BSTJ 10: 1. January 1931: Condenser and Carbon Microphones - Their Construction and Use. (Jones, W.C.)

10.9 MB·English

Condenser and Carbon Microphones Their Construction
and Tse’

By Wc JONES

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OR samo sortase wid Nave Le deco sie
‘Bos orignal work on the talepione, aly xo ate us ester
sively in sound recording tor motion pictures, namely, the condenser
mnicrephone and the carton microphone. It has therefore been
suggested that ie would be fiting to review ac this cme the coa-
Srution of thew jnirumenes ad conser nome ol their trans
ssion chagneceriacice al the precuations which should be exercised
in their we
Coxnensee Micsamnoxe

In 188i, AE. Dolbear' proposed ¢ telephone instrument which

could be usd cicher as an electrostatic mierepione or receiver, This

ese at Sea Maan Tet Foge” Comsowr, O9, 180
SERS Sone a Toate a
‘nstument consis of two plates iesulaeed (tom one anocher and
Camp together sit the perivhery. The back plate was held in a
fixed pesition whoceas the frost was Free to vibrate and served as a
fliaphragn._ Leis bin th! ithe diapheayn were set in vibration
Sy wma pressure, the elcerical raped ance hecwee the to pla
‘would Ie changed ia responce to the sound waves, and fa source of
Tlertical potential were emesis series with fhe instrument
‘hanging euurens wow Howe mhic would Ta fay fathtul copy of
the ines due to the sare wave. pparesdy Detbesr realized
thatthe enrvent developed i his way won! be ifoute, for im the
fete system which he proposed a subsite for the ane sing
Tells mace imasunsnts Fe emploved the electrostatic instrument
ly are receiver snd adopted che lone vontact type of microphone.
Ai anpenianately the same time as arele aqpeared in the Preach
vest elling acteaion tothe use ofa condenate a= a micrepune sit
Fmemeating on the fact tos! This tye af snicrapuone hae sen Fare
To ess stasiive dan the [ne contac: type

(ining. tthe lew sensitivity of the condenser microphone, the fei

usloess ofthis nscruen was exiremely Hated fer & member
UF yr ant ft diel aot aseume a position 2: imparcance among the
Fhstrumests used in acoustic measurements and sound repreition
util stable sappliers mac bees developed. The develoament of
the viet tube amplifier, huwever. tel vhis need. Ip 1917 B,C.
‘Weste plished am acount of the work: aie he Fall done on an
innproved condenser microphone having 2 stretched diaphragm and
Ihack sre so ncaa relative to hic <Bapieagen that i addition to
Serving i one place of the comers i aldo suicient a damping
oviiee ce eect of Gisphragm resonance to a miaimum’ The
Fesponae of ns oaicument was suiciently uniform over a wide rang
Uf frequencies go make et ey war im Ih quali cound repro=
uct het 2 alae sol scout meaeureasacs in general

"The szajor portion of the condenser microphones ted tnday
ound erring esviy the essetia features of the Weate micro
hone.” Markell peogces bas, however, been mae athe design ape
Cmtruction of uhew inetcrmente xara the ints! ecloure an it
Tell no doubt be of incest tr many ln eonsides briefly che macure of
thi alvance

SR ide at tits tsa Urns Seti Lt neo fe the At
SETI nb hin Ec. Wt, Man, Ma 92, S
BBL. SYSTRM TROHNFEAL JOURNAL

In the early microiees employing ai damping che diaphragm wae
comporel fa thin slit sesl which war steed to give fet
frlatively high atiiuess, When seeembled in the microphone the
ifoess was futher ineceased by thet of che air lm between diaphragia
fant che sasning plate with the result char tae raeonaat frequency
‘was well above the frequencies whieh it waa desired to transmit and
The daptnagan vibvated in its nocial mode over a wide fomqueny
ringe. Tn ich a sisucture the mechasiral impedance for equencies
‘low resonance ie du almost eniely to stiffness reactance. Hence &
fustant snd presse previuces substantially the same displasement
1 the diiragn ab all Seequcaies within this range en uniform
esponae results excepcac che very low fequarcics where a xppreciable
Fesuctinn inthe stillness ofthe aie fm occurs. ‘The effective mass of
fatal diaphragm: fey however, relatively large and neceesitates a
comparatively hig eines to tosure the desired reeonane Frequency
Tom the standpoint of eecting maximum rensitvisy ofthe micro=
plioae, ie. csplacement of the diaphragm pec unit force, it is ofcourse
mporcint to make the stifiess low nepal employ ema
44 value of mechanical resncince ax x osietent with the degree of
damping required. An improvement in uth seqcts can be effected
hy decreasing the was of the daphrogus for with a reduced mass a
ven resonant lequeney ean be obcained with lower values of stiiaese
the desire damping eoustont accused with Ieee mcchosica!

