Iec 61094 3 2016

ELECTROACOUSTICS – MEASUREMENT MICROPHONES – Part 3: Primary method for free-field cal bration of laboratory standard microphones by the reciprocity technique This p rt of IEC 610 4 • sp

Part 3: Primary method for free-field cal bration of laboratory standard

Partie 3: Méthode primaire pour l étalonnage en champ l bre des microphones

étalons de laboratoire par la méthode de réciprocité

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Part 3: Primary method for free-field cal bration of laboratory standard

Partie 3: Méthode primaire pour l étalonnage en champ l bre des microphones

étalons de laboratoire par la méthode de réciprocité

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CONTENTS

FOREWORD 4

1 Sco e 6

2 Normative referen es 6

3 Terms an definition 6

4 Referen e en ironmental con ition 7

5 Prin iples of fre - ield calbration by reciprocity 7

5.1 General prin iples 7

5.1.1 General 7

5.1.2 General prin iples u in thre micro hones 7

5.1.3 General prin iples u in two micro hones an an au i ary sou d source 8

5.2 Basic expres ion 8

5.3 In ert voltage tec niq e 9

5.4 Fre - ield receivin c aracteristic of a micro hone 9

5.5 Fre - ield tran mit in c aracteristic of a micro hone 10 5.6 Reciprocity proced re 1

5.7 Final expres ion for the fre - ield sen itivity 1

5.7.1 Method u in thre micro hones 1

5.7.2 Method u in two micro hones an an au i ary sou d source 12 6 Factors influen in the fre - ield sen itivity 12 6.1 General 12 6.2 Polarizin voltage 12 6.3 Shield config ration 12 6.4 Acou tic con ition 13 6.5 Position of the acou tic centre of a micro hone 13 6.6 De en en e on en ironmental con ition 14 6.6.1 General 14 6.6.2 Static pres ure 14 6.6.3 Temp rature 14 6.6.4 Humidity 14 6.6.5 Tran formation to referen e en ironmental con ition 14 6.7 Con ideration con ernin me s rement sp ce 15 7 Calbration u certainty comp nents 15 7.1 General 15 7.2 Electrical tran fer imp dan e 15 7.3 Deviation from ideal fre - ield con ition 15 7.4 At en ation of sou d in air 16 7.5 Polarizin voltage 16 7.6 Ph sical pro erties of air 16 7.7 Imp rfection of the ry 16 7.8 Un ertainty on fre - ield sen itivity level 17 An ex A (informative) Values for the p sition of the acou tic centre 19 An ex B (normative) Values of the air at en ation co ficient 2

B.1 General 2

An ex C (informative) En ironmental influen e on the sen itivity of micro hones 2

C.1 General 2

C.2 De en en e on static pres ure 2

C.3 De en en e on temp rature 2

An ex D (informative) Ap l cation of time selective tec niq es for removal of u wanted reflection an acou tic interferen e b twe n micro hones 2

D.1 General 2

D.2 Practical con ideration 2

D.2.1 Sig al- o-noise ratio 2

D.2.2 Reflection from wal s an me s rement rig 25 D.3 Freq en y l mitation 2

D.3.1 General 2

D.3.2 Me s rements b sed on frequen y swe ps 2

D.3.3 Me s rements b sed on pure tones 2

D.4 Generatin mis in p rtion of the freq en y resp n e previou to tran formin to the time-domain 2

D.4.1 General 2

D.4.2 Mis in freq en ies b low the minimum me s rement freq en y 2

D.4.3 Mis in freq en ies a ove the maximum me s red freq en y 2

D.4.4 Fi terin the exten ed freq en y resp n e 2

Bibl ogra h 2

Fig re 1 – Eq ivalent circ it for a receivin micro hone u der fre - ield con ition 9

Fig re 2 – Eq ivalent circ it for a tran mitin micro hone u der fre - ield con ition 10 Fig re A.1 – Example of the estimated values of the acou tic centres of LS1P an LS2 P micro hones given in the bibl ogra hical referen es for An ex A 1

9

Ta le 1 – Un ertainty comp nents 17

Ta le B.1 – Values for aten ation of sou d pres ure in air (in dB/m) 22

INTERNATIONAL ELECTROTECHNICAL COMMISSION

Part 3: Primary method for free-field cal bration of laboratory

standard microphones by the reciprocity technique

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International Stan ard IEC 610 4-3 has b en pre ared by IEC tec nical commite 2 :

Electro cou tic

This secon edition can els an re laces the first edition publ s ed in 19 5 This edition

con titutes a tec nical revision

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

edition:

a) a new informative an ex des ribin the u e of time-selective tec niq es to minimize the

influen e of acou tic reflection from the me s rement setup;

b) provision for the cal bration of micro hones in driven s ield config ration

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

Ful information on the votin for the a proval of this stan ard can b fou d in the re ort on

votin in icated in the a ove ta le

This publcation has b en drafed in ac ordan e with the ISO/IEC Directives, Part 2

A l st of al p rts in the IEC 610 4 series, publ s ed u der the general title Ele ctro coustc –

Me sureme nt microp ho es, can b fou d on the IEC we site

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

the sta i ty date in icated on the IEC we site un er "htp:/we store.iec.c " in the data

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

• reconfirmed,

• with rawn,

• re laced by a revised edition, or

• amen ed

IMPORTANT – The 'colour in ide' logo on the cov r pa e of this publ c tion indic te

that it contain colours whic are consid re to be us ful for th cor e t

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

colour printer

ELECTROACOUSTICS – MEASUREMENT MICROPHONES –

Part 3: Primary method for free-field cal bration of laboratory

standard microphones by the reciprocity technique

This p rt of IEC 610 4

• sp cifies a primary method of determinin the complex fre - ield sen itivity of la oratory

stan ard micro hones so as to establ s a re rod cible an ac urate b sis for the

me s rement of sou d pres ure u der fre - ield con ition ,

• is a pl ca le to la oratory stan ard micro hones me tin the req irements of

IEC 610 4-1,

• is inten ed for u e by la oratories with hig ly exp rien ed staf an sp cial zed

eq ipment

NOT Th c lbratio prin iple d scrib d in this p rt of IEC 610 4 is also a plc ble to workin sta d rd

micro h n s, prefera ly use with ut th ir prote tio grid

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

are in isp n a le for its a pl cation For dated referen es, only the edition cited a pl es For

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

amen ments) a pl es

IEC 610 4-1:2 0 , Me su reme nt microp ho e s – Part 1: Sp cificato s for lab oratory sta dard

microp ho es

IEC 610 4-2:2 0 , Electro coustc – Me asureme t microp ho e s – Part 2: Primary meth d

for p re ssure calb rato of lab oratory sta dard microp ho es b y th recip rociy tech ique

