s ould b placed in a prominent p sition close to the entran e, or entran es where

there are more than one, of the are where an in u tion lo p is in tal ed The sig s al b of

s ff i ient size to b e si y re d an con tru ted of d ra le material An example of s c a

sig is given in Fig re C.1 The same s mb l s ould b u ed to in icate the presen e of

in u tion coupl n on tele hone han sets

A plan in icatin the u eful mag etic f ield volume s ould b placed b side the a

ove-mentioned sig or in orp rated in it

The name, or p sition, of the p rson resp n ible f or the pro er o eration of the lo p s stem

an how to contact them s ould also b given

For smal are in u tion-lo p s stems e.g win ow cou ters, a sig s ould b placed in a

prominent p sition where the he rin aid u er is exp cted to b normal y situated

Cle r in tru tion on how to u e the in u tion-lo p s stem s ould b avai a le to he rin aid

u ers on req est

SOURCE: E SI TR 101 7 7.[7]

Figure C.1 – Graphic l s mb l: indu tiv coupl ng

C.3 Inf ormation to be provided to system instal ers and by them to users

The f ol owin information s ould b provided:

• the plan sp cified in Clau e C.2;

• the sp cification f or the amplfier an as ociated eq ipment as given in Clau e C.4;

• the field stren th set up as des rib d in 10.4 (in lu in the notes);

• the set in of the control p sition to provide the req ired field stren th in the sp cif ied

mag etic field volume;

• the method by whic the mag etic field stren th can b monitored to en ure con istent

day-to-day o eration of the s stem;

• the a pro riate p sition of micro hones, the sig al req irements for external playb c

devices an the set in of a pro riate controls to en ure that the sp cified mag etic f ield

is prod ced u der normal o eration;

• the eff ect of other electrical eq ipment u ed in the are where the lo p s stem is in tal ed

C.4 Inf ormation to be provided by the manufa turer of the ampl fying

equipment

Se IEC 6 4 9-1

Annex D

(inf or mative)

Measuring speech signals

The previou edition of this stan ard refer ed to a 'on -term average level of sp ec sig als

as a referen e value However, this 'on -term average level can ot b formal y defined, as

s own by these me s rements of a sp ec sig al, u in a true r.m.s meter with diff erent

Furthermore, a meter with a lon averagin time can ot practica ly b u ed f or setin the

s stem gain control so as to o tain 10 mA/m 'on term average' at the referen e p int Even

with a 15 s averagin time, this would b a dif fic lt an inac urate proces , f or thre re son

• After every adju tment, it is neces ary to wait f or at le st 3 time-con tants, i.e 4 s, for

the voltages in the time-con tant circ it to sta i ze at the new input level (This eff ect can

b se n on s c a meter if a pin noise sig al is me s red.)

• Even with a 15 s averagin time, the meter re din is far f om con tant, an a c oice has

to b made b twe n estimatin an 'average' re din or takin the maximum re din over

a certain (but u sp cified an u recorded) time p riod

• The me s rement do s not determine whether the in u tion-lo p s stem can prod ce the

hig er field stren th neces ary for the re rod ction of the sp ec sig als without

u ac e ta le ampl tu e distortion

Usin a 0,12 s time-con tant, the re din is, of course, varia le f om moment to moment, so

that the u er also has to u e some s bjective averagin in this case

On the other han , the p ak programme meter is desig ed so that a rel a le re din of the

maximum level can b o tained without u d e o erator f atig e Numerou exp riments have

s own that a p ak programme f ield stren th meter, s bstantial y of the Typ 2 sp cif ied in

IEC 6 2 8-10, s aled in r.m.s values f or a sine-wave sig al, re d 5 0 mA/m when the s

ort-term r.m.s (12 ms average) f ield stren th of a typical sp ec sig al is 4 0 mA/m

In u tion-lo p s stems, u in no compres ion (as distin t fom automatic gain control) or only

a moderate amou t, set up in this way are fou d in practice to b eminently satisfactory in

terms of sig al level The meter resp n s immediately to c an es in the gain control set in

If the amplfier is overloaded on programme p ak , this is s own by the fai ure to ac ieve

5 0 mA/m

Annex E

(inf or mative)

Basic theory and practice of audio-f requency induction-loop systems

E.1 Properties of the loop and its magnetic f ield

A b sic in u tion lo p is a con u tor car yin an au io-f req en y c r ent whic s r ou d the

are where rece tion is req ired A c r ent f lowin in the lo p con u tor prod ces a mag etic

f ield, whose stren th is me s red in amp res p r metre The f ield stren th prod ced by a

given c r ent varies gre tly fom place to place within an outside the lo p Se [8] [9] an