“The alnndaum alloys have Uherefoce placed stel ia che diaphragms
vf mogtuf the couleuser ieruplonoria Use taday. typical example
he such a ankropkoze is the Western Elec:re Company's instrament
{G9t-tyee) shown in the photogeaph, Fig. 1, and the cross-sectional
‘ew, Fig. 2," "The daphragin of this inatument ix mace from ale
taint slay sheet 0011 inch in thickress "The eyes are elenped
securely between chreadeal ring, gaskets of ufter aluminum being
provided fo prevent darsage at the clamping surface. The requisite
fifiness is obtained by advancing the seething Fg ils rexinant
frequency of S000 cycles ie obtine’. The methal of determining
the ecorane Frequency ofthe diaphragen eas ‘follows. Thedlianhragin
Dovenly tbe ene x conplel to a coelenser microphone which ix

provided with oullable crew fre meaning ie output, A special
lalepone zeseiver in laced in contact with che diaphragm on the
side opposite co the coupler. Current from a vaczum tube osillacer
ia then passed through the winding of the resiver, setting up eddy
cucrenttin the disphragen unde fst. The forees which are developed
45 result of the reaction of che magnetic ell produced by the eddy
currents end that of the permanent magnet of the receiver set the
test diaphragm in motion. The resonant frequency is determined yy
ning the requener at which te oxtput from the colenser miro
phone sa maimum.

Th the cary Wente microphone dh damping plage was continuous
sucfase, Suheequent work by 1 B, Crandall” ehowed tar che re
tired amount of desnping athe resonant frequency eon be ale
‘thout adding vray to the impedance at other Crequencies by eu
fing groovesin the plate, ‘This ree he sts iatredced By the
tie nl aa seceaoed she ireglarty in reas ac Tow Eequencies
previously mentioned, The grooves in the damping plase of the

Western Eloise Compaay's SMeevnm rieraphone ate eat at sight
snes. Holes, tapeced as th nner eal bn reduce reonant eects,
ce red through te plate #1 the intermetion of the grooves co form
fonaccting passages betwen the air lan at te front and the cavity
tte back, Invondes to prevent che msunanee whic wld reall
(ihe groves ented Into the portion nf the ehanuber suerounding
‘ecasing plate, the outer er are loved by an snoular ring which
1 feened over ashouléer on the late, The ssface of te damping:
plate fs plane within € x 10 *inch, The depaeture from a plane in
ny iiliviual ease is determined commercially by the inteference
pattern developed when ea rpally flat plate is placed over the
Uapine place under tet

eer
% HELE SYSTRM TREWMICAS JOUSWAL

A duralumin spaving rig 001 inch in thickness separates the
Shaping plate form the diaphragm, Tei essential that all dust and
tbe exetaalFrna sin seen Th preven reign aera From
fenecingtheengh the holes ine ple plere of sie be fastened over
Uke outer surface. ‘The demablyof Une digphrugm aml the damming
plates made ia a dust pac glass cabinet

ihe hack wel f ve randenser rviropiowe wee righ changes in
tie separatica between the damping plate and ebe diaphragm of
suiciens magnitude affect not only the sensitivity of the stent
Do also its srequeney eaposse characteristic woul? resale Irom vais
tions ia Decometris pressure. Complete coupensution for these

&

Ss

sd
Wi

a

:

3}

Fig. 2 Cerwatal wo the JBL alee mish

chcgns in prestre can only be obsnined ly pevaittiag free ineee=
Sange of air beowrer doth sides of the mictophore dixparigm
‘This is, however, objectionable owing to tae faet that soficient
ruintre i likely to be introduced to tact comcsion and afer the
insulation beeween te damping place and the diaphcagm. 8 cor:
ensiting ciphragm of organic matte has theceore bea introduced
‘which prevents ehicwnderirahle effect of humidiey but i auficienty
Tow in stisnes to equstize the changes in premre evenara! im Ube
ovina ace of the miceophone