IEC TS 610 4-7:2 0 , Me asureme t microp ho e s – Part 7: Values for th difere ce b etwee n

free-field a d p re ssure se si viy le vels of lab oratory sta dard micro h ne s

ISO 9 13-1, Acoustc – Ate uato of sound durin p ro a ato ou tdo rs – Part 1:

Calcu lato ofth ab sorpto of sound b y th atmosp e re

ISO/IEC Guide 9 -3, Uncertainty of me sureme t – Part 3: G uide to th e p re ssio of

uncertainty in me sureme t (GUM:19 5)

3 Terms and definitions

For the purp ses of this doc ment, the terms an definition given in IEC 610 4-1,

IEC 610 4-2, ISO/IEC Guide 9 -3 an the fol owin a ply

3.1

pha e

<fre - ield sen itivity of a micro hone phase an le b twe n the o en-circ it voltage an the

sou d pres ure that would exist at the p sition of the acou tic centre of the micro hone in the

a sen e of the micro hone, for a sin soidal plane progres ive wave of given freq en y an

3.2

a oustic c ntre

<micro hone p int from whic a proximately spherical wavefronts from a sou d-emit in

tran d cer prod cin a sin soidal sig al at a given freq en y a p ar to diverge with resp ct

to a smal region arou d an o servation p int at a sp cified direction an distan e from the

sou d source

Note 1 to e try: Th a o stic c ntre of a re ipro al tra sd c r wh n use as a re eiv r is c in id nt with th

a o stic c ntre wh n use as a tra smiter

Note 2 to e try: This d finitio o ly a ples to re io s of th so n field wh re sp eric l or a pro imately

sp eric l wa efro ts are o serv d

3.3

e uiv le t point-tra s uc r

notional tran d cer oc upyin a sin le p int that, when located at the p sition of its acou tic

centre, simulates the tran mit in or receivin c aracteristic of a micro hone, for a

sin soidal sig al of given freq en y, an for a given o servation direction an distan e

fre -field conditions

airb rne sou d- ield en ironment where sou d waves can pro agate fre ly without

disturb n es of an kin

4 Reference environmental conditions

The referen e en ironmental con ition are:

• static pres ure 101,3 5 kPa

• relative h midity 5 %

5 Principles of free- ield cal bration by reciprocity

5.1 Ge eral principle

5.1.1 Ge eral

A reciprocity cal bration of micro hones may b car ied out by me n of thre micro hones,

two of whic s al b reciprocal, or by me n of an au i ary sou d source an two

micro hones, one of whic s al b reciprocal

NOT 1 If o e of th micro h n s isn t re ipro al it c n o ly b use as a so n re eiv r

NOT 2 L b ratory sta d rd micro h n s are re ipro al wh n use within th ir ln ar o eratin ra g

5.1.2 Ge eral principle using thre microphone

L t two of the micro hones b coupled acou tical y u der fre - ield con ition Usin one of

them as a sou d source an the other as a sou d receiver, the electrical tran fer imp dan e

is me s red When the acou tic tran fer imp dan e of the s stem is known, the prod ct of

the fre - ield sen itivities of the two coupled micro hones can b determined Usin p ir-wise

avai a le, from whic an expres ion for the fre - ield sen itivity of e c of the thre

micro hones can b derived

5.1.3 Ge eral principle using two micro hone a d a a xi ary sou d sourc

First, let the two micro hones b coupled acou ticaly u der fre - ield con ition , an the

prod ct of the fre - ield sen itivities of the two micro hones b determined as des rib d in

5.1.2 Next, let the two micro hones b seq ential y presented to the same sou d pres ure,

set up by the au i ary sou d source u der identical fre - ield con ition The ratio of the two

output voltages wi then eq al the ratio of the fre - ield sen itivities of the two micro hones

Th s, from the prod ct an the ratio of the fre - ield sen itivities of the two micro hones, an

expres ion for the fre - ield sen itivity of e c of the two micro hones can b derived

NOT In ord r to o tain th ratio of fre - ield se sitivities, a dire t c mp riso meth d c n b use , a d th

a xi ary so n so rc c n b a oth r ty e of tra sd c r or a third micro h n h vin me h nic l or a o stic l

c ara teristics whic difer from th seof th micro h n s b in c lbrate

5.2 Ba ic e pre sions

L b ratory stan ard micro hones are con idered reciprocal an th s the two-p rt formula of

the micro hones can b writ en as:

q is the volume velocity throu h the acou tical terminals (dia hragm) of the

micro hone, in c bic metres p r secon (m

is the acou tic imp dan e of the micro hone when the electrical

terminals are u lo ded, in p s al-secon s p r c bic metre (Pa⋅s⋅m

NOT Un erln d symb ls re rese t c mple q a tities

Formula (1) may then b rewrit en as:

whic con titute the formula of reciprocity for the micro hone

When the sou d pres ure p is not u iform over the s rface of the dia hragm, as wi b the

case at hig freq en ies when the micro hone is located in a plane progres ive wave, the

the micro hone In this case, Formula (1) wi also b val d for the re l micro hone throu h a

sp cial interpretation of , se 5.4 an 5.5

5.3 Ins rt volta e te hniq e

The in ert voltage tec niq e is u ed to determine the o en-circ it voltage of a micro hone

when it is electrical y lo ded

L t a micro hone havin a certain o en-circ it voltage an internal electrical imp dan e b

con ected to an external electrical lo d imp dan e To me s re the o en-circ it voltage, an

imp dan e, smal comp red to the lo d imp dan e, is con ected in series with the

micro hone an a cal bratin voltage a pled acros it

L t a sou d pres ure an a cal bratin voltage of the same freq en y b a pled alternately

When the calbratin voltage is adju ted u ti it gives the same voltage dro acros the lo d

imp dan e as res lts from the sou d pres ure on the micro hone, the o en-circ it voltage

wi b eq al in mag itu e to the cal bratin voltage

5.4 Fre -field re eiving c ara teristic of a microphone

L t a micro hone b placed in a progres ive plane wave of sou d pres ure p

0 The eq ivalent

circ it of the micro hone is given in Fig re 1, where is the sou d pres ure when the

dia hragm is block d an the actual sou d pres ure at the acou tic terminals of the

microphon und r free-field condition

where S(f,θ) is the s aterin factor an de en s on the ge metrical config ration of the

micro hone It is a fu ction of freq en y f an an le of in iden e θ of the sou d wave

impin in on the dia hragm of the micro hone

an th s, from the b sic definition, the fre - ield sen itivity is given by:

(4)