[10] Su h lo ps prod ce detecta le mag etic field outside the req ired volume, an size

con traints can have a sig if i ant imp ct on the desig of s c lo ps Tec niq es exist f or

red cin the spi of sig al outside the wanted volume, an for coverin very large are s Se

An ex I an [1 ]

Fig re E.1 s ows a lo p, an a diagrammatic re resentation of the mag etic vectors The

direction of these vectors folows the circ lar lnes, an th s there are vertical an horizontal

comp nents The variation of the f ield stren th in sp ce is very large (as s own in Fig re E.2)

Alon lne Z1, whic is in the plane of the lo p, the field stren th reac es an extremely hig

value close to the wire Displacement f om the lo p plane (s c as s own by l ne Z2) as ists

in o tainin a more ac e ta le f ield distribution The l ne la el ed 's stem n l l ne' s ows that

the p ints at whic the vertical comp nent of the mag etic f ield is zero l e at in re sin

distan es outside the lo p p rimeter as the heig t of the l stenin p sition a ove the lo p

plane in re ses

For most au io- req en y in u tion lo p s stems, the l steners are stan in or sit in , so that

the axes of the telecoi s in the he rin aid are normal y vertical, an th s resp n to the

vertical comp nent of the mag etic f ield of the lo p In hospitals an places of wors ip,

however, the axes of some telecoi s may b horizontal or at some intermediate an le, so the

relevant comp nent of the mag etic field is then imp rtant At the ge metrical centre of a

sin le-turn s uare lo p of side d (in m), the f ield stren th H (in A/m) prod ced by a c r ent I

(in A) is given by H = 2 2 I/πd ) (in A/m), an is vertical (strictly, it is p rp n ic lar to the

plane of the lo p) Provided that the sides d

1, d

2

of a rectan ular lo p are not extremely

dif ferent, this formula is a pl ca le if d is taken as √(d

1d

2)

For inf ormation des s, etc the lo p is often p sitioned in a vertical plane, an then the

eff ective vertical comp nent whic exists at the level of the to wire, an b yon , is u ed to

drive the he rin aid Care is neces ary, as lowerin the l stenin p sition to the axis of the

lo p res lts in no u e ble sig al b in received

It can b s own that the most u if orm vertical field stren th within the projected are of a

rectan ular lo p (provided the ratio of the len th to width is not very large) is o tained at a

distan e fom the plane of the lo p of 0,12 times to 0,16 times the width of the lo p

Fig re E.2 s ows the distribution of the vertical comp nent acros a lo p as a f un tion of the

p sition an elevation of the lstenin p sition This has b en calc lated f or a lo p 1,5 times

lon er than the width, but the c an e f rom ne r s uare to an aspect ratio of 4 is q ite smal

This f i ure also s ows that gre ter sp cin of the l stenin p sition f om the lo p plane (or a

lo p of smaler dimen ion ) can b u ed at the exp n e of los of f ield stren th The worst

location f or a lo p is at he d-heig t ( his s ould b b rne in min if there is a pro osal to

route a lo p con u tor f rom f lo r level up an over a do rway) The c rves are la el ed with

the distan e f om the lo p as a p rcentage of the lo p width, whi e the horizontal axis gives

p sition as a p rcentage of the lo p width The vertical s ale in icates the field stren th

variation in decib ls

E.2 Dire tional response of the telecoi of a hearing aid

Whi e the directional resp n e of a telecoi may b slg tly af fected by metal p rts in the

he rin aid, the the retical resp n e fol ows a cosine law This me n that the resp n e is

red ced by only 3 dB at an an le of 4 ° to the mag etic axis, an red ced by only 9,3 dB at

a) Geometry of the lo p an p sitio s of me surement of the f ield paterns sh wn in b)

b) Paterns of he vertic l an h rizo tal comp nents

Figure E.2 – Stre gths of the compone ts of the ma netic field d e to c r e t in a

horizontal re ta gular loo at points in a pla e abov or below the lo p pla e

Fig re E.3 s ows the sp tial variation of the vertical field, s owin that, at large se aration

f om the lo p plane, the n l p ints are wel outside the lo p p rimeter, an that the field

stren th outside the lo p may b hig enou h to b u a le, or, hig enou h to interf ere with

other lo p s stems in the vicinity, or b th

Horiz nt al field stre gth me s re alo g a ln p ralel

to this a d 1,2 u its a ov e or b low t he lo p pla e

V ertic l field stre gt h me s re

alo g a ln p ralel to t his