In order ty prevent tinamisson lees ae voice frequensies de to
the presence of the compensating diaphragm, an acoustic valve is
CORDENER AND CLNNON AticROPNONES a

inserted berweea the damping plate and this diaphragm, ‘This valve
Couniste of a dge of silk lampal besween two aluminum plates of
taarqual digmeiers. Gas in passing trom the camping plate 4 the
ompensttg dlaphragm roves laterally from the wage of the saller
plate through the sill to Pole in The center of the larger plate, ‘The
EEnpedance af his path ie high at voie tequenclee but low enol for
steadily applied pressuredillerences er

in baroetrie preseur,

Alter the slamping plate and diapragrn ice sew le the sane
hereon the clamping wings ill with Sesno tn wee the Se
govlight anil estiade meistine. tale i, however, provided for
filling the wiophone wich aitregea. ‘The purpose ofthe nteugen i
prevent eoemaion of the comping plate ard lzphragm sures
find climate any eeduction in orare ue we esicatin of the sealing
compu

TH has been customary far some time 10 determine the reqwne
characteristics cf i enmlener atksaphone by the thermophone
tuettiod* In meking this measurement the glean af the ie
Phone ie coupled oust vally co the shermopie in whe mganee
Sliowa in Fig, 3. The theraaphone consists of te ettipe of gold Feil

came for change

sesitin, |t[peite

which sce mounted on a plute and Gi isto the recess in the frat af
The miesophone, “Cepiuary tubs ae provided for Alling the spaze
fnelosed beeween the plats and the snieruphowe Wapheagan wth

The There a Hosen Seng uf Sou” HD al a 1h
Iydrogea, This is done in evder to make the wave-length of ue soul
developed in the recess as Iasge ae posible cumipared wil duvensons
fof che chamber, It thie were not tke case the sound presiore at dt
Serene postion in the chembee would nat be in phase and the condi-
tions on which the compucarione of the mognitade af the sonnd
pressure are haved would nol he set. A dtect current of known
Yalieiepaseedthuough theft. Superaypered upon te lrect erent
isan alernatingeuorent of the dexsed frequency whith causes fact
ations in The txapexature of she foil art io che yaw dmmedalely
furrowading it. “Uhese duciuations in tepesalure Se carn cance
changes in the prose on the micreptone diaphragm. The mag
tude of the presare developed on te diaphragn can be compte
from the sonetanin of the thermnghine and the emsphng tsity, al
the volzage developed by the mierophene to a given pressure deter
mmived with sulablemeasu-ing circuits" Obyiovsly. sucka elibration
‘fords a measure of the response of the microphone in terms ofthe
Actual pressure developed on the diaphragan and ix iadepndent of the
ester dimension of the frame, Hen, Stes nt tale it
faccoun: zay effect which the microphone may have on the sound fed
‘hen used a3 a pice up instrument for recccding brileasting pte
poses, The thermoplione calibration is often refewel to ana "pee=
fore" calibration andthe respon obtained By placing the fnatrament
inscsennd eld af enmstant presse, a eld" calienion, A thermo=
phone calibration of eprerenlative Weslers Flecsie 10 Ltype con
lenses inierophone i shun on ig

Far sn’ of the uaes co which the eadenseravcrophone is put, for
cesmiple he eallnation of lal Iype telephone secuivers the cl
tons under which i= eperaes agree Wa Uhese under wr Uther
plows valibration is made. There ase, however, casey winse tie
fisoement docs uot exit, tor when a microphone is inserted iu
une eld oF unilorm intensity che rewire on the diaphragm mar
Hieart rather widely from a eonat vale i veniam requenes
anges. Severs] aneles "have seeent'y eppaated calling sttention to
thin dixctepancyTetween the prcssice al Geld celdnatioas aa
inting cut thal a preure cation of @averaphone way not be
fatirely representative of its peefoemance under the cnditions which

Hs Belen Spe

{ee Tlotane Troan! I, Main ad
ote pe Ae Pregnies i
“The dilflesence between the yresure el fel elibraons is due
several factoce, In the Aree plnce the sound ie fected aroued the
twicrapione diferenely st eittrene feqoenies. AL Frequeneice where
the wavelength is large ce compared wath ite exer dimensios the
presse ie ie same se shat of the andaturhed wave. Ae the higher
[mquencies where the microphre lange in emacaon wil the wave
length of the sound, che restr is tier at develop at the lower
Frequencies In the D¢-type mierephone the elfet of direction

isd These elnino oc recone kane

frst become noticeale in the sesion of 1200 eyees and reaches a
‘iimurs of 6 ab at spprosimaely 2200 eyelos. The senond factor
ic causes a difereace beeween the pressure ane Sed eatwain i
eonelie esumaice in Lhe shallow caviey ia tront of the microphone
"This eanses the pressure actuating the diaphragm to be higher ¢has
tiva ul the forklent sound wave in t3e fequency regia of 1500 ta
S500 vyelen "The misma increase in pressure oocurs at aoprox
‘nately 8500 geles, tthe sunt sone et Inet mative the