Formula (4) s ows that the dif eren e b twe n the pres ure sen itivity an the fre - ield

sen itivity is determined not only by the ge metry of the micro hone throu h the s aterin

factor S(f,θ) but also by the relation b twe n the acou tic imp dan e of the micro hone an

the radiation imp dan e

NOT Th efe t of th micro h n v ntin me h nism is n t a c u te for in th mo el prese te a d wi also

influ n e th difere c b twe n th pres ure se sitivity a d fre -ield se sitivity at lowfre u n ies (se 6.1)

5.5 Fre -field tra smit in c ara teristic of a microphon

L t a micro hone b u ed as a tran miter u der fre - ield con ition The eq ivalent circ it

of the micro hone is given in Fig re 2

IEC

Ke

1 micro h n

Figure 2 – Equiv le t circ it for a tra smit ing

micro hon und r fre -field condition

As , the two-p rt formula of tran mitin micro hone can b writ en as:

(5)

so that:

From the general prin iple of reciprocity, it can b ded ced that at a remote p int, the

eq ivalent p int tran d cer wi act as a simple source of stren th S(f,θ) = M

f

i an the

sou d pres ure at the distan e d b twe n this p int an the eq ivalent p int tran d cer

NOT Deriv tio of Formula (6) giv n a o e is b se o a lump d p rameter re rese tatio of th micro h n

(se Formula (1) A more rig ro s d riv tio c n b o tain d b usin a inte ral form of re rese tatio of th

formula of th micro h n

, resp ctively, b situated in a fre field facin e c other an with coin ident

prin ip l axes A c r ent i throu h the electrical terminals of micro hone 1 wi prod ce a

sou d pres ure

0

p given by Formula (6) at a distan e d from its acou tic centre, u der fre

-field con ition When introd cin micro hone 2 into the sou d field, neglectin los es in the

medium an as umin no interaction ta es place b twe n the two micro hones, the o en

-circ it voltage of micro hone 2 wi b :

t kdf

b in the distan e b twe n the acou tic centres of micro hone 1 an micro hone 2

At hig freq en ies the molec lar relaxation efects an vis ou los es in air can ot b

neglected an th s, the prod ct of the fre - ield sen itivities is given by:

e

d

kdUd

is the ph sical distan e b twe n the dia hragms of micro hone 1 an

micro hone 2, b in the actual distan e the sou d wave has pro agated

5.7 Fin l e pre sions for the fre -field s nsitivity

5.7.1 Method using thre microphone

Implementin the prin iples in 5.1.2, let the electrical tran fer imp dan e U

2 /

1

b denoted by

Z

e,12

with simi ar expres ion for micro hone p irs in olvin the third micro hone,

micro hone 3 The final expres ion for the complex fre - ield sen itivity of micro hone 1 is

e

(8)

Simi ar expres ion a ply for micro hone 2 an micro hone 3

The mod lu an phase of the fre - ield sen itivity can b derived from Formula (8),

whereup n the phase s ould b refer ed to the ful four-q adrant phase ran e, i.e 0 to 2 rad

5.7.2 Method using two microphone a d a a xi ary soun sourc

If only two micro hones an an au i ary sou d source are u ed, then implementin the

prin iples in 5.1.3, the final expres ion for the mod lu of the fre - ield sen itivity is:

d

kdd

comp rison again t the au i ary source, se 5.1.3

6 Factors influencing the fre - ield sensitivity

The fre - ield sen itivity of a la oratory stan ard micro hone de en s on p larizin voltage,

as it has an electrostatic tran d ction mec anism, an the en ironmental con ition

The b sic mode of o eration of a p larized electrostatic micro hone as umes that the

electrical c arge on the micro hone is k pt con tant at al freq en ies This con ition can ot

b maintained at very low freq en ies an the prod ct of the micro hone ca acitan e an the

p larizin resistan e determines the time con tant for the flow of c arge to an from the

micro hone Whie the o en-circ it sen itivity of the micro hone, as o tained u in the in ert

voltage tec niq e, wi b determined cor ectly, the a solute output from an as ociated

pre mpl fier to the micro hone wi decre se at low freq en ies in ac ordan e with this time

con tant

The con tru tion prin iples of la oratory stan ard micro hones imply that the static pres ure

b hin an in front of the dia hragm s al remain the same To comply with this a pres ure

eq al zin tub is u ed to con ect the b ck cavity of the micro hone to the external medium

The ef ect of this tub is that the fre - ield sen itivity wi a pro c zero at very low

freq en ies (b low a few hertz) The tec niq e des rib d in this stan ard is not s ita le for

determinin the fre - ield sen itivity in this freq en y ran e

Furthermore, the definition of the fre - ield sen itivity impl es that certain req irements b

fulfi ed by the me s rements It is es ential d rin a cal bration that these con ition are

controled s f iciently wel so that the res ltin u certainty comp nents are smal

6.2 Polarizing volta e

The sen itivity of a la oratory stan ard micro hone is a proximately pro ortional to the

p larizin voltage an th s the p larizin voltage actual y u ed d rin the cal bration s al b

re orted

To comply with IEC 610 4-1, a p larizin voltage of 2 0,0 V is recommen ed

6.3 Shield configuration

The o en-circ it voltage, an therefore the fre - ield sen itivity, de en s on the s ield

config ration Con eq ently, IEC 610 4-1 sp cifies a referen e mec anical config ration for

the s ield for u e in determinin the o en-circ it voltage Whi e the referen e mec anical

config ration is es ential, the s ield can either b grou ded (grou ded-s ield config ration),

or the output voltage from the micro hone can b a pl ed to the s ield (driven-s ield

config ration) It s al b stated whether the driven-s ield or grou ded-s ield config ration

The same s ield config ration s al a ply to b th tran mit er an receiver micro hones d rin

the cal bration

If an non-stan ard config ration is u ed, the res lts of a cal bration s al b refer ed to the

referen e mec anical config ration

If the man facturer sp cifies a maximum mec anical force to b a pl ed to the central

electrical contact of the micro hone, this l mit s al not b ex e ded

NOT 1 Wh n th shield is driv n, th lo din imp d n e as se n from th micro h n is ma imize , a d it c n

b d scrib d more a c rately th n in th c se of usin th gro n e shield c nfig ratio In th id al c se, in

whic th micro h n is a p rfe tly ln ar a d p s iv d vic a d th shield is eith r gro n e , or driv n from a

zero so rc imp d n e,th re is n difere c b twe n th o e -circ it se sitivity with gro n e or driv nshield

NOT 2 In th driv n-shield c nfig ratio , a plyin th o tp t v lta e from th micro h n to th shield me ns

th t a y difere c b twe n th sig al a ple o th shield a d o th c ntre-pin of th micro h n is n glgible

NOT 3 If a micro h n is c n e te to a pre mplfier b me ns of a a a ter th re is th p s ibi ty th t th

o e -circ it v lta e of th micro h n is n t d termin d pro erly b th insert v lta e te h iq e at hig

fre u n ies Th d viatio s d p n o th lo d imp d n e asse n from th micro h n

6.4 Acoustic con itions

The fre - ield sen itivity of a micro hone de en s on the ge metrical config ration of the

hou in containin the pre mpl fier For this re son, the micro hone an the s ield

config ration s al b atac ed to a c ln er whose diameter is eq al to the nominal diameter

of the micro hone, se Ta le 1 an Ta le 2 in IEC 610 4-1:2 0 The len th of the c l n er

s al b lon comp red to the diameter of the micro hone A minimum len th of twenty times

the diameter of the micro hone with a grad aly ta ered tran ition to the s p ortin stru ture

is recommen ed This ar an ement s al also a ply to the tran mit er micro hone

The definition of the fre - ield sen itivity of a micro hone refers to the sou d pres ure in an

u disturb d plane progres ive wave In the far field of a sou d source located u der fre - ield

con ition , spherical waves are en ou tered whic , at a s ficient distan e from the source,

are a proximately plane waves in a l mited region Th s, the distan e b twe n the receiver

micro hone an the tran miter micro hone s al b gre t enou h to en ure a proximately

plane waves in a s ita le region arou d the receiver micro hone (se 7.3) Con ersely, the

influen e of reflection from the interior s rfaces of an anec oic c amb r u ual y in re ses as

the distan e b twe n the two micro hones is in re sed Also the s at erin factor S(f,θ)

de en s on the c aracter of the sou d field an can only b u ambig ou ly defined for a true

plane progres ive wave Therefore, the metrological con ition s ould b careful y c osen

an it may b prefera le to car y out cal bration at more than one distan e to as es the

cal bration u certainty atributa le to de en en e on these con ition

6.5 Position of th a ou tic c ntre of a micro hone

The p sition of the acou tic centre of a micro hone can b determined from me s rements of

the sou d pres ure prod ced by the micro hone when u ed as a sou d source in a fre field,

as a fu ction of distan e r from an arbitrari y c osen referen e p int of the micro hone In a

l mited region of the far field, the sou d pres ure, cor ected for the efect of sou d

at en ation, wi fol ow the 1/r-law, r b in refer ed now to the acou tic centre of the

micro hone Th s, when plot in the in erse value of the me s red sou d pres ure as a

fu ction of the distan e from an arbitrari y c osen referen e p int of the micro hone (most

con eniently the centre of the dia hragm), a straig t lne can b fit ed (e.g by the method of

le st s uares) throu h the ploted values The intersection of this straig t l ne an the

a s is a axis determines the p sition of the acou tic centre relative to the referen e p int

The acou tic centres u ed to determine d

12 (se 5.7) s al relate to the orientation an

se aration u ed d rin the fre - ield calbration

An ex A contain information on typical values for the p sition of the acou tic centre for

NOT Sp cific a plc tio s ma re uire d finin th a o stic c ntre of th micro h n at a fix d p sitio , for

insta c , at th c ntre of th micro h n dia hra m Wh re s this wo ld result in th mo ulus of th se sitivity

h vin a d p n e c o th dista c b twe n micro h n a d so rc , it ma pro id a useful refere c for th

p ase resp nse Whie usin this fix d a o stic c ntre in th c lc latio s is p s ible, a altern tiv is to d termin

th se sitivity usin th a o stic c ntre b se o th in erse-dista c law a d later a ply a c re tio to th

p ase resp nse to th prefere p sitio of th a o stic c ntre

6.6 Depe de c on e vironme tal conditions

The general de en en e of the pres ure sen itivity on en ironmental con ition is given in

6.5 of IEC 610 4-2:2 0 In ad ition to this, the fre - ield sen itivity further de en s on

en ironmental con ition throu h the relation given in Formula (4) In this formula, the

radiation imp dan e is a fu ction of the air den ity an sp ed of sou d Simi arly the

s at erin factor S(f,θ) de en s on the wavelen th an th s on the sp ed of sou d in air

6.6.2 Static pre s re

In ad ition to the de en en e des rib d in IEC 610 4-2:2 0 (6.5 an An ex D), a further

de en en e is cau ed by the relation hip b twe n the acou tic imp dan e of the micro hone

an its radiation imp dan e d e to the c an e in the den ity of air with static pres ure The

major influen e of the mas term of the radiation imp dan e is to lower the resonan e

freq en y of the micro hone sl g tly

6.6.3 Temperature

In ad ition to the de en en e des rib d in IEC 610 4-2:2 0 (6.5 an An ex D), a further

de en en e is cau ed by the relation hip b twe n the acou tic imp dan e of the micro hone

an its radiation imp dan e d e to the c an e in the den ity an the sp ed of sou d in air

with temp rature In ad ition, a de en en e is cau ed by the s aterin factor S(f,θ) ac ordin

to Formula (4) d e to the c an e in the sp ed of sou d in air with temp rature

IEC TS 610 4-7:2 0 , Clau e 6 des rib s the temp rature de en en e of the diferen e

b twe n the fre - ield sen itivity an the pres ure sen itivity that s al b con idered in

ad ition to the temp rature de en en e of the pres ure sen itivity itself

NOT If a micro h n is e p se to e c s iv temp rature v riatio s, a p rma e t c a g in se sitivity c n

result

6.6.4 Humidity

Ac ordin to Formula (4) a sl g t ef ect may b fou d on the fre - ield sen itivity in ad ition to

influen e on the pres ure sen itivity, cau ed by the influen e of h midity on the den ity an

sp ed of sou d in air

NOT Certain c n itio s c n influ n e th sta i ty of p larizin v lta e a d b c plate c arg a d th refore

influ n e th se sitivity For e ample th surfa e resista c of th insulatio material b twe n th b c plate a d

th h usin of th micro h n c n d teriorate u d r e c s iv ly h mid c n itio s, p rtic larly if th material is

c ntamin te Th surfa e resista c h s a n tic a le efe t o th se sitivity of th micro h n at low

fre u n ies, esp cialy o th p ase resp nse

6.6.5 Tra sformation to refere c e vironme tal conditions

When re ortin the res lts of a cal bration, the fre - ield sen itivity s ould b refer ed to the

referen e en ironmental con ition if rel a le cor ection data are avai a le For moderate

deviation from referen e en ironmental con ition , the cor ection u ed for the pres ure