Wi I annie
Sp ap io ae | TNs
in ST AEN. #

tuiesophome chat the waves supeoacs Icom a destin normal to the
Aiaphsagon ancl cellection irom surrounding walls and wise is
Depligible, she sombiasd elect of lteaccion and reeonsnce ht
predice @ attiinum departure trom Hates of apyrovimacely Lah

Tsshonn by the fil calibstion Pig, 3° Ihe sod ave wave

thse cen are tne rom segblhes wrk uf P. 8. Br of Cie Rel
slong he diaphragea the ellentive presure ie reed a the higher
Frequencies due i diference ia plane. Hence, if the direction of
‘pyro af the sound wave i parallel tothe pave ofthe diaphragm,
‘he departure from dacacsa is mater‘e'y reduced. This ia brought
tu quite elecly by the held ealityation for sound approaching fromm
{ dizeetion parallel to the diaphragm, Vg, 6

‘Te discrepancy beewven the pressure and Geld calibrations of the
condenser microphone involves two important assumptions, namely
{plane sound wave ano relection [ram wall er surending object,
{When the microphone is wed in a seucio muck of the eoune veanaes
Ue displragan byway of reeton ean the walle of the vi Tae
fequivement of no fetecton i cherefore noe mice snd the inflee of
Ie aoountie properties of the reflecting sufeces is added tothe char
seteiaticn of the mienpane. The efleet of che diluion che

“ oe
tad RH 1

: —

H Siesta —t -

x TE aasisll

sono Geld ad the tenency for mont uacerials toe ere absorbent
for sounds of high feequercy appeare co cause the response under
studio conditis to be more acl ike det abzained when the sured
fnptoackes in a direction pole to the diaphragm si make the
Stpertires from the peseuneraliation Wess waked chan the 5

talibmation for a etn sur to te digheagen yield etic,

"The pethape accounts in part at lest fr the instances fm which a
corrective network designed W canenwate for the beld calibration
dural to te dlaphneye Fichal vo er atealinpeaventea i
‘iy

"The acoustic conditions wader which a aicsophone i used cover a
wide cage, 1 oud theveove be difalt not impceble to adopt
‘ect of conditions for wee in connection witha Red ealibention ofthe
Ccndenser micron which wo! be own tt ie representalive
of those encuuntered in practice, Phe prseure method w elbration
tn the orher band is definite. simple, and capable of being arcuately
‘halicsted in dierent laboratories, Ln view of thie stuaion fe woul
sco viene to retin, at vat for cae pros, cae thernalyne
pressure method of ealibration for yeneral se. Tu cases where previ
fwanditadive measurements are required @ fed calibration of the
thieeophone should of ecurse be were sles he condos of actual
tse. Varicus methods of making sch 2 calibration have been ge
toed, “The Rayleigh dise bas been wed extensively in thie work
thus fer ut there sve ertain very definite Titations to he extent
in whieh it can be applied, Si intresting claconson of he se af
tie Kayleigt diss may be found in yspers by E. J. Basues and W,
Wert" and L. J Sivan!

Te would ecortreazonable to expect chat fatare design work would
hncaeectlthwardvesicing cransition, resonant aul phase dierence
fects toa minimum, ‘The mss of wore along this ne have been
Fepocted hy S. Ballantine ® and D. A. ORven To both ieatanees
Ive mechar eal desig fs euck that the reoneat eavity ia rent ol He
‘diaphragm is eliminate vt the busing © spherical ce serearline
tor aluce the dfraction effet. Teer has as get een Teele oppor
ftv ity to determine che extent w the pratial iospravernentelfeeted
by these ehacges ia design sd the whole discussion continues to be
savewhat aden i character,

Cxmsa Mecaanas

Let's orisnal micraphone wis exetially generator and hence
‘was Tice in is output to Lae stair so5009 power aealable nt
Ite digphragat, The denne for Inleshosie crennupication over
Touger distances led tothe erly intrdurtion ofa varben microphone,
In this instrument {ne cesntance of the carbon eleme® is cre te
yin eeeprnse tu Pe wn yoesauce on the diaphragen ane pruduces
Changes in the curent suppdin] (rom aa extecal source of electrical
potential whieh are fae fief copies ofthe pressure anges which
Ceritute the enurel weve. The eaten microphone fs tereure
iowral amplifier wil Teal source of power is controll by
fhe acoustic power of he sound wre

The earhen elemet of "tutton” of the frat recophoncs (Edizon,
1877) wa made from plumbago compzessed Inco cysndrieal form

‘te ap pear te Rey DB. Mar an
SE er lalla teil Wee”. an
Pcie te tase

rca RE Reals ye HS Meeewmena” PA oe