sen itivity may b a pl ed with a pro riately in re sed u certainty

The actual con ition d rin the cal bration s ould b re orted

6.7 Consid rations conc rnin me s reme t spa e

The test sp ce s al b as fre as p s ible of an ef ects that cau e in ta i ties in the sou d

field, for example b twe n me s rements of micro hone combination These in lu e

c an in en ironment con ition , air flows, thermal gradients an electromag etic

disturb n e

It s al have a level of b ckgrou d noise an vibration that ena les the me s rements to

me t the sig al- o-noise req irements of the me s rement s stem u ed In practice, ste s

s ould b ta en to red ce the b ckgrou d noise as mu h as p s ible

7 Cal bration uncertainty compone ts

In ad ition to the factors mentioned in Clau e 6, whic afect the fre - ield sen itivity, further

u certainty comp nents are introd ced by the method, the eq ipment an the degre of care

u der whic the cal bration is car ied out Factors whic afect the calbration in a known way

s al b me s red or calc lated with as hig ac urac as practica le in order to minimize

their influen e on the res ltin u certainty

7.2 Ele tric l tra sfer impe a c

Variou method are u ed for me s rin the electrical tran fer imp dan e with the neces ary

ac urac , an no preferen e is given

The c r ent throu h the tran miter is u ual y determined by me s rin the voltage acros a

cal brated imp dan e in series with the tran miter micro hone To en ure a cor ect

determination of the c r ent, the grou ded or driven s ield config ration s al b atac ed to

the tran miter micro hone The calbration of the series imp dan e s al in lu e an ca le

ca acitan e an other lo d imp dan e present when me s rin the voltage acros the

imp dan e This al ows the electrical tran fer imp dan e to b determined from a voltage

ratio me s rement an the cal brated value of series imp dan e

The voltage u ed to ex ite the tran mit er micro hone s al b s c that the u certainty

arisin from harmonic , either from this source or generated by the micro hone, is smal

comp red to the ran om u certainty

Noise or other interferen e s c as cros - alk, whether of acou tical or other origin, s al not

u d ly afect the determination of the fre - ield sen itivity

The presen e of electrical cros - alk res lts in a p rent deviation from the 1/r-law havin a

p riodicity of one wavelen th

NOT 1 Fre u n y sele tiv te h iq es c n b use to impro e th sig al-o-n ise ratio

NOT 2 Time sele tiv te h iq es c n b use to minimize th influ n e of cros -alk a d refle tio s from th

e viro me t a d th me h nic l su p rts in th c lbratio set u Se An e D

NOT 3 Cros -alk c n b me sure with th micro h n s p sitio e as d rin a c lbratio , b t with th

re eiv r micro h n su stitute b a d mmy micro h n h vin th same c p cita c a d e tern l g ometry as

th re eiv r micro h n , a d th n d terminin th resultin difere c in th ele tric tra sfer imp d n e

Altern tiv ly, cros -alk c n b d termin d b setin th p larizin v lta e to zero d rin a c lbratio In b th

meth ds, fre u n y sele tiv te h iq es area v nta e us

7.3 De iations from ide l fre -field conditions

Durin a calbration, certain acou tic req irements s al b fulfi ed (se 6.4) The calbration

s al b car ied out in a fre field This con ition is most practical y o tained in an anec oic

ro m where win is a sent an airb rne noise is minimal Due to the non-ide l p rforman e

de en ent reflection from the wals wi oc ur an res lt in deviation from the 1/r-law for the

mod lu of sou d pres ure The mag itu e of these deviation wi de en on the distan e

an direction from the sou d source At a given freq en y, the deviation as a fu ction of

distan e wi s ow a p riodicity b twe n a half an one wavelen th, cor esp n in to a

reflected wave that is p ralel or p rp n ic lar to the direct wave Normal y, the smal est

deviation are o tained in a region arou d the centre of the ro m an where the prin iple axis

is neither p ral el nor p rp n ic lar to an wal s of the ro m However, the presen e of

braces, gri work, ca les, s p orts of variou kin s, l g tin , venti ation, etc may give rise to

disturb n es in the sou d field

The influen e of these disturbin reflection can b red ced to ac e ta le levels only by

exercisin the utmost care when esta l s in the cal bration s stem an proced re

An ex D gives examples of time selective tec niq es that may also b u ed to red ce the

influen e of deviation from ide l fre - ield con ition

Further, in order to en ure an a proximately plane wave in a s ita le region arou d the

receivin micro hone, it is recommen ed that the distan e b twe n the two micro hones

d rin the cal bration s ould b gre ter than ten times the nominal diameter of the

micro hones

7.4 At e uation of sou d in air

Un er practical con ition , sou d waves pro agatin in a fre field wi b at en ated d e to

molec lar relaxation ef ects an to thermal-vis ou los es

This aten ation de en s on the freq en y of the sou d, the water va our content an the

temp rature of the air It is determined by the re l p rt α, the air aten ation co f icient, of the

complex pro agation co ficient γ in Formula (6)

Values of the air aten ation co ficient expres ed in ne ers p r metre s al b calc lated

from the formula given in An ex B

7.5 Polarizing volta e

In order to determine the p larizin voltage, provision can b made for me s rin this voltage

directly at the terminals of the micro hone This is imp rtant when the polarizin voltage is

o tained from a hig -imp dan e source, d e to the finite in ulation resistan e of the

micro hone Alternatively, the in ulation resistan e of the micro hone can b me s red an

verified to b s f iciently hig that a me s rement of the p larizin voltage s p ly with the

micro hone removed, or a me s rement at a low imp dan e p rt of the p larizin voltage

s p ly, are val d

7.6 Phy ic l propertie of air

Certain q antities des ribin the ph sical pro erties of air are req ired for calc latin the

sen itivity of a micro hone or its de en en e on en ironmental con ition These q antities

de en on en ironmental con ition s c as static pres ure, temp rature an h midity

Values of the q antities an their de en en ies on en ironmental con ition are des rib d in

An ex B an in IEC 610 4-2:2 0 , An ex F

7.7 Imperfe tion of theory

The practical implementation of the reciprocity the rem an the derivation of the acou tic

tran fer imp dan e are b sed on some ide l zed as umption a out the micro hones an the

movement of the micro hone dia hragm The fol owin are examples where these

• Smal s ale imp rfection in the micro hones may le d to diferent movement p tern of

the dia hragm b twe n in ivid al samples of a given micro hone model, res ltin in

variation in the p sition of the acou tic centre, in p rtic lar at hig freq en ies

• Micro hones may not b reciprocal The efect of this can b minimized by combinin only

micro hones of the same model

7.8 Unc rtainty on fre -field s nsitivity le el

The u certainty on the fre - ield sen itivity level s ould b determined in ac ordan e with

ISO/IEC Guide 9 -3 When re ortin the res lts of a cal bration, the u certainty, as a fu ction

of freq en y, s al b stated as the exp n ed u certainty of me s rement u in a coverage

factor k cor esp n in to a 9 % confiden e pro a i ty

Due to the complexity of the final expres ion for the fre - ield sen itivity in Formula (8), the

u certainty analy is of the acou tic tran fer imp dan e is u ual y p rformed by re e tin a

calc lation whie the variou comp nents are c an ed one at a time by their as ociated

u certainty The diferen e from the res lt derived by the u c an ed comp nents is then

u ed to determine the stan ard u certainty related to the variou comp nents

Ta le 1 l sts a n mb r of comp nents af ectin the u certainty of a calbration Not al of the

comp nents may b relevant in a given cal bration setup b cau e variou method are u ed

for me s rin the electrical tran fer imp dan e, for minimizin the influen e of cor elated

reflection from the en ironment an for determinin the acou tic centres of the micro hones

Table 1 – Unc rtainty comp ne ts

The u certainty comp nents l sted in Ta le 1 are general y a fu ction of freq en y an s al

b derived as a stan ard u certainty The u certainty comp nents s ould b expres ed in a

l ne r form but a logarithmic form is also ac e ta le as the values are typical y very smal an

the derived final exp n ed u certainty of me s rement is then es ential y the same

Annex A

(informativ )

Values for the position of the acoustic centre

As defined, the acou tic centre de en s on orientation, on freq en y an on the distan e of

the o servation p int from the micro hone At s f iciently remote o servation p ints, the

ef ect of the acou tic centre p sition on the cal bration u certainty diminis es At s c

distan es, the centre of the dia hragm may b ta en as the acou tic centre For distan es in

the ran e 15 mm to 5 0 mm, normal y u ed when car yin out reciprocity cal bration , the

values given in the biblogra h an s own in Fig re A.1 may b a pled The values for the

p sition of the acou tic centre refers to the prin ip l axis an are given relative to the s rface

of the dia hragm as a fu ction of freq en y for micro hones typ LS1P an LS2 P A p sitive

sig in icates that the acou tic centre is in front of the dia hragm The u certainty of the

values in Fig re A.1 is estimated to b les than 2 mm b low the resonan e freq en y of the

micro hones At present s c data are not avai a le for other typ s of micro hones

NOT In g n ral th a o stic c ntre wi b difere t for in ivid al micro h n s of th same ty e, p rtic larly at

hig fre u n ies aro n a d a o e th reso a c fre u n y of th micro h n In pra tic , use of a a era e or

ty ic l v lu of th a o stic c ntre ma simplfy th c lc latio of th fre -ield se sitivity In this c se it is

n c s ary to a d a a ditio al u c rtainty c mp n nt as o iate with th v ria i ty of th a o stic c ntre to th

u c rtainty b d et

Figure A.1 – Ex mple of th e timate v lue of the a oustic c ntre of LS1P a d

LS2 P micro hon s giv n in the bibl ographic l refere c s for Anne A

Annex B

(normative)

Values of the air at enuation coef icient

Certain q antities, des ribin the pro erties of air, enter the expres ion for calc latin the

fre - ield sen itivity of the micro hones, se Formula (6) an (7) Method for calc latin the

den ity an sp ed of sou d in h mid air (in lu in disp rsion ef ects) are des rib d in

IEC 610 4-2 Method for calc latin the air at en ation co f icient are given in ISO 9 13-1

Its An ex A des rib s the ph sical mec anisms for the phenomenon, an 5.2 an An ex B

give formula for calc latin the at en ation co ficient as a fu ction of freq en y,

temp rature, static pres ure an relative h midity The calc lation proced re in B.2 fol ows

the g idel nes in ISO 9 13-1 with a few adju tments to comply with the proced res given in

IEC 610 4-2

B.2 Calculation procedure

The formula given in this an ex are b sed on the me s red en ironmental varia les:

t temp rature, in degre Celsiu (°C);

p

s

static pres ure, in p s als (Pa);

H relative h midity, as a p rcentage (%)

The calc lation of the air a sorption ta es into ac ou t that the h mid air is not an ide l gas

an th s some ad itional q antities an con tants are u ed:

T = 2 3,15 + t , the thermod namic temp rature, in k lvin (K);

vib,O+ α

The ac urac of the calc lated air at en ation co ficient is estimated to b ±10 % for

variation within the fol owin ran es:

Freq en y- o-pres ure ratio: 0,4 Hz/kPa to 10

4

Hz/kPa

Ta le B.1 gives values for the at en ation of sou d pres ure in air calc lated ac ordin to

ISO 9 13-1 u der the en ironmental con ition most relevant for reciprocity fre - ield

cal bration in a la oratory

The ta ulated values are expres ed in decib ls p r metre as 8,6 6 α

Table B.1 – Value for at e uation of sound pre s re in air (in dB/m)

Annex C

(informativ )

Environmental influence on the sensitivity of microphones

An ex C gives information on the influen e of static pres ure an temp rature on the fre

-field sen itivity of typ LS1P an LS2P micro hones At present, s c data are not avai a le

for other typ s of LS micro hones

A comprehen ive des ription of the influen e of the en ironmental con ition on the pres ure

sen itivity of micro hones is given in An ex D of IEC 610 4-2:2 0 In ad ition to this

influen e, the radiation imp dan e an the dif raction arou d the micro hone also de en on

the en ironmental con ition

Con tru tional detai s of the micro hone determine the relative influen e of the en ironmental

con ition The sp ed of sou d, the den ity an the vis osity of air are con idered l ne r

fu ction of temp rature an /or static pres ure The res ltin static pres ure an temp rature

co f icients of the micro hone are then con idered to b determined by the ratio of the fre

-field sen itivity at referen e con ition to the fre - ield sen itivity at the relevant static

pres ure an temp rature, resp ctively The examples a ply to the fre - ield sen itivity for

sou d pro agation alon the prin ip l axis toward the front of dia hragm

C.2 Dependence on static pres ure

Examples of the static pres ure co ficient refer in to the pres ure sen itivity of micro hones

are s own in Fig re D.1 of IEC 610 4-2:2 0 The ad itional influen e from the radiation

imp dan e wi modify this fig re primariy by lowerin the resonan e freq en y sl g tly Th s,

in the a sen e of a detai ed knowled e of the static pres ure co ficients for the fre - ield

sen itivities the cor esp n in values for the pres ure sen itivities may b u ed with an

in re sed u certainty at hig freq en ies In general, the static pres ure co f icient de en s

on con tru tional detai s of the micro hone an the actual values may dif er con idera ly for

two micro hones of diferent man facture althou h the micro hones may b lon to the same

typ Con eq ently, the static pres ure co f icients s own on Fig re D.1 of IEC 610 4-2:2 0

s ould not b a pl ed to in ivid al micro hones

The low- req en y value of the static pres ure co ficient general y l es b twe n

−0,01 dB/kPa an − ,0 dB/kPa for LS1P micro hones, an b twe n −0,0 3 dB/kPa an

−0,0 8 dB/kPa for LS2P micro hones

Examples of the temp rature co f icient refer in to the pres ure sen itivity of micro hones

are s own in Fig re D.2 of IEC 610 4-2:2 0 For the fre - ield sen itivity, temp rature

variation influen e the fre - ield sen itivity in two way : via the ad itional radiation

imp dan e an via the s aterin factor S(f,θ) (se Formula (4)

The influen e on the fre - ield sen itivity arisin from the radiation imp dan e fol ows the

same proced re as outl ned in C.2 a ove, i.e the temp rature co ficients val d for the

pres ure sen itivities may b a pl ed with an in re sed u certainty at hig freq en ies

The influen e of temp rature on the sp ed of sou d, i.e on the wavelen th, also afects the

s at erin factor S(f,θ) (se Formula (4) This ef ect wi de en on the an le of sou d

in iden e an may le d to hig values at hig freq en ies at certain an les of minimum

pres ure sen itivity level diferen es for typ LS1P an LS2P micro hones an their

de en en e on temp rature

The low- req en y value of the temp rature co ficient general y l es b twe n − ,0 5 dB/K

an + ,0 5 dB/K for b th LS1P an LS2P micro hones

The practical implementation of the fre - ield reciprocity method can s fer from a n mb r of

pro lems that wi have a degradin efect on the ac urac of the fre - ield sen itivity

determined from these me s rements These p rturb tion in lu e electrical cros - alk,

reflection from the mec anical elements u ed to hold the micro hones in place an from

ro m wal s, an the acou tic interferen e b twe n micro hones These p rturb tion can b

total y or p rtial y removed by fi terin in the time domain ( ime-selective or time-win owin

It is common, d e to the very low sig al- o-noise ratio (SNR) d rin the me s rement of the

electrical tran fer imp dan e, that pure sin soidal sig als are u ed for drivin the tran miter

micro hone L rge drivin sig als can introd ce harmonic artefacts that wi afect the level of

the fu damental freq en y comp nent, an th s s ould b avoided A p s ibi ty for

improvin the SNR is to a ply nar ow-b n fi terin either by hardware fi ters, by sp ctral

averagin , or by s n hronou averagin in the time domain

Sig al- o-noise ratio is freq en y de en ent At low an very hig freq en ies, the eficien y

of micro hones as sou d tran mit ers is very low, an the res ltin SNR is p or; it would

req ire impractical y lon averagin times to o tain a s f iciently nar ow fi terin b n width

Ac e ta le SNR’s are general y o tained in the freq en y ran e from 0,1 times to 3 times the

resonan e freq en y of the micro hones

This presents a l mitation on the freq en y ran e where me s rements of the electrical

tran fer imp dan e yield an ac e ta le ac urac , regardles of the sig al typ u ed

D.2.2 Refle tions from wal s a d me s reme t rig

The in tant in whic reflection from wals an other mec anical elements of the

me s rement rig re c the micro hones helps to decide how lon the total impulse resp n e

s ould b in order to ful y se arate early an late reflection from the direct impulse

resp n e Besides the practical desig con ideration that define the p sition of p tential y

reflective elements in the me s rement set up, a decision on the sou d a sorbin material

u ed in the me s rement ro ms wi have an influen e in the selection of the me s rement

p rameters

Circ lar con olution can oc ur when the tran fer fu ction is me s red in the freq en y

domain, an then tran formed into the time domain Con eq ently, for in uf iciently lon

impulse resp n es, or in ersely in uficiently smal freq en y sp cin , late-ar ivin secon ary

or tertiary reflection at the receiver micro hone wi b mis ed out, whic then fold over the

o tained impulse resp n e an most l k ly afect the final res lts This is p rtic larly critical in

• decre sin the size of the freq en y ste s, hen e in re sin the len th of the impulse

resp n e ( his wi also improve sampl n of the reflection an other disturb n es), an

• havin the wal s of the me s rement ro m covered with a sorb nt material that red ces

the ampl tu e of the in ident wave by at le st 3 dB

Circ lar con olution wi not oc ur when u in swe ps providin that s ficient zero-p d in is

u ed However, it is imp rtant to en ure that the impulse resp n e is s f iciently lon as to

in lu e al reflection

D.3 Frequenc l mitations

D.3.1 Ge eral

Swe t sine sig als can b u ed to me s re the freq en y resp n e in a finite freq en y

ran e that can in lu e low freq en ies, an up to a freq en y l mit pres rib d by the

in trumentation u ed Me s rements b sed on pure tones or bro db n sig als, l k pseu

o-ran om noise, can also start at low freq en ies, an go up to the hig freq en y l mit dictated

by the in trumentation Due to the sig al- o-noise l mitation dis u sed in D.2.1, the practical

dif ic lties of in lu in low freq en y me s rements may outweig the b nefits

In ad ition, the nature of e c me s rement method req ires that low freq en y l mitation

are tre ted diferently

Swe p sig als can b desig ed to have a freq en y content confined within a finite freq en y

b n The sig al proces in res ltin in the determination of the impulse resp n e is b sed

on the decon olution of the me s red resp n e with the in erse fi ter; therefore, the desig of

the sig al can b nefit from the fol owin con ideration

• The in erse fi ter wi amplfy those freq en y comp nents outside of the freq en y ran e

of interest This can p tential y mask the retrieved impulse resp n e if the sig al- o-noise

ratio (SNR) outside the freq en ies of interest is not hig enou h to cou terb lan e the

ampl fication provided by the in erse fi ter

• An dis ontin ity in the ex itation sig al wi yield artefacts in the form of impulsive noise

This is p rtic larly imp rtant when desig in the on et an en of the ex itation sig al

Ta erin the start an en of the ex itation sig al with a time win ow can avoid s c

pro lems

• If the time- req en y selectivity fe tured by swe p sig als is to b ful y exploited,

p rtic larly for distortion analy is, it is imp rtant to implement the decon olution proces

as a lne r con olution To do this in the freq en y domain by me n of the Dis rete

Fourier Tran form (DFT) or the Fast Fourier Tran form (FF ) algorithms, one has to ad

zero s at the en of the temp ral sig als ( his proced re is of en refer ed to as

“zero-p d in ”) so that there is no wrap-arou d ef ect when p rformin the con olution

When the freq en y resp n e is me s red with pure tones, the start an en p ints of the

freq en y ran e me s red can b c osen arbitrari y Due to the presen e of cros - alk,

b ckgrou d noise, an the p or tran mis ion ca a i ties of the micro hones, it is impractical

an p s ibly time-con umin to ma e me s rements at freq en ies b low one-eig th of the

resonan e freq en y for LS1 an LS2 micro hones

The hig est me s rement freq en y s ould b c osen s c that a more re l stic

re resentation of the direct impulse resp n e b twe n micro hones is o tained This u ualy

oc urs when the hig est me s red freq en y is more than 2 to 3 times the resonan e

D.4 Ge erating missing portions of the frequency response pre ious to

transforming to the time-domain

D.4.1 Ge eral

To ma e the tran formation from the freq en y domain to the time domain an o tain a

re resentation of the impulse resp n e that is fre of artefacts, it is neces ary to me s re the

tran fer imp dan e at freq en ies from −∞ to +∞ (or f rom 0 to +∞ when u in a one-sided

freq en y resp n e) This can ot b done in practice, an an me s rement wi b s bject

to wel-defined freq en y l mits, (f

min, f

ma) as dis u sed a ove A les ac urate but sti

rel a le re resentation can b o tained by u in two proced res: fi in the mis in freq en y

ran es with the retical y determined values of the freq en y resp n e, an a plyin either a

low-p s freq en y fi ter or a b n -p s freq en y fi ter b fore the tran formation to the time

domain

In prin iple, the me s red freq en y resp n e can b re resented as the multiplcation of the

infinite freq en y resp n e an a rectan ular b n -p s freq en y fi ter However, the u e of

this a pro c is not recommen ed b cau e s c a fiter has p orly at en ated side lo es that

can res lt in an impulse resp n e contaminated with spuriou comp nents

D.4.2 Mis ing fre ue cie below th minimum me s reme t freq e c

The electrical tran fer imp dan e from f= 0 to f= f

mincan b generated u in an expres ion

o tained from re-ar an in Formula (7):

=

(D.1)

At low freq en ies up to a out a q arter of the resonan e freq en y, the fre - ield sen itivity

can b calc lated by u in Formula (4) At these freq en ies the lo d of the radiation

imp dan e can b neglected, an the fre - ield sen itivity can b calc lated from the prod ct

of the pres ure sen itivity an the s aterin factor Furthermore, at these freq en ies air

a sorption can also b neglected in Formula (D.1)

The pres ure sen itivity can b determined exp rimental y from electrostatic actuator

me s rements or reciprocity cal bration However, it is imp rtant to note that in a fre - ield

cal bration, the static pres ure eq al zation tub of the micro hone is exp sed to the sou d

field The pres ure sen itivity can also b determined analyticaly from lump d p rameter

models

The difraction factor can b determined either n merical y, u in the Bou dary Element

Method, the Finite Element Method or an other integral formulation, or it can b determined

exp rimental y from me s rements in a large stan in wave tub

NOT In ord r to a oid stro g disc ntin ities b twe n th c lc late a d th me sure tra sfer imp d n es, a

smo thin tra sitio c n b a hie e b d finin a o erla fre u n y ra g in whic th fre u n y resp nse is

c lc late as a pro res iv gldin a era e

D.4.3 Mis ing fre ue cie ab v the ma imum me s re fre ue c

The hig freq en y data can ot in practice b determined from Formula (D.1) d e to the

man complexities of the b haviour of the micro hone: resonan es in the b ck cavity, vis ou

los es, etc A simpl fied a pro c may b to con ider the micro hone as a sin le degre of

fre dom s stem, where the sen itivity fal s by 12 dB p r octave a ove its resonan e

freq en y A simple an practical a pro c is to extra olate the complex freq en y resp n e

u in the last few me s rement p ints in whic the me s red freq en y resp n e decay

D.4.4 Fi tering the e te de fre ue c re pons

An ad itional me s re to ac elerate the decay of the freq en y resp n e at hig freq en ies

is to a ply a low-p s freq en y fi ter A low-p s fi ter desig ed for this purp se s ould have

hig ly aten ated secon ary lo es (at le st 8 dB), minimal rip le (0,01 dB maximum), an

l ne r phase It is also recommen ed that the rol -of freq en y of the fiter l es within the

extra olated freq en y ran e

NOT A b n -p s h vin simiar pro erties to th low-p s fiter c n also b use

Bibl ography

Relevant for An ex A:

WAGNER, R.P an NEDZELNITSKY, V Determination of acou tic center cor ection values

for typ LS2 P micro hones at normal in iden e, J Acoust Soc Am 10 , 19 8, 19 -2 3

BARRERA-FIGUEROA, S RASMUSSEN, K an JACOBSEN, F Th acoustc ce ter of

lab oratory sta dard microp ho es J Acoust Soc Am 12 , 2 0 , 2 6 -2 7

RODRIGUES, D DUROCHER, J.-N BRUNEAU, M an BRUNEAU A.-M A new method

for the determination of the acou tic center of acou tic tran d cers, Acta Acustca unie d wih

Acustca 9 , 2 10, 3 0-3 5

Relevant for An ex D:

A general biblogra h con ernin time selective method a pl ed to micro hone calbration is

presented in IEC 610 4-8 The bibl ogra h b low s p lements this with l terature that is

sp cifical y related to the a pl cation of time-selective tec niq es to fre - ield reciprocity

cal bration

IEC 610 4-8:2 12, Me su reme t microp ho e s – Part 8: Meth ds for determinin th fre -field

s ensi viy of workin sta dard micro h n s b y comp riso

LAMBERT, J.-M an DUROCHER, J.-N An lyse des pe rtu rb ato s acoustques lors de

létalo n ge e ch mp lb re des microp ho es étalo s a co de sateurs dis d’u n p uce p r la

tech iqu e de la ré cip rocié, L b ratoire National d’Es ais, 19 9

VORLÄNDER, M an BIETZ, H Novel bro d b n reciprocity tec niq e for simultane u

fre - ield an difu e- ield micro hone cal bration, Acustca 8 , 19 4, 3 5-3 7

BARRERA-FIGUEROA, S RASMUSSEN, K an JACOBSEN, F A time-selective tec niq e

for fre - ield reciprocity cal bration of con en er micro hones, J Acoust Soc Am 1 4, 2 0 ,

14 7-14 6

KWON, H.S SUH, S.J an SUH, J.G Freq en y Win owin Tec niq e for Red cin Multi

-Path Noise in Fre -Field Reciprocity Micro hone Cal bration Method, Ke Engin erin

Materials 3 1-3 3, 2 0 , 12 5-12 8